Biology

Biology is the scientific study of life.[1][2][3] It is a natural science with a broad scope but has several
unifying themes that tie it together as a single, coherent field.[1][2][3] For instance, all organisms are
made up of cells that process hereditary information encoded in genes, which can be transmitted to
future generations. Another major theme is evolution, which explains the unity and diversity of life.[1][2]
[3] Energy processing is also important to life as it allows organisms to move, grow, and reproduce.[1][2]
[3] Finally, all organisms are able to regulate their own internal environments.[1][2][3][4][5]

Biologists are able to study life at multiple levels of organization,[1] from the molecular biology of a cell
to the anatomy and physiology of plants and animals, and evolution of populations.[1][6] Hence, there
are multiple subdisciplines within biology, each defined by the nature of their research questions and
the tools that they use.[7][8][9] Like other scientists, biologists use the scientific method to make
observations, pose questions, generate hypotheses, perform experiments, and form conclusions about
the world around them.[1]

Life on Earth, which emerged more than 3.7 billion years ago,[10] is immensely diverse. Biologists have
sought to study and classify the various forms of life, from prokaryotic organisms such as archaea and
bacteria to eukaryotic organisms such as protists, fungi, plants, and animals. These various organisms
contribute to the biodiversity of an ecosystem, where they play specialized roles in the cycling of
nutrients and energy through their biophysical environment.

History
The earliest of roots of science, which included medicine, can be traced to
ancient Egypt and Mesopotamia in around 3000 to 1200 BCE.[11][12] Their
contributions shaped ancient Greek natural philosophy.[11][12][13][14]
Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed
extensively to the development of biological knowledge. He explored
biological causation and the diversity of life. His successor, Theophrastus,
began the scientific study of plants.[15] Scholars of the medieval Islamic
world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī
(828–896), who wrote on botany,[16] and Rhazes (865–925) who wrote on
anatomy and physiology. Medicine was especially well studied by Islamic
scholars working in Greek philosopher traditions, while natural history
drew heavily on Aristotelian thought.

Biology began to quickly develop with Anton van Leeuwenhoek's dramatic Diagram of a fly from
improvement of the microscope. It was then that scholars discovered Robert Hooke's innovative
spermatozoa, bacteria, infusoria and the diversity of microscopic life. Micrographia, 1665

Investigations by Jan Swammerdam led to new interest in entomology and

helped to develop techniques of microscopic dissection and staining.[17] Advances in microscopy had a
profound impact on biological thinking. In the early 19th century, biologists pointed to the central
importance of the cell. In 1838, Schleiden and Schwann began promoting the now universal ideas that
(1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life,
although they opposed the idea that (3) all cells come from the division of other cells, continuing to
support spontaneous generation. However, Robert Remak and Rudolf Virchow were able to reify the
third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into cell
theory.[18][19]
Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published
a basic taxonomy for the natural world in 1735, and in the 1750s introduced scientific names for all his
species.[20] Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living
forms as malleable—even suggesting the possibility of common descent.[21]

Serious evolutionary thinking originated with the works of Jean-Baptiste

Lamarck, who presented a coherent theory of evolution.[23] The British
naturalist Charles Darwin, combining the biogeographical approach of
Humboldt, the uniformitarian geology of Lyell, Malthus's writings on
population growth, and his own morphological expertise and extensive
natural observations, forged a more successful evolutionary theory based on
natural selection; similar reasoning and evidence led Alfred Russel Wallace
to independently reach the same conclusions.[24][25]

The basis for modern genetics began with the work of Gregor Mendel in
1865.[26] This outlined the principles of biological inheritance.[27] However,
In 1842, Charles Darwin
the significance of his work was not realized until the early 20th century
penned his first sketch of
when evolution became a unified theory as the modern synthesis reconciled
On the Origin of Species.
Darwinian evolution with classical genetics.[28] In the 1940s and early [22]
1950s, a series of experiments by Alfred Hershey and Martha Chase pointed
to DNA as the component of chromosomes that held the trait-carrying units
that had become known as genes. A focus on new kinds of model organisms such as viruses and bacteria,
along with the discovery of the double-helical structure of DNA by James Watson and Francis Crick in
1953, marked the transition to the era of molecular genetics. From the 1950s onwards, biology has been
vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana, Robert
W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons. The Human
Genome Project was launched in 1990 to map the human genome.[29]

Chemical basis

Atoms and molecules

All organisms are made up of chemical elements;[30] oxygen, carbon, hydrogen, and nitrogen account
for most (96%) of the mass of all organisms, with calcium, phosphorus, sulfur, sodium, chlorine, and
magnesium constituting essentially all the remainder. Different elements can combine to form
compounds such as water, which is fundamental to life.[30] Biochemistry is the study of chemical
processes within and relating to living organisms. Molecular biology is the branch of biology that seeks
to understand the molecular basis of biological activity in and between cells, including molecular
synthesis, modification, mechanisms, and interactions.

Water
Life arose from the Earth's first ocean, which formed some 3.8 billion years ago.[31] Since then, water
continues to be the most abundant molecule in every organism. Water is important to life because it is
an effective solvent, capable of dissolving solutes such as sodium and chloride ions or other small
molecules to form an aqueous solution. Once dissolved in water, these solutes are more likely to come in
contact with one another and therefore take part in chemical reactions that sustain life.[31] In terms of
its molecular structure, water is a small polar molecule with a bent shape formed by the polar covalent
bonds of two hydrogen (H) atoms to one oxygen (O) atom (H2O).[31] Because the O–H bonds are polar,
the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge.
[31] This polar property of water allows it to attract other water
molecules via hydrogen bonds, which makes water cohesive.[31]
Surface tension results from the cohesive force due to the attraction
between molecules at the surface of the liquid.[31] Water is also
adhesive as it is able to adhere to the surface of any polar or charged
non-water molecules.[31] Water is denser as a liquid than it is as a
solid (or ice).[31] This unique property of water allows ice to float
above liquid water such as ponds, lakes, and oceans, thereby
insulating the liquid below from the cold air above.[31] Water has the
capacity to absorb energy, giving it a higher specific heat capacity
than other solvents such as ethanol.[31] Thus, a large amount of
energy is needed to break the hydrogen bonds between water Model of hydrogen bonds (1)
molecules to convert liquid water into water vapor. [31] As a between molecules of water

molecule, water is not completely stable as each water molecule

continuously dissociates into hydrogen and hydroxyl ions before reforming into a water molecule again.
[31] In pure water, the number of hydrogen ions balances (or equals) the number of hydroxyl ions,

resulting in a pH that is neutral.

Organic compounds
Organic compounds are molecules that contain carbon bonded to
another element such as hydrogen.[31] With the exception of water,
nearly all the molecules that make up each organism contain carbon.
[31][32] Carbon can form covalent bonds with up to four other atoms,

enabling it to form diverse, large, and complex molecules.[31][32] For

example, a single carbon atom can form four single covalent bonds
such as in methane, two double covalent bonds such as in carbon
dioxide (CO2), or a triple covalent bond such as in carbon monoxide
(CO). Moreover, carbon can form very long chains of
Organic compounds such as
interconnecting carbon–carbon bonds such as octane or ring-like
glucose are vital to organisms.
structures such as glucose.

The simplest form of an organic molecule is the hydrocarbon, which is a large family of organic
compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon
backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and
sulfur (S), which can change the chemical behavior of that compound.[31] Groups of atoms that contain
these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called
functional groups.[31] There are six prominent functional groups that can be found in organisms: amino
group, carboxyl group, carbonyl group, hydroxyl group, phosphate group, and sulfhydryl group.[31]

In 1953, the Miller–Urey experiment showed that organic compounds could be synthesized abiotically
within a closed system mimicking the conditions of early Earth, thus suggesting that complex organic
molecules could have arisen spontaneously in early Earth (see abiogenesis).[33][31]

Macromolecules
Macromolecules are large molecules made up of smaller subunits or monomers.[34] Monomers include
sugars, amino acids, and nucleotides.[35] Carbohydrates include monomers and polymers of sugars.[36]
Lipids are the only class of macromolecules that are not made up of polymers. They include steroids,
phospholipids, and fats,[35] largely nonpolar and hydrophobic (water-repelling) substances.[37] Proteins
are the most diverse of the macromolecules. They include enzymes,
transport proteins, large signaling molecules, antibodies, and
structural proteins. The basic unit (or monomer) of a protein is an
amino acid.[34] Twenty amino acids are used in proteins.[34] Nucleic
acids are polymers of nucleotides.[38] Their function is to store,
transmit, and express hereditary information.[35]

Cells
Cell theory states that cells are the fundamental units of life, that all
The (a) primary, (b) secondary, (c)
living things are composed of one or more cells, and that all cells
tertiary, and (d) quaternary
arise from preexisting cells through cell division.[39] Most cells are structures of a hemoglobin protein
very small, with diameters ranging from 1 to 100 micrometers and
are therefore only visible under a light or electron microscope.[40]
There are generally two types of cells: eukaryotic cells, which contain a nucleus, and prokaryotic cells,
which do not. Prokaryotes are single-celled organisms such as bacteria, whereas eukaryotes can be
single-celled or multicellular. In multicellular organisms, every cell in the organism's body is derived
ultimately from a single cell in a fertilized egg.

Cell structure
Every cell is enclosed within a cell membrane that separates its
cytoplasm from the extracellular space.[41] A cell membrane consists
of a lipid bilayer, including cholesterols that sit between
phospholipids to maintain their fluidity at various temperatures. Cell
membranes are semipermeable, allowing small molecules such as
oxygen, carbon dioxide, and water to pass through while restricting
the movement of larger molecules and charged particles such as
ions.[42] Cell membranes also contain membrane proteins, including Structure of an animal cell depicting
various organelles
integral membrane proteins that go across the membrane serving as
membrane transporters, and peripheral proteins that loosely attach
to the outer side of the cell membrane, acting as enzymes shaping the cell.[43] Cell membranes are
involved in various cellular processes such as cell adhesion, storing electrical energy, and cell signalling
and serve as the attachment surface for several extracellular structures such as a cell wall, glycocalyx,
and cytoskeleton.

Within the cytoplasm of a cell, there are many biomolecules such as

proteins and nucleic acids.[44] In addition to biomolecules,
eukaryotic cells have specialized structures called organelles that
have their own lipid bilayers or are spatially units.[45] These
organelles include the cell nucleus, which contains most of the cell's
DNA, or mitochondria, which generate adenosine triphosphate
(ATP) to power cellular processes. Other organelles such as
endoplasmic reticulum and Golgi apparatus play a role in the
Structure of a plant cell synthesis and packaging of proteins, respectively. Biomolecules such
as proteins can be engulfed by lysosomes, another specialized
organelle. Plant cells have additional organelles that distinguish
them from animal cells such as a cell wall that provides support for the plant cell, chloroplasts that
harvest sunlight energy to produce sugar, and vacuoles that provide storage and structural support as
well as being involved in reproduction and breakdown of plant seeds.[45] Eukaryotic cells also have
cytoskeleton that is made up of microtubules, intermediate filaments, and microfilaments, all of which
provide support for the cell and are involved in the movement of the cell and its organelles.[45] In terms
of their structural composition, the microtubules are made up of tubulin (e.g., α-tubulin and β-tubulin)
whereas intermediate filaments are made up of fibrous proteins.[45] Microfilaments are made up of actin
molecules that interact with other strands of proteins.[45]

Metabolism
All cells require energy to sustain cellular processes. Metabolism is
the set of chemical reactions in an organism. The three main
purposes of metabolism are: the conversion of food to energy to run
cellular processes; the conversion of food/fuel to monomer building
blocks; and the elimination of metabolic wastes. These enzyme-
catalyzed reactions allow organisms to grow and reproduce,
maintain their structures, and respond to their environments.
Metabolic reactions may be categorized as catabolic—the breaking
down of compounds (for example, the breaking down of glucose to
Example of an enzyme-catalysed
pyruvate by cellular respiration); or anabolic—the building up exothermic reaction
(synthesis) of compounds (such as proteins, carbohydrates, lipids,
and nucleic acids). Usually, catabolism releases energy, and
anabolism consumes energy. The chemical reactions of metabolism are organized into metabolic
pathways, in which one chemical is transformed through a series of steps into another chemical, each
step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow
organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling
them to spontaneous reactions that release energy. Enzymes act as catalysts—they allow a reaction to
proceed more rapidly without being consumed by it—by reducing the amount of activation energy
needed to convert reactants into products. Enzymes also allow the regulation of the rate of a metabolic
reaction, for example in response to changes in the cell's environment or to signals from other cells.

Cellular respiration
Cellular respiration is a set of metabolic reactions and
processes that take place in cells to convert chemical
energy from nutrients into adenosine triphosphate
(ATP), and then release waste products.[46] The reactions
involved in respiration are catabolic reactions, which
break large molecules into smaller ones, releasing
energy. Respiration is one of the key ways a cell releases
chemical energy to fuel cellular activity. The overall
reaction occurs in a series of biochemical steps, some of
which are redox reactions. Although cellular respiration
is technically a combustion reaction, it clearly does not Respiration in a eukaryotic cell
resemble one when it occurs in a cell because of the slow,
controlled release of energy from the series of reactions.

Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular
respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis, citric acid
cycle (or Krebs cycle), electron transport chain, and oxidative phosphorylation.[47] Glycolysis is a
metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates, with
two net molecules of ATP being produced at the same time.[47] Each pyruvate is then oxidized into
acetyl-CoA by the pyruvate dehydrogenase complex, which also generates NADH and carbon dioxide.
Acetyl-CoA enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of
the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH2, and 2 ATP molecules.
Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the mitochondrial
cristae. Oxidative phosphorylation comprises the electron transport chain, which is a series of four
protein complexes that transfer electrons from one complex to another, thereby releasing energy from
NADH and FADH2 that is coupled to the pumping of protons (hydrogen ions) across the inner
mitochondrial membrane (chemiosmosis), which generates a proton motive force.[47] Energy from the
proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs.
The transfer of electrons terminates with molecular oxygen being the final electron acceptor.

If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a
process of fermentation. The pyruvate is not transported into the mitochondrion but remains in the
cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the
purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the
excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis. In the absence
of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD+ for
glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product
is lactic acid. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when
energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms
joined by NADH. During anaerobic glycolysis, NAD+ regenerates when pairs of hydrogen combine with
pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible
reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when
oxygen becomes available, NAD+ attaches to hydrogen from lactate to form ATP. In yeast, the waste
products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol
fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does
not require oxygen.

Photosynthesis
Photosynthesis is a process used by plants and other organisms to
convert light energy into chemical energy that can later be released
to fuel the organism's metabolic activities via cellular respiration.
This chemical energy is stored in carbohydrate molecules, such as
sugars, which are synthesized from carbon dioxide and water.[48][49]
[50] In most cases, oxygen is released as a waste product. Most

plants, algae, and cyanobacteria perform photosynthesis, which is

largely responsible for producing and maintaining the oxygen
content of the Earth's atmosphere, and supplies most of the energy
necessary for life on Earth.[51]

Photosynthesis has four stages: Light absorption, electron transport,

ATP synthesis, and carbon fixation.[47] Light absorption is the initial
step of photosynthesis whereby light energy is absorbed by
Photosynthesis changes sunlight
chlorophyll pigments attached to proteins in the thylakoid into chemical energy, splits water to
membranes. The absorbed light energy is used to remove electrons liberate O2, and fixes CO2 into
from a donor (water) to a primary electron acceptor, a quinone sugar.
designated as Q. In the second stage, electrons move from the
quinone primary electron acceptor through a series of electron
carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP+, which is
reduced to NADPH, a process that takes place in a protein complex called photosystem I (PSI). The
transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the
thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more
concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated
across the inner mitochondrial membrane in aerobic respiration.[47]

During the third stage of photosynthesis, the movement of protons down their concentration gradients
from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by
that same ATP synthase.[47] The NADPH and ATPs generated by the light-dependent reactions in the
second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose
by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose
bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle.[52]

Cell signaling
Cell signaling (or communication) is the ability of cells to receive, process, and transmit signals with its
environment and with itself.[53][54] Signals can be non-chemical such as light, electrical impulses, and
heat, or chemical signals (or ligands) that interact with receptors, which can be found embedded in the
cell membrane of another cell or located deep inside a cell.[55][54] There are generally four types of
chemical signals: autocrine, paracrine, juxtacrine, and hormones.[55] In autocrine signaling, the ligand
affects the same cell that releases it. Tumor cells, for example, can reproduce uncontrollably because
they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to
nearby cells and affects them. For example, brain cells called neurons release ligands called
neurotransmitters that diffuse across a synaptic cleft to bind with a receptor on an adjacent cell such as
another neuron or muscle cell. In juxtacrine signaling, there is direct contact between the signaling and
responding cells. Finally, hormones are ligands that travel through the circulatory systems of animals or
vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence
the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that
bind with an inotropic receptor can alter the excitability of a target cell. Other types of receptors include
protein kinase receptors (e.g., receptor for the hormone insulin) and G protein-coupled receptors.
Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which
a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal
transduction.

Cell cycle
The cell cycle is a series of events that take place in a cell
that cause it to divide into two daughter cells. These
events include the duplication of its DNA and some of its
organelles, and the subsequent partitioning of its
cytoplasm into two daughter cells in a process called cell
division.[56] In eukaryotes (i.e., animal, plant, fungal, and
protist cells), there are two distinct types of cell division:
mitosis and meiosis.[57] Mitosis is part of the cell cycle, in
which replicated chromosomes are separated into two
new nuclei. Cell division gives rise to genetically identical In meiosis, the chromosomes duplicate and the
cells in which the total number of chromosomes is homologous chromosomes exchange genetic
maintained. In general, mitosis (division of the nucleus) information during meiosis I. The daughter cells
is preceded by the S stage of interphase (during which divide again in meiosis II to form haploid gametes.
the DNA is replicated) and is often followed by telophase
and cytokinesis; which divides the cytoplasm, organelles and cell membrane of one cell into two new
cells containing roughly equal shares of these cellular components. The different stages of mitosis all
together define the mitotic phase of an animal cell cycle—the division of the mother cell into two
genetically identical daughter cells.[58] The cell cycle is a vital process by which a single-celled fertilized
egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some
internal organs are renewed. After cell division, each of the daughter cells begin the interphase of a new
cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of
DNA replication followed by two divisions.[59] Homologous chromosomes are separated in the first
division (meiosis I), and sister chromatids are separated in the second division (meiosis II). Both of
these cell division cycles are used in the process of sexual reproduction at some point in their life cycle.
Both are believed to be present in the last eukaryotic common ancestor.

Prokaryotes (i.e., archaea and bacteria) can also undergo cell division (or binary fission). Unlike the
processes of mitosis and meiosis in eukaryotes, binary fission in prokaryotes takes place without the
formation of a spindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly
coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it
increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium
(triggered by FtsZ polymerization and "Z-ring" formation).[60] The new cell wall (septum) fully
develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled
DNA rods, ribosomes, and plasmids.

Sexual reproduction and meiosis

Meiosis is a central feature of sexual reproduction in eukaryotes, and the most fundamental function of
meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by
parents.[61][62] Two aspects of sexual reproduction, meiotic recombination and outcrossing, are likely
maintained respectively by the adaptive advantages of recombinational repair of genomic DNA damage
and genetic complementation which masks the expression of deleterious recessive mutations.[63]

The beneficial effect of genetic complementation, derived from outcrossing (cross-fertilization) is also
referred to as hybrid vigor or heterosis. Charles Darwin in his 1878 book The Effects of Cross and Self-
Fertilization in the Vegetable Kingdom[64] at the start of chapter XII noted “The first and most
important of the conclusions which may be drawn from the observations given in this volume, is that
generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on
which I experimented.” Genetic variation, often produced as a byproduct of sexual reproduction, may
provide long-term advantages to those sexual lineages that engage in outcrossing.[63]

Genetics

Inheritance
Genetics is the scientific study of inheritance.[65][66][67] Mendelian inheritance, specifically, is the
process by which genes and traits are passed on from parents to offspring.[27] It has several principles.
The first is that genetic characteristics, alleles, are discrete and have alternate forms (e.g., purple vs.
white or tall vs. dwarf), each inherited from one of two parents. Based on the law of dominance and
uniformity, which states that some alleles are dominant while others are recessive; an organism with at
least one dominant allele will display the phenotype of that dominant allele. During gamete formation,
the alleles for each gene segregate, so that each gamete carries only one allele for each gene.
Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, the law of
independent assortment, states that genes of different traits can segregate independently during the
formation of gametes, i.e., genes are unlinked. An exception to this
rule would include traits that are sex-linked. Test crosses can be
performed to experimentally determine the underlying genotype of
an organism with a dominant phenotype.[68] A Punnett square can
be used to predict the results of a test cross. The chromosome theory
of inheritance, which states that genes are found on chromosomes,
was supported by Thomas Morgans's experiments with fruit flies,
which established the sex linkage between eye color and sex in these
insects.[69]

Genes and DNA Punnett square depicting a cross

A gene is a unit of heredity that corresponds to a region of between two pea plants
deoxyribonucleic acid (DNA) that carries genetic information that heterozygous for purple (B) and
controls form or function of an organism. DNA is composed of two white (b) blossoms

polynucleotide chains that coil around each other to form a double

helix.[70] It is found as linear chromosomes in eukaryotes, and circular
chromosomes in prokaryotes. The set of chromosomes in a cell is
collectively known as its genome. In eukaryotes, DNA is mainly in the cell
nucleus.[71] In prokaryotes, the DNA is held within the nucleoid.[72] The
genetic information is held within genes, and the complete assemblage in
an organism is called its genotype.[73] DNA replication is a
semiconservative process whereby each strand serves as a template for a
new strand of DNA.[70] Mutations are heritable changes in DNA.[70] They
can arise spontaneously as a result of replication errors that were not
corrected by proofreading or can be induced by an environmental mutagen
such as a chemical (e.g., nitrous acid, benzopyrene) or radiation (e.g., x-ray,
gamma ray, ultraviolet radiation, particles emitted by unstable isotopes).
[70] Mutations can lead to phenotypic effects such as loss-of-function, gain-

of-function, and conditional mutations.[70] Some mutations are beneficial,

as they are a source of genetic variation for evolution.[70] Others are Bases lie between two
harmful if they were to result in a loss of function of genes needed for spiraling DNA strands.
survival.[70]

Gene expression
Gene expression is the molecular process by which a
genotype encoded in DNA gives rise to an observable
phenotype in the proteins of an organism's body. This
process is summarized by the central dogma of molecular
biology, which was formulated by Francis Crick in 1958.
[74][75][76] According to the Central Dogma, genetic

information flows from DNA to RNA to protein. There

are two gene expression processes: transcription (DNA to
The extended central dogma of molecular biology
RNA) and translation (RNA to protein).[77]
includes all the processes involved in the flow of
genetic information.
Gene regulation
The regulation of gene expression by environmental factors and during different stages of development
can occur at each step of the process such as transcription, RNA splicing, translation, and post-
translational modification of a protein.[78] Gene expression can be influenced by positive or negative
regulation, depending on which of the two types of regulatory proteins called transcription factors bind
to the DNA sequence close to or at a promoter.[78] A cluster of genes that share the same promoter is
called an operon, found mainly in prokaryotes and some lower eukaryotes (e.g., Caenorhabditis
elegans).[78][79] In positive regulation of gene expression, the activator is the transcription factor that
stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation
occurs when another transcription factor called a repressor binds to a DNA sequence called an operator,
which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called
inducers (e.g., allolactose), thereby allowing transcription to occur.[78] Specific genes that can be
activated by inducers are called inducible genes, in contrast to constitutive genes that are almost
constantly active.[78] In contrast to both, structural genes encode proteins that are not involved in gene
regulation.[78] In addition to regulatory events involving the promoter, gene expression can also be
regulated by epigenetic changes to chromatin, which is a complex of DNA and protein found in
eukaryotic cells.[78]

Genes, development, and evolution

Development is the process by which a multicellular organism (plant or animal) goes through a series of
changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.
[80] There are four key processes that underlie development: Determination, differentiation,

morphogenesis, and growth. Determination sets the developmental fate of a cell, which becomes more
restrictive during development. Differentiation is the process by which specialized cells arise from less
specialized cells such as stem cells.[81][82] Stem cells are undifferentiated or partially differentiated cells
that can differentiate into various types of cells and proliferate indefinitely to produce more of the same
stem cell.[83] Cellular differentiation dramatically changes a cell's size, shape, membrane potential,
metabolic activity, and responsiveness to signals, which are largely due to highly controlled
modifications in gene expression and epigenetics. With a few exceptions, cellular differentiation almost
never involves a change in the DNA sequence itself.[84] Thus, different cells can have very different
physical characteristics despite having the same genome. Morphogenesis, or the development of body
form, is the result of spatial differences in gene expression.[80] A small fraction of the genes in an
organism's genome called the developmental-genetic toolkit control the development of that organism.
These toolkit genes are highly conserved among phyla, meaning that they are ancient and very similar in
widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and
the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox
genes. Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a
developing embryo or larva.[85]

Evolution

Evolutionary processes
Evolution is a central organizing concept in biology. It is the change in heritable characteristics of
populations over successive generations.[86][87] In artificial selection, animals were selectively bred for
specific traits. [88] Given that traits are inherited, populations contain a varied mix of traits, and
reproduction is able to increase any population, Darwin argued that in the natural world, it was nature
that played the role of humans in selecting for specific traits.[88] Darwin inferred that individuals who
possessed heritable traits better adapted to their environments are more likely to survive and produce
more offspring than other individuals.[88] He further inferred that this would lead to the accumulation
of favorable traits over successive generations, thereby increasing the match between the organisms and
their environment.[89][90][91][88][92]

Speciation
A species is a group of organisms that mate with one another and
speciation is the process by which one lineage splits into two
lineages as a result of having evolved independently from each other.
[93] For speciation to occur, there has to be reproductive isolation.
[93] Reproductive isolation can result from incompatibilities between

genes as described by Bateson–Dobzhansky–Muller model.

Reproductive isolation also tends to increase with genetic Natural selection for darker traits
divergence. Speciation can occur when there are physical barriers
that divide an ancestral species, a process known as allopatric
speciation.[93]

Phylogeny
A phylogeny is an evolutionary
history of a specific group of
organisms or their genes.[94] It can
be represented using a phylogenetic
tree, a diagram showing lines of
descent among organisms or their
genes. Each line drawn on the time
axis of a tree represents a lineage of
descendants of a particular species
or population. When a lineage
divides into two, it is represented as
a fork or split on the phylogenetic
tree.[94] Phylogenetic trees are the Phylogenetic tree showing the domains of bacteria, archaea, and
basis for comparing and grouping eukaryotes

different species.[94] Different

species that share a feature inherited from a common ancestor are described as having homologous
features (or synapomorphy).[95][96][94] Phylogeny provides the basis of biological classification.[94] This
classification system is rank-based, with the highest rank being the domain followed by kingdom,
phylum, class, order, family, genus, and species.[94] All organisms can be classified as belonging to one
of three domains: Archaea (originally Archaebacteria), bacteria (originally eubacteria), or eukarya
(includes the fungi, plant, and animal kingdoms).[97]

History of life
The history of life on Earth traces how organisms have evolved from the earliest emergence of life to
present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct,
descended from a last universal common ancestor that lived about 3.5 billion years ago.[98][99]
Geologists have developed a geologic time scale that divides the history of the Earth into major
divisions, starting with four eons (Hadean, Archean, Proterozoic, and Phanerozoic), the first three of
which are collectively known as the Precambrian, which lasted approximately 4 billion years.[100] Each
eon can be divided into eras, with the Phanerozoic eon that began 539 million years ago[101] being
subdivided into Paleozoic, Mesozoic, and Cenozoic eras.[100] These three eras together comprise eleven
periods (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic,
Cretaceous, Tertiary, and Quaternary).[100]

The similarities among all known present-day species indicate that they have diverged through the
process of evolution from their common ancestor.[102] Biologists regard the ubiquity of the genetic code
as evidence of universal common descent for all bacteria, archaea, and eukaryotes.[103][10][104][105]
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean
eon and many of the major steps in early evolution are thought to have taken place in this environment.
[106] The earliest evidence of eukaryotes dates from 1.85 billion years ago,[107][108] and while they may

have been present earlier, their diversification accelerated when they started using oxygen in their
metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with
differentiated cells performing specialised functions.[109]

Algae-like multicellular land plants are dated back to about 1 billion years ago,[110] although evidence
suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 billion years ago.
[111] Microorganisms are thought to have paved the way for the inception of land plants in the

Ordovician period. Land plants were so successful that they are thought to have contributed to the Late
Devonian extinction event.[112]

Ediacara biota appear during the Ediacaran period,[113] while vertebrates, along with most other modern
phyla originated about 525 million years ago during the Cambrian explosion.[114] During the Permian
period, synapsids, including the ancestors of mammals, dominated the land,[115] but most of this group
became extinct in the Permian–Triassic extinction event 252 million years ago.[116] During the recovery
from this catastrophe, archosaurs became the most abundant land vertebrates;[117] one archosaur group,
the dinosaurs, dominated the Jurassic and Cretaceous periods.[118] After the Cretaceous–Paleogene
extinction event 66 million years ago killed off the non-avian dinosaurs,[119] mammals increased rapidly
in size and diversity.[120] Such mass extinctions may have accelerated evolution by providing
opportunities for new groups of organisms to diversify.[121]

Diversity

Bacteria and Archaea

Bacteria are a type of cell that constitute a large domain of
prokaryotic microorganisms. Typically a few micrometers in length,
bacteria have a number of shapes, ranging from spheres to rods and
spirals. Bacteria were among the first life forms to appear on Earth,
and are present in most of its habitats. Bacteria inhabit soil, water,
acidic hot springs, radioactive waste,[122] and the deep biosphere of
the Earth's crust. Bacteria also live in symbiotic and parasitic
relationships with plants and animals. Most bacteria have not been
characterised, and only about 27 percent of the bacterial phyla have Bacteria – Gemmatimonas
species that can be grown in the laboratory.[123] aurantiaca (-=1 Micrometer)

Archaea constitute the other domain of prokaryotic cells and were

initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom), a
term that has fallen out of use.[124] Archaeal cells have unique properties separating them from the other
two domains, Bacteria and Eukaryota. Archaea are further divided into multiple recognized phyla.
Archaea and bacteria are generally similar in size and shape, although a few archaea have very different
shapes, such as the flat and square cells of Haloquadratum walsbyi.[125] Despite this morphological
similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely
related to those of eukaryotes, notably for the enzymes involved in
transcription and translation. Other aspects of archaeal
biochemistry are unique, such as their reliance on ether lipids in
their cell membranes,[126] including archaeols. Archaea use more
energy sources than eukaryotes: these range from organic
compounds, such as sugars, to ammonia, metal ions or even
hydrogen gas. Salt-tolerant archaea (the Haloarchaea) use sunlight
as an energy source, and other species of archaea fix carbon, but
unlike plants and cyanobacteria, no known species of archaea does
both. Archaea reproduce asexually by binary fission, fragmentation,
or budding; unlike bacteria, no known species of Archaea form Archaea – Haloarchaea
endospores.

The first observed archaea were extremophiles, living in extreme environments, such as hot springs and
salt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in
almost every habitat, including soil, oceans, and marshlands. Archaea are particularly numerous in the
oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the
planet.

Archaea are a major part of Earth's life. They are part of the microbiota of all organisms. In the human
microbiome, they are important in the gut, mouth, and on the skin.[127] Their morphological, metabolic,
and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen
cycling; organic compound turnover; and maintaining microbial symbiotic and syntrophic communities,
for example.[128]

Eukaryotes
Eukaryotes are hypothesized to have split from archaea,
which was followed by their endosymbioses with bacteria
(or symbiogenesis) that gave rise to mitochondria and
chloroplasts, both of which are now part of modern-day
eukaryotic cells.[129] The major lineages of eukaryotes
diversified in the Precambrian about 1.5 billion years ago
and can be classified into eight major clades: alveolates,
excavates, stramenopiles, plants, rhizarians,
amoebozoans, fungi, and animals. [129] Five of these
clades are collectively known as protists, which are
Euglena, a single-celled eukaryote that can both
mostly microscopic eukaryotic organisms that are not
move and photosynthesize
plants, fungi, or animals.[129] While it is likely that
protists share a common ancestor (the last eukaryotic
common ancestor), [130] protists by themselves do not constitute a separate clade as some protists may
be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such
as algae, invertebrates, or protozoans, the protist grouping is not a formal taxonomic group but is used
for convenience.[129][131] Most protists are unicellular; these are called microbial eukaryotes.[129]

Plants are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the kingdom
Plantae, which would exclude fungi and some algae. Plant cells were derived by endosymbiosis of a
cyanobacterium into an early eukaryote about one billion years ago, which gave rise to chloroplasts.[132]
The first several clades that emerged following primary endosymbiosis were aquatic and most of the
aquatic photosynthetic eukaryotic organisms are collectively described as algae, which is a term of
convenience as not all algae are closely related.[132] Algae comprise several distinct clades such as
glaucophytes, which are microscopic freshwater algae that may have resembled in form to the early
unicellular ancestor of Plantae.[132] Unlike glaucophytes, the other algal clades such as red and green
algae are multicellular. Green algae comprise three major clades: chlorophytes, coleochaetophytes, and
stoneworts.[132]

Fungi are eukaryotes that digest foods outside their bodies,[133] secreting digestive enzymes that break
down large food molecules before absorbing them through their cell membranes. Many fungi are also
saprobes, feeding on dead organic matter, making them important decomposers in ecological systems.
[133]

Animals are multicellular eukaryotes. With few exceptions, animals consume organic material, breathe
oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula,
during embryonic development. Over 1.5 million living animal species have been described—of which
around 1 million are insects—but it has been estimated there are over 7 million animal species in total.
They have complex interactions with each other and their environments, forming intricate food webs.
[134]

Viruses
Viruses are submicroscopic infectious agents that replicate inside
the cells of organisms.[135] Viruses infect all types of life forms, from
animals and plants to microorganisms, including bacteria and
archaea.[136][137] More than 6,000 virus species have been described
in detail.[138] Viruses are found in almost every ecosystem on Earth
and are the most numerous type of biological entity.[139][140]

The origins of viruses in the evolutionary history of life are unclear:

some may have evolved from plasmids—pieces of DNA that can
move between cells—while others may have evolved from bacteria.
In evolution, viruses are an important means of horizontal gene
transfer, which increases genetic diversity in a way analogous to
sexual reproduction.[141] Because viruses possess some but not all
characteristics of life, they have been described as "organisms at the Bacteriophages attached to a
bacterial cell wall
edge of life",[142] and as self-replicators.[143]

Ecology
Ecology is the study of the distribution and abundance of life, the interaction between organisms and
their environment.[144]

Ecosystems
The community of living (biotic) organisms in conjunction with the nonliving (abiotic) components
(e.g., water, light, radiation, temperature, humidity, atmosphere, acidity, and soil) of their environment
is called an ecosystem.[145][146][147] These biotic and abiotic components are linked together through
nutrient cycles and energy flows.[148] Energy from the sun enters the system through photosynthesis
and is incorporated into plant tissue. By feeding on plants and on one another, animals move matter and
energy through the system. They also influence the quantity of plant and microbial biomass present. By
breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate
nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used
by plants and other microbes.[149]

Populations
A population is the group of organisms of the same species that
occupies an area and reproduce from generation to generation.[150]
[151][152][153][154] Population size can be estimated by multiplying

population density by the area or volume. The carrying capacity of

an environment is the maximum population size of a species that
can be sustained by that specific environment, given the food,
habitat, water, and other resources that are available.[155] The
carrying capacity of a population can be affected by changing
environmental conditions such as changes in the availability of Reaching carrying capacity through
resources and the cost of maintaining them. In human populations, a logistic growth curve
new technologies such as the Green revolution have helped increase
the Earth's carrying capacity for humans over time, which has
stymied the attempted predictions of impending population decline, the most famous of which was by
Thomas Malthus in the 18th century.[150]

Communities
A community is a group of populations of
species occupying the same geographical area
at the same time. A biological interaction is the
effect that a pair of organisms living together in
a community have on each other. They can be
either of the same species (intraspecific
interactions), or of different species
(interspecific interactions). These effects may
A (a) trophic pyramid and a (b) simplified food web. The
be short-term, like pollination and predation,
trophic pyramid represents the biomass at each level.[156]
or long-term; both often strongly influence the
evolution of the species involved. A long-term
interaction is called a symbiosis. Symbioses range from mutualism, beneficial to both partners, to
competition, harmful to both partners.[157] Every species participates as a consumer, resource, or both
in consumer–resource interactions, which form the core of food chains or food webs.[158] There are
different trophic levels within any food web, with the lowest level being the primary producers (or
autotrophs) such as plants and algae that convert energy and inorganic material into organic
compounds, which can then be used by the rest of the community.[51][159][160] At the next level are the
heterotrophs, which are the species that obtain energy by breaking apart organic compounds from other
organisms.[158] Heterotrophs that consume plants are primary consumers (or herbivores) whereas
heterotrophs that consume herbivores are secondary consumers (or carnivores). And those that eat
secondary consumers are tertiary consumers and so on. Omnivorous heterotrophs are able to consume
at multiple levels. Finally, there are decomposers that feed on the waste products or dead bodies of
organisms.[158] On average, the total amount of energy incorporated into the biomass of a trophic level
per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead
material used by decomposers as well as heat lost from metabolism make up the other ninety percent of
energy that is not consumed by the next trophic level.[161]
Biosphere
In the global ecosystem or biosphere, matter exists as different
interacting compartments, which can be biotic or abiotic as well
as accessible or inaccessible, depending on their forms and
locations.[163] For example, matter from terrestrial autotrophs
are both biotic and accessible to other organisms whereas the
matter in rocks and minerals are abiotic and inaccessible. A
biogeochemical cycle is a pathway by which specific elements of
matter are turned over or moved through the biotic (biosphere)
and the abiotic (lithosphere, atmosphere, and hydrosphere)
compartments of Earth. There are biogeochemical cycles for
nitrogen, carbon, and water. Fast carbon cycle showing the movement
of carbon between land, atmosphere, and
oceans in billions of tons per year. Yellow
Conservation numbers are natural fluxes, red are
Conservation biology is the study of the conservation of Earth's human contributions, white are stored
biodiversity with the aim of protecting species, their habitats, carbon. Effects of the slow carbon cycle,
such as volcanic and tectonic activity, are
and ecosystems from excessive rates of extinction and the
not included.[162]
erosion of biotic interactions.[164][165][166] It is concerned with
factors that influence the maintenance, loss, and restoration of
biodiversity and the science of sustaining evolutionary processes that engender genetic, population,
species, and ecosystem diversity.[167][168][169][170] The concern stems from estimates suggesting that up
to 50% of all species on the planet will disappear within the next 50 years,[171] which has contributed to
poverty, starvation, and will reset the course of evolution on this planet.[172][173] Biodiversity affects the
functioning of ecosystems, which provide a variety of services upon which people depend. Conservation
biologists research and educate on the trends of biodiversity loss, species extinctions, and the negative
effect these are having on our capabilities to sustain the well-being of human society. Organizations and
citizens are responding to the current biodiversity crisis through conservation action plans that direct
research, monitoring, and education programs that engage concerns at local through global scales.[174]
[167][168][169]

References
1. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "Evolution, the
themes of biology, and scientific inquiry". Campbell Biology (11th ed.). New York: Pearson. pp. 2–26.
ISBN 978-0134093413.
2. Hillis, David M.; Heller, H. Craig; Hacker, Sally D.; Laskowski, Marta J.; Sadava, David E. (2020).
"Studying life". Life: The Science of Biology (12th ed.). W. H. Freeman. ISBN 978-1319017644.
3. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Biology and the three of life". Biological Science (6th ed.). Hoboken, N.J.:
Pearson. pp. 1–18. ISBN 978-0321976499.
4. Modell, Harold; Cliff, William; Michael, Joel; McFarland, Jenny; Wenderoth, Mary Pat; Wright, Ann
(December 2015). "A physiologist's view of homeostasis" (https://www.ncbi.nlm.nih.gov/pmc/articles/
PMC4669363). Advances in Physiology Education. 39 (4): 259–266. doi:10.1152/advan.00107.2015
(https://doi.org/10.1152%2Fadvan.00107.2015). PMC 4669363 (https://www.ncbi.nlm.nih.gov/pmc/ar
ticles/PMC4669363). PMID 26628646 (https://pubmed.ncbi.nlm.nih.gov/26628646).
 5. Davies, PC; Rieper, E; Tuszynski, JA (January 2013). "Self-organization and entropy reduction in a
living cell" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712629). Bio Systems. 111 (1): 1–10.
Bibcode:2013BiSys.111....1D (https://ui.adsabs.harvard.edu/abs/2013BiSys.111....1D). doi:10.1016/
j.biosystems.2012.10.005 (https://doi.org/10.1016%2Fj.biosystems.2012.10.005). PMC 3712629 (htt
ps://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712629). PMID 23159919 (https://pubmed.ncbi.nlm.ni
h.gov/23159919).
6. Based on definition from: "Aquarena Wetlands Project glossary of terms" (https://web.archive.org/we
b/20040608113114/http://www.bio.txstate.edu/~wetlands/Glossary/glossary.html). Texas State
University at San Marcos. Archived from the original (http://www.bio.txstate.edu/~wetlands/Glossary/
glossary.html) on 2004-06-08.
7. Craig, Nancy (2014). Molecular Biology, Principles of Genome Function. OUP Oxford.
ISBN 978-0-19-965857-2.
8. Mosconi, Francesco; Julou, Thomas; Desprat, Nicolas; Sinha, Deepak Kumar; Allemand, Jean-
François; Vincent Croquette; Bensimon, David (2008). "Some nonlinear challenges in biology".
Nonlinearity. 21 (8): T131. Bibcode:2008Nonli..21..131M (https://ui.adsabs.harvard.edu/abs/2008No
nli..21..131M). doi:10.1088/0951-7715/21/8/T03 (https://doi.org/10.1088%2F0951-7715%2F21%2F
8%2FT03). S2CID 119808230 (https://api.semanticscholar.org/CorpusID:119808230).
9. Howell, Elizabeth (8 December 2014). "How Did Life Become Complex, And Could It Happen
Beyond Earth?" (https://web.archive.org/web/20180817193332/https://www.astrobio.net/origin-and-e
volution-of-life/life-become-complex-happen-beyond-earth/). Astrobiology Magazine. Archived from
the original on 17 August 2018. Retrieved 14 February 2018.
10. Pearce, Ben K.D.; Tupper, Andrew S.; Pudritz, Ralph E.; et al. (March 1, 2018). "Constraining the
Time Interval for the Origin of Life on Earth". Astrobiology. 18 (3): 343–364. arXiv:1808.09460 (http
s://arxiv.org/abs/1808.09460). Bibcode:2018AsBio..18..343P (https://ui.adsabs.harvard.edu/abs/201
8AsBio..18..343P). doi:10.1089/ast.2017.1674 (https://doi.org/10.1089%2Fast.2017.1674).
PMID 29570409 (https://pubmed.ncbi.nlm.nih.gov/29570409). S2CID 4419671 (https://api.semantics
cholar.org/CorpusID:4419671).
11. Lindberg, David C. (2007). "Science before the Greeks". The beginnings of Western science: the
European Scientific tradition in philosophical, religious, and institutional context (2nd ed.). Chicago,
Illinois: University of Chicago Press. pp. 1–20. ISBN 978-0-226-48205-7.
12. Grant, Edward (2007). "Ancient Egypt to Plato". A History of Natural Philosophy: From the Ancient
World to the Nineteenth Century (https://archive.org/details/historynaturalph00gran). New York:
Cambridge University Press. pp. 1 (https://archive.org/details/historynaturalph00gran/page/n16)–26.
ISBN 978-052-1-68957-1.
13. Magner, Lois N. (2002). A History of the Life Sciences, Revised and Expanded (https://books.googl
e.com/books?id=YKJ6gVYbrGwC). CRC Press. ISBN 978-0-203-91100-6. Archived (https://web.arc
hive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC) from the
original on 2015-03-24.
14. Serafini, Anthony (2013). The Epic History of Biology (https://books.google.com/books?id=Z3oECAA
AQBAJ&q=biology%20egypt&pg=PA2). Springer. ISBN 978-1-4899-6327-7. Archived (https://web.ar
chive.org/web/20210415122005/https://books.google.com/books?id=Z3oECAAAQBAJ&q=biolog
y%20egypt&pg=PA2) from the original on 15 April 2021. Retrieved 14 July 2015.
15. One or more of the preceding sentences incorporates text from a publication now in the public
domain: Chisholm, Hugh, ed. (1911). "Theophrastus". Encyclopædia Britannica (11th ed.).
Cambridge University Press.
16. Fahd, Toufic (1996). "Botany and agriculture". In Morelon, Régis; Rashed, Roshdi (eds.).
Encyclopedia of the History of Arabic Science. Vol. 3. Routledge. p. 815. ISBN 978-0-415-12410-2.
17. Magner, Lois N. (2002). A History of the Life Sciences, Revised and Expanded (https://books.googl
e.com/books?id=YKJ6gVYbrGwC). CRC Press. pp. 133–44. ISBN 978-0-203-91100-6. Archived (htt
ps://web.archive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC)
from the original on 2015-03-24.
18. Sapp, Jan (2003). "7". Genesis: The Evolution of Biology. New York: Oxford University Press.
ISBN 978-0-19-515618-8.
19. Coleman, William (1977). Biology in the Nineteenth Century: Problems of Form, Function, and
Transformation. New York: Cambridge University Press. ISBN 978-0-521-29293-1.
20. Mayr, Ernst. The Growth of Biological Thought, chapter 4
21. Mayr, Ernst. The Growth of Biological Thought, chapter 7
22. * Darwin, Francis, ed. (1909). The foundations of The origin of species, a sketch written in 1842 (htt
p://darwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf) (PDF). Cambridge: Printed at
the University Press. p. 53. LCCN 61057537 (https://lccn.loc.gov/61057537). OCLC 1184581 (http
s://www.worldcat.org/oclc/1184581). Archived (https://web.archive.org/web/20160304111606/http://d
arwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf) (PDF) from the original on 4 March
2016. Retrieved 27 November 2014.
23. Gould, Stephen Jay. The Structure of Evolutionary Theory. The Belknap Press of Harvard University
Press: Cambridge, 2002. ISBN 0-674-00613-5. p. 187.
24. Mayr, Ernst. The Growth of Biological Thought, chapter 10: "Darwin's evidence for evolution and
common descent"; and chapter 11: "The causation of evolution: natural selection"
25. Larson, Edward J. (2006). "Ch. 3" (https://books.google.com/books?id=xzLRvxlJhzkC). Evolution:
The Remarkable History of a Scientific Theory. Random House Publishing Group.
ISBN 978-1-58836-538-5. Archived (https://web.archive.org/web/20150324124009/http://books.goog
le.com/books?id=xzLRvxlJhzkC) from the original on 2015-03-24.
26. Henig (2000). Op. cit (http://archive.org/details/monkingardenlost00heni). pp. 134–138.
27. Miko, Ilona (2008). "Gregor Mendel's principles of inheritance form the cornerstone of modern
genetics. So just what are they?" (https://www.nature.com/scitable/topicpage/gregor-mendel-and-th
e-principles-of-inheritance-593/). Nature Education. 1 (1): 134. Archived (https://web.archive.org/we
b/20190719224056/http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-in
heritance-593) from the original on 2019-07-19. Retrieved 2021-05-13.
28. Futuyma, Douglas J.; Kirkpatrick, Mark (2017). "Evolutionary Biology". Evolution (4th ed.).
Sunderland, Mass.: Sinauer Associates. pp. 3–26.
29. Noble, Ivan (2003-04-14). "Human genome finally complete" (https://news.bbc.co.uk/2/hi/science/nat
ure/2940601.stm). BBC News. Archived (https://web.archive.org/web/20060614141605/http://new
s.bbc.co.uk/2/hi/science/nature/2940601.stm) from the original on 2006-06-14. Retrieved
2006-07-22.
30. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "The chemical
context of life". Campbell Biology (11th ed.). New York: Pearson. pp. 28–43. ISBN 978-0134093413.
31. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Water and carbon: The chemical basis of life". Biological Science (6th ed.).
Hoboken, N.J.: Pearson. pp. 55–77. ISBN 978-0321976499.
32. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "Carbon and
the molecular diversity of life". Campbell Biology (11th ed.). New York: Pearson. pp. 56–65.
ISBN 978-0134093413.
33. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Carbon and molecular
diversity of life". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 56–65.
ISBN 978-1464175121.
34. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Protein structure and function". Biological Science (6th ed.). Hoboken,
N.J.: Pearson. pp. 78–92. ISBN 978-0321976499.
35. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "The structure
and function of large biological molecules". Campbell Biology (11th ed.). New York: Pearson. pp. 66–
92. ISBN 978-0134093413.
36. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "An introduction to carbohydrate". Biological Science (6th ed.). Hoboken,
N.J.: Pearson. pp. 107–118. ISBN 978-0321976499.
37. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Lipids, membranes, and the first cells". Biological Science (6th ed.).
Hoboken, N.J.: Pearson. pp. 119–141. ISBN 978-0321976499.
38. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Nucleic acids and the RNA world". Biological Science (6th ed.). Hoboken,
N.J.: Pearson. pp. 93–106. ISBN 978-0321976499.
39. Mazzarello, P. (May 1999). "A unifying concept: the history of cell theory". Nature Cell Biology. 1 (1):
E13–15. doi:10.1038/8964 (https://doi.org/10.1038%2F8964). PMID 10559875 (https://pubmed.ncb
i.nlm.nih.gov/10559875). S2CID 7338204 (https://api.semanticscholar.org/CorpusID:7338204).
40. Campbell, Neil A.; Williamson, Brad; Heyden, Robin J. (2006). Biology: Exploring Life (http://www.ph
school.com/el_marketing.html). Boston: Pearson Prentice Hall. ISBN 978-0132508827. Archived (htt
ps://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html) from the
original on 2014-11-02. Retrieved 2021-05-13.
41. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "Membrane
structure and function". Campbell Biology (11th ed.). New York: Pearson. pp. 126–142.
ISBN 978-0134093413.
42. Alberts, B.; Johnson, A.; Lewis, J.; et al. (2002). Molecular Biology of the Cell (https://www.ncbi.nl
m.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%20Cell) (4th ed.). New
York: Garland Science. ISBN 978-0-8153-3218-3. Archived (https://web.archive.org/web/201712200
92628/https://www.ncbi.nlm.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%2
0Cell) from the original on 2017-12-20.
43. Tom Herrmann; Sandeep Sharma (March 2, 2019). "Physiology, Membrane" (https://www.ncbi.nlm.ni
h.gov/books/NBK538211/). StatPearls. PMID 30855799 (https://pubmed.ncbi.nlm.nih.gov/30855799)
. Archived (https://web.archive.org/web/20220217034021/https://www.ncbi.nlm.nih.gov/books/NBK5
38211/) from the original on February 17, 2022. Retrieved May 14, 2021.
44. Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002).
"Cell Movements and the Shaping of the Vertebrate Body" (https://www.ncbi.nlm.nih.gov/books/NBK
26863/). Molecular Biology of the Cell (4th ed.). Archived (https://web.archive.org/web/20200122055
346/https://www.ncbi.nlm.nih.gov/books/NBK26863/) from the original on 2020-01-22. Retrieved
2021-05-13. The Alberts text discusses how the "cellular building blocks" move to shape developing
embryos. It is also common to describe small molecules such as amino acids as "molecular building
blocks (https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBook
HL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.sec
tion.4#23) Archived (https://web.archive.org/web/20200122055404/https://www.ncbi.nlm.nih.gov/boo
ks?cmd=Search&doptcmdl=GenBookHL&term=%22all%2Bcells%22%2BAND%2Bmboc4%5Bboo
k%5D%2BAND%2B372023%5Buid%5D&rid=mboc4.section.4#23) 2020-01-22 at the Wayback
Machine".
45. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Cells: The working units of
life". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 60–81.
ISBN 978-1464175121.
46. Bailey, Regina. "Cellular Respiration" (http://biology.about.com/od/cellularprocesses/a/cellrespiratio
n.htm). Archived (https://web.archive.org/web/20120505043947/http://biology.about.com/od/cellularp
rocesses/a/cellrespiration.htm) from the original on 2012-05-05.
47. Lodish, Harvey; Berk, Arnold.; Kaiser, Chris A.; Krieger, Monty; Scott, Matthew P.; Bretscher,
Anthony; Ploegh, Hidde; Matsudaira, Paul (2008). "Cellular energetics". Molecular Cell Biology
(6th ed.). New York: W.H. Freeman and Company. pp. 479–532. ISBN 978-0716776017.
48. "photosynthesis" (http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0)
. Online Etymology Dictionary. Archived (https://web.archive.org/web/20130307020959/http://www.et
ymonline.com/index.php?term=photosynthesis&allowed_in_frame=0) from the original on
2013-03-07. Retrieved 2013-05-23.
49. φῶς (https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=fw=s2).
Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
50. σύνθεσις (https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=su/nqes
is). Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
51. Bryant, D. A.; Frigaard, N. U. (Nov 2006). "Prokaryotic photosynthesis and phototrophy illuminated".
Trends in Microbiology. 14 (11): 488–496. doi:10.1016/j.tim.2006.09.001 (https://doi.org/10.1016%2F
j.tim.2006.09.001). PMID 16997562 (https://pubmed.ncbi.nlm.nih.gov/16997562).
52. Reece, J.; Urry, L.; Cain, M. (2011). Biology (https://archive.org/details/isbn_9781256158769/page/2
35) (International ed.). Upper Saddle River, New Jersey: Pearson Education. pp. 235, 244 (https://ar
chive.org/details/isbn_9781256158769/page/235). ISBN 978-0-321-73975-9. "This initial
incorporation of carbon into organic compounds is known as carbon fixation."
53. Neitzel, James; Rasband, Matthew. "Cell communication" (https://www.nature.com/scitable/topic/cel
l-communication-14122659/). Nature Education. Archived (https://web.archive.org/web/2010092911
0101/https://www.nature.com/scitable/topic/cell-communication-14122659/) from the original on 29
September 2010. Retrieved 29 May 2021.
54. "Cell signaling" (https://www.nature.com/scitable/topicpage/cell-signaling-14047077/). Nature
Education. Archived (https://web.archive.org/web/20101031053612/https://www.nature.com/scitable/
topicpage/cell-signaling-14047077/) from the original on 31 October 2010. Retrieved 29 May 2021.
55. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Cell membranes and
signaling". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 82–104.
ISBN 978-1464175121.
56. Martin, E. A.; Hine, R. (2020). A dictionary of biology (6th ed.). Oxford: Oxford University Press.
ISBN 978-0199204625. OCLC 176818780 (https://www.worldcat.org/oclc/176818780).
57. Griffiths, A. J. (2012). Introduction to genetic analysis (10th ed.). New York: W.H. Freeman.
ISBN 978-1429229432. OCLC 698085201 (https://www.worldcat.org/oclc/698085201).
58. "10.2 The Cell Cycle – Biology 2e | OpenStax" (https://openstax.org/books/biology-2e/pages/10-2-th
e-cell-cycle). openstax.org. 28 March 2018. Archived (https://web.archive.org/web/20201129223722/
https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle) from the original on 2020-11-29.
Retrieved 2020-11-24.
59. Freeman, Scott; Quillin, Kim; Allison, Lizabeth; Black, Michael; Podgorski, Greg; Taylor, Emily;
Carmichael, Jeff (2017). "Meiosis". Biological Science (6th ed.). Hoboken, New Jersey: Pearson.
pp. 271–289. ISBN 978-0321976499.
60. Casiraghi, A.; Suigo, L.; Valoti, E.; Straniero, V. (February 2020). "Targeting Bacterial Cell Division: A
Binding Site-Centered Approach to the Most Promising Inhibitors of the Essential Protein FtsZ" (http
s://www.ncbi.nlm.nih.gov/pmc/articles/PMC7167804). Antibiotics. 9 (2): 69. doi:10.3390/
antibiotics9020069 (https://doi.org/10.3390%2Fantibiotics9020069). PMC 7167804 (https://www.ncb
i.nlm.nih.gov/pmc/articles/PMC7167804). PMID 32046082 (https://pubmed.ncbi.nlm.nih.gov/320460
82).
61. Brandeis M. New-age ideas about age-old sex: separating meiosis from mating could solve a
century-old conundrum. Biol Rev Camb Philos Soc. 2018 May;93(2):801-810. doi: 10.1111/
brv.12367. Epub 2017 Sep 14. PMID 28913952
62. Hörandl E. Apomixis and the paradox of sex in plants. Ann Bot. 2024 Mar 18:mcae044. doi:
10.1093/aob/mcae044. Epub ahead of print. PMID 38497809
63. Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex.
Science. 1985 Sep 20;229(4719):1277-81. doi: 10.1126/science.3898363. PMID 3898363
64. Darwin, C. R. 1878. The effects of cross and self fertilisation in the vegetable kingdom. London:
John Murray. darwin-online.org.uk
65. Griffiths, Anthony J.; Wessler, Susan R.; Carroll, Sean B.; Doebley, John (2015). "The genetics
revolution". An Introduction to Genetic Analysis (11th ed.). Sunderland, Massachusetts: W.H.
Freeman & Company. pp. 1–30. ISBN 978-1464109485.
66. Griffiths, Anthony J. F.; Miller, Jeffrey H.; Suzuki, David T.; Lewontin, Richard C.; Gelbart, William M.,
eds. (2000). "Genetics and the Organism: Introduction" (https://www.ncbi.nlm.nih.gov/books/bv.fcgi?r
id=iga.section.60). An Introduction to Genetic Analysis (7th ed.). New York: W. H. Freeman.
ISBN 978-0-7167-3520-5.
67. Hartl, D.; Jones, E (2005). Genetics: Analysis of Genes and Genomes (https://archive.org/details/ge
netics00dani) (6th ed.). Jones & Bartlett. ISBN 978-0-7637-1511-3.
68. Miko, Ilona (2008). "Test crosses" (https://www.nature.com/scitable/topicpage/test-crosses-585/).
Nature Education. 1 (1): 136. Archived (https://web.archive.org/web/20210521003428/https://www.n
ature.com/scitable/topicpage/test-crosses-585/) from the original on 2021-05-21. Retrieved
2021-05-28.
69. Miko, Ilona (2008). "Thomas Hunt Morgan and sex linkage" (https://www.nature.com/scitable/topicpa
ge/thomas-hunt-morgan-and-sex-linkage-452/). Nature Education. 1 (1): 143. Archived (https://we
b.archive.org/web/20210520234008/https://www.nature.com/scitable/topicpage/thomas-hunt-morga
n-and-sex-linkage-452/) from the original on 2021-05-20. Retrieved 2021-05-28.
70. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "DNA and its role in
heredity". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 172–193.
ISBN 978-1464175121.
71. Russell, Peter (2001). iGenetics (https://archive.org/details/igenetics0000russ_v6o1). New York:
Benjamin Cummings. ISBN 0-8053-4553-1.
72. Thanbichler, M; Wang, SC; Shapiro, L (October 2005). "The bacterial nucleoid: a highly organized
and dynamic structure" (https://doi.org/10.1002%2Fjcb.20519). Journal of Cellular Biochemistry. 96
(3): 506–21. doi:10.1002/jcb.20519 (https://doi.org/10.1002%2Fjcb.20519). PMID 15988757 (https://
pubmed.ncbi.nlm.nih.gov/15988757). S2CID 25355087 (https://api.semanticscholar.org/CorpusID:25
355087).
73. "Genotype definition – Medical Dictionary definitions" (http://www.medterms.com/script/main/art.as
p?articlekey=8472). Medterms.com. 2012-03-19. Archived (https://web.archive.org/web/2013092105
4803/http://www.medterms.com/script/main/art.asp?articlekey=8472) from the original on
2013-09-21. Retrieved 2013-10-02.
74. Crick, Francis H. (1958). "On protein synthesis". Symposia of the Society for Experimental Biology.
12: 138–63. PMID 13580867 (https://pubmed.ncbi.nlm.nih.gov/13580867).
75. Crick, Francis H. (August 1970). "Central dogma of molecular biology". Nature. 227 (5258): 561–3.
Bibcode:1970Natur.227..561C (https://ui.adsabs.harvard.edu/abs/1970Natur.227..561C).
doi:10.1038/227561a0 (https://doi.org/10.1038%2F227561a0). PMID 4913914 (https://pubmed.ncb
i.nlm.nih.gov/4913914). S2CID 4164029 (https://api.semanticscholar.org/CorpusID:4164029).
76. "Central dogma reversed". Nature. 226 (5252): 1198–9. June 1970. Bibcode:1970Natur.226.1198. (h
ttps://ui.adsabs.harvard.edu/abs/1970Natur.226.1198.). doi:10.1038/2261198a0 (https://doi.org/10.1
038%2F2261198a0). PMID 5422595 (https://pubmed.ncbi.nlm.nih.gov/5422595). S2CID 4184060 (h
ttps://api.semanticscholar.org/CorpusID:4184060).
77. Lin, Yihan; Elowitz, Michael B. (2016). "Central Dogma Goes Digital" (https://doi.org/10.1016%2Fj.m
olcel.2016.03.005). Molecular Cell. 61 (6): 791–792. doi:10.1016/j.molcel.2016.03.005 (https://doi.or
g/10.1016%2Fj.molcel.2016.03.005). PMID 26990983 (https://pubmed.ncbi.nlm.nih.gov/26990983).
78. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Regulation of gene
expression". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 215–233.
ISBN 978-1464175121.
79. Keene, Jack D.; Tenenbaum, Scott A. (2002). "Eukaryotic mRNPs may represent posttranscriptional
operons" (https://doi.org/10.1016%2Fs1097-2765%2802%2900559-2). Molecular Cell. 9 (6): 1161–
1167. doi:10.1016/s1097-2765(02)00559-2 (https://doi.org/10.1016%2Fs1097-2765%2802%290055
9-2). PMID 12086614 (https://pubmed.ncbi.nlm.nih.gov/12086614).
80. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Genes, development, and
evolution". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 273–298.
ISBN 978-1464175121.
81. Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.
82. Slack, J.M.W. (2007). "Metaplasia and transdifferentiation: from pure biology to the clinic". Nature
Reviews Molecular Cell Biology. 8 (5): 369–378. doi:10.1038/nrm2146 (https://doi.org/10.1038%2Fn
rm2146). PMID 17377526 (https://pubmed.ncbi.nlm.nih.gov/17377526). S2CID 3353748 (https://ap
i.semanticscholar.org/CorpusID:3353748).
83. Atala, Anthony; Lanza, Robert (2012). Handbook of Stem Cells (https://books.google.com/books?id=
wm-K_dKpjBAC&pg=RA1-PA451). Academic Press. p. 452. ISBN 978-0-12-385943-3. Archived (htt
ps://web.archive.org/web/20210412065854/https://books.google.com/books?id=wm-K_dKpjBAC&pg
=RA1-PA451) from the original on 2021-04-12. Retrieved 2021-05-28.
84. Yanes, Oscar; Clark, Julie; Wong, Diana M.; Patti, Gary J.; Sánchez-Ruiz, Antonio; Benton, H. Paul;
Trauger, Sunia A.; Desponts, Caroline; Ding, Sheng; Siuzdak, Gary (June 2010). "Metabolic
oxidation regulates embryonic stem cell differentiation" (https://www.ncbi.nlm.nih.gov/pmc/articles/P
MC2873061). Nature Chemical Biology. 6 (6): 411–417. doi:10.1038/nchembio.364 (https://doi.org/1
0.1038%2Fnchembio.364). PMC 2873061 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2873061)
. PMID 20436487 (https://pubmed.ncbi.nlm.nih.gov/20436487).
85. Carroll, Sean B. "The Origins of Form" (http://www.naturalhistorymag.com/features/061488/the-origi
ns-of-form). Natural History. Archived (https://web.archive.org/web/20181009154501/http://www.nat
uralhistorymag.com/features/061488/the-origins-of-form) from the original on 9 October 2018.
Retrieved 9 October 2016. "Biologists could say, with confidence, that forms change, and that
natural selection is an important force for change. Yet they could say nothing about how that change
is accomplished. How bodies or body parts change, or how new structures arise, remained complete
mysteries."
86. Hall, Brian K.; Hallgrímsson, Benedikt (2007). Strickberger's Evolution (https://books.google.com/bo
oks?id=jrDD3cyA09kC). Jones & Bartlett Publishers. pp. 4–6. ISBN 978-1-4496-4722-3. Archived (ht
tps://web.archive.org/web/20230326093707/https://books.google.com/books?id=jrDD3cyA09kC)
from the original on 2023-03-26. Retrieved 2021-05-27.
87. "Evolution Resources" (http://www.nas.edu/evolution/index.html). Washington, D.C.: National
Academies of Sciences, Engineering, and Medicine. 2016. Archived (https://web.archive.org/web/20
160603230514/http://www.nas.edu/evolution/index.html) from the original on 2016-06-03.
88. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "Descent with
modifications: A Darwinian view of life". Campbell Biology (11th ed.). New York: Pearson. pp. 466–
483. ISBN 978-0134093413.
89. Lewontin, Richard C. (November 1970). "The Units of Selection" (http://joelvelasco.net/teaching/167/
lewontin%2070%20-%20the%20units%20of%20selection.pdf) (PDF). Annual Review of Ecology
and Systematics. 1: 1–18. doi:10.1146/annurev.es.01.110170.000245 (https://doi.org/10.1146%2Fan
nurev.es.01.110170.000245). JSTOR 2096764 (https://www.jstor.org/stable/2096764).
S2CID 84684420 (https://api.semanticscholar.org/CorpusID:84684420). Archived (https://web.archiv
e.org/web/20150206172942/http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20unit
s%20of%20selection.pdf) (PDF) from the original on 2015-02-06.
90. Darwin, Charles (1859). On the Origin of Species, John Murray.
91. Futuyma, Douglas J.; Kirkpatrick, Mark (2017). "Evolutionary biology". Evolution (4th ed.).
Sunderland, Mass.: Sinauer Associates. pp. 3–26.
92. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Processes of evolution".
Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 299–324.
ISBN 978-1464175121.
93. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Speciation". Principles of
Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 343–356. ISBN 978-1464175121.
94. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Reconstructing and using
phylogenies". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 325–342.
ISBN 978-1464175121.
95. Kitching, Ian J.; Forey, Peter L.; Williams, David M. (2001). "Cladistics" (https://www.sciencedirect.co
m/science/article/pii/B9780123847195000228). In Levin, Simon A. (ed.). Encyclopedia of
Biodiversity (2nd ed.). Elsevier. pp. 33–45. doi:10.1016/B978-0-12-384719-5.00022-8 (https://doi.or
g/10.1016%2FB978-0-12-384719-5.00022-8). ISBN 9780123847201. Archived (https://web.archiv
e.org/web/20210829234556/https://www.sciencedirect.com/science/article/pii/B97801238471950002
28) from the original on 29 August 2021. Retrieved 29 August 2021.)
96. Futuyma, Douglas J.; Kirkpatrick, Mark (2017). "Phylogeny: The unity and diversity of life". Evolution
(4th ed.). Sunderland, Mass.: Sinauer Associates. pp. 401–429.
 97. Woese, CR; Kandler, O; Wheelis, ML (June 1990). "Towards a natural system of organisms:
proposal for the domains Archaea, Bacteria, and Eucarya" (https://www.ncbi.nlm.nih.gov/pmc/article
s/PMC54159). Proceedings of the National Academy of Sciences of the United States of America.
87 (12): 4576–79. Bibcode:1990PNAS...87.4576W (https://ui.adsabs.harvard.edu/abs/1990PNAS...8
7.4576W). doi:10.1073/pnas.87.12.4576 (https://doi.org/10.1073%2Fpnas.87.12.4576). PMC 54159
(https://www.ncbi.nlm.nih.gov/pmc/articles/PMC54159). PMID 2112744 (https://pubmed.ncbi.nlm.ni
h.gov/2112744).
98. Montévil, M; Mossio, M; Pocheville, A; Longo, G (October 2016). "Theoretical principles for biology:
Variation" (https://www.academia.edu/27942089). Progress in Biophysics and Molecular Biology.
From the Century of the Genome to the Century of the Organism: New Theoretical Approaches. 122
(1): 36–50. doi:10.1016/j.pbiomolbio.2016.08.005 (https://doi.org/10.1016%2Fj.pbiomolbio.2016.0
8.005). PMID 27530930 (https://pubmed.ncbi.nlm.nih.gov/27530930). S2CID 3671068 (https://api.se
manticscholar.org/CorpusID:3671068). Archived (https://web.archive.org/web/20180320150224/htt
p://www.academia.edu/27942089/Theoretical_principles_for_biology_Variation) from the original on
2018-03-20.
99. De Duve, Christian (2002). Life Evolving: Molecules, Mind, and Meaning (https://archive.org/details/li
feevolvingmole00duve_331). New York: Oxford University Press. p. 44 (https://archive.org/details/life
evolvingmole00duve_331/page/n60). ISBN 978-0-19-515605-8.
100. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The history of life on Earth".
Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 357–376.
ISBN 978-1464175121.
101. "Stratigraphic Chart 2022" (https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf) (PDF).
International Stratigraphic Commission. February 2022. Archived (https://web.archive.org/web/2022
0402100018/https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf) (PDF) from the original
on 2 April 2022. Retrieved 25 April 2022.
102. Futuyma 2005
103. Futuyma, DJ (2005). Evolution (https://archive.org/details/evolution0000futu). Sinauer Associates.
ISBN 978-0-87893-187-3. OCLC 57311264 (https://www.worldcat.org/oclc/57311264).
104. Rosing, Minik T. (January 29, 1999). "13C-Depleted Carbon Microparticles in >3700-Ma Sea-Floor
Sedimentary Rocks from West Greenland". Science. 283 (5402): 674–676.
Bibcode:1999Sci...283..674R (https://ui.adsabs.harvard.edu/abs/1999Sci...283..674R). doi:10.1126/
science.283.5402.674 (https://doi.org/10.1126%2Fscience.283.5402.674). PMID 9924024 (https://pu
bmed.ncbi.nlm.nih.gov/9924024).
105. Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; et al. (January 2014). "Evidence for biogenic
graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. 7 (1): 25–28.
Bibcode:2014NatGe...7...25O (https://ui.adsabs.harvard.edu/abs/2014NatGe...7...25O). doi:10.1038/
ngeo2025 (https://doi.org/10.1038%2Fngeo2025).
106. Nisbet, Euan G.; Fowler, C.M.R. (December 7, 1999). "Archaean metabolic evolution of microbial
mats" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1690475). Proceedings of the Royal Society
B. 266 (1436): 2375–2382. doi:10.1098/rspb.1999.0934 (https://doi.org/10.1098%2Frspb.1999.093
4). PMC 1690475 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1690475).
107. Knoll, Andrew H.; Javaux, Emmanuelle J.; Hewitt, David; et al. (June 29, 2006). "Eukaryotic
organisms in Proterozoic oceans" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1578724).
Philosophical Transactions of the Royal Society B. 361 (1470): 1023–1038. doi:10.1098/
rstb.2006.1843 (https://doi.org/10.1098%2Frstb.2006.1843). PMC 1578724 (https://www.ncbi.nlm.ni
h.gov/pmc/articles/PMC1578724). PMID 16754612 (https://pubmed.ncbi.nlm.nih.gov/16754612).
108. Fedonkin, Mikhail A. (March 31, 2003). "The origin of the Metazoa in the light of the Proterozoic
fossil record" (https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin
_2003.pdf) (PDF). Paleontological Research. 7 (1): 9–41. doi:10.2517/prpsj.7.9 (https://doi.org/10.25
17%2Fprpsj.7.9). S2CID 55178329 (https://api.semanticscholar.org/CorpusID:55178329). Archived
from the original (http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf) (PDF) on 2009-02-26. Retrieved
2008-09-02.
109. Bonner, John Tyler (January 7, 1998). "The origins of multicellularity". Integrative Biology. 1 (1): 27–
36. doi:10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 (https://doi.org/10.1002%2
F%28SICI%291520-6602%281998%291%3A1%3C27%3A%3AAID-INBI4%3E3.0.CO%3B2-6).
110. Strother, Paul K.; Battison, Leila; Brasier, Martin D.; et al. (May 26, 2011). "Earth's earliest non-
marine eukaryotes". Nature. 473 (7348): 505–509. Bibcode:2011Natur.473..505S (https://ui.adsab
s.harvard.edu/abs/2011Natur.473..505S). doi:10.1038/nature09943 (https://doi.org/10.1038%2Fnatu
re09943). PMID 21490597 (https://pubmed.ncbi.nlm.nih.gov/21490597). S2CID 4418860 (https://ap
i.semanticscholar.org/CorpusID:4418860).
111. Beraldi-Campesi, Hugo (February 23, 2013). "Early life on land and the first terrestrial ecosystems"
(https://doi.org/10.1186%2F2192-1709-2-1). Ecological Processes. 2 (1): 1–17.
Bibcode:2013EcoPr...2....1B (https://ui.adsabs.harvard.edu/abs/2013EcoPr...2....1B).
doi:10.1186/2192-1709-2-1 (https://doi.org/10.1186%2F2192-1709-2-1).
112. Algeo, Thomas J.; Scheckler, Stephen E. (January 29, 1998). "Terrestrial-marine teleconnections in
the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic
events" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692181). Philosophical Transactions of the
Royal Society B. 353 (1365): 113–130. doi:10.1098/rstb.1998.0195 (https://doi.org/10.1098%2Frst
b.1998.0195). PMC 1692181 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1692181).
113. Jun-Yuan, Chen; Oliveri, Paola; Chia-Wei, Li; et al. (April 25, 2000). "Precambrian animal diversity:
Putative phosphatized embryos from the Doushantuo Formation of China" (https://www.ncbi.nlm.ni
h.gov/pmc/articles/PMC18256). Proc. Natl. Acad. Sci. U.S.A. 97 (9): 4457–4462.
Bibcode:2000PNAS...97.4457C (https://ui.adsabs.harvard.edu/abs/2000PNAS...97.4457C).
doi:10.1073/pnas.97.9.4457 (https://doi.org/10.1073%2Fpnas.97.9.4457). PMC 18256 (https://ww
w.ncbi.nlm.nih.gov/pmc/articles/PMC18256). PMID 10781044 (https://pubmed.ncbi.nlm.nih.gov/1078
1044).
114. D-G., Shu; H-L., Luo; Conway Morris, Simon; et al. (November 4, 1999). "Lower Cambrian
vertebrates from south China" (https://web.archive.org/web/20090226122732/http://www.bios.niu.ed
u/davis/bios458/Shu1.pdf) (PDF). Nature. 402 (6757): 42–46. Bibcode:1999Natur.402...42S (https://
ui.adsabs.harvard.edu/abs/1999Natur.402...42S). doi:10.1038/46965 (https://doi.org/10.1038%2F46
965). S2CID 4402854 (https://api.semanticscholar.org/CorpusID:4402854). Archived from the
original (http://www.bios.niu.edu/davis/bios458/Shu1.pdf) (PDF) on 2009-02-26. Retrieved
2015-01-22.
115. Hoyt, Donald F. (February 17, 1997). "Synapsid Reptiles" (https://web.archive.org/web/20090520072
737/http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML). ZOO 138 Vertebrate
Zoology (Lecture). Pomona, Calif.: California State Polytechnic University, Pomona. Archived from
the original (http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML) on 2009-05-20.
Retrieved 2015-01-22.
116. Barry, Patrick L. (January 28, 2002). Phillips, Tony (ed.). "The Great Dying" (https://science.nasa.go
v/science-news/science-at-nasa/2002/28jan_extinction/). Science@NASA. Marshall Space Flight
Center. Archived (https://web.archive.org/web/20100410015208/https://science.nasa.gov/science-ne
ws/science-at-nasa/2002/28jan_extinction/) from the original on 2010-04-10. Retrieved 2015-01-22.
117. Tanner, Lawrence H.; Lucas, Spencer G.; Chapman, Mary G. (March 2004). "Assessing the record
and causes of Late Triassic extinctions" (https://web.archive.org/web/20071025225841/http://nmnat
uralhistory.org/pdf_files/TJB.pdf) (PDF). Earth-Science Reviews. 65 (1–2): 103–139.
Bibcode:2004ESRv...65..103T (https://ui.adsabs.harvard.edu/abs/2004ESRv...65..103T).
doi:10.1016/S0012-8252(03)00082-5 (https://doi.org/10.1016%2FS0012-8252%2803%2900082-5).
Archived from the original (http://nmnaturalhistory.org/pdf_files/TJB.pdf) (PDF) on 2007-10-25.
Retrieved 2007-10-22.
118. Benton, Michael J. (1997). Vertebrate Palaeontology (2nd ed.). London: Chapman & Hall.
ISBN 978-0-412-73800-5. OCLC 37378512 (https://www.worldcat.org/oclc/37378512).
119. Fastovsky, David E.; Sheehan, Peter M. (March 2005). "The Extinction of the Dinosaurs in North
America" (https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf) (PDF). GSA
Today. 15 (3): 4–10. doi:10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 (https://doi.org/10.113
0%2F1052-5173%282005%29015%3C4%3ATEOTDI%3E2.0.CO%3B2). Archived (https://web.archi
ve.org/web/20190322190338/https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-1
5-3-4.pdf) (PDF) from the original on 2019-03-22. Retrieved 2015-01-23.
120. Roach, John (June 20, 2007). "Dinosaur Extinction Spurred Rise of Modern Mammals" (https://we
b.archive.org/web/20080511161825/https://news.nationalgeographic.com/news/2007/06/070620-ma
mmals-dinos.html). National Geographic News. Washington, D.C.: National Geographic Society.
Archived from the original (https://news.nationalgeographic.com/news/2007/06/070620-mammals-di
nos.html) on 2008-05-11. Retrieved 2020-02-21.
▪ Wible, John R.; Rougier, Guillermo W.; Novacek, Michael J.; et al. (June 21, 2007). "Cretaceous
eutherians and Laurasian origin for placental mammals near the K/T boundary". Nature. 447
(7147): 1003–1006. Bibcode:2007Natur.447.1003W (https://ui.adsabs.harvard.edu/abs/2007Nat
ur.447.1003W). doi:10.1038/nature05854 (https://doi.org/10.1038%2Fnature05854).
PMID 17581585 (https://pubmed.ncbi.nlm.nih.gov/17581585). S2CID 4334424 (https://api.sema
nticscholar.org/CorpusID:4334424).
121. Van Valkenburgh, Blaire (May 1, 1999). "Major Patterns in the History of Carnivorous Mammals" (htt
ps://zenodo.org/record/890156). Annual Review of Earth and Planetary Sciences. 27: 463–493.
Bibcode:1999AREPS..27..463V (https://ui.adsabs.harvard.edu/abs/1999AREPS..27..463V).
doi:10.1146/annurev.earth.27.1.463 (https://doi.org/10.1146%2Fannurev.earth.27.1.463). Archived
(https://web.archive.org/web/20200229201201/https://zenodo.org/record/890156) from the original
on February 29, 2020. Retrieved May 15, 2021.
122. Fredrickson, J. K.; Zachara, J. M.; Balkwill, D. L. (July 2004). "Geomicrobiology of high-level nuclear
waste-contaminated vadose sediments at the Hanford site, Washington state" (https://www.ncbi.nl
m.nih.gov/pmc/articles/PMC444790). Applied and Environmental Microbiology. 70 (7): 4230–41.
Bibcode:2004ApEnM..70.4230F (https://ui.adsabs.harvard.edu/abs/2004ApEnM..70.4230F).
doi:10.1128/AEM.70.7.4230-4241.2004 (https://doi.org/10.1128%2FAEM.70.7.4230-4241.2004).
PMC 444790 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC444790). PMID 15240306 (https://pub
med.ncbi.nlm.nih.gov/15240306).
123. Dudek, N. K.; Sun, C. L.; Burstein, D. (2017). "Novel Microbial Diversity and Functional Potential in
the Marine Mammal Oral Microbiome" (https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pd
f?t=pghuwe) (PDF). Current Biology. 27 (24): 3752–3762. Bibcode:2017CBio...27E3752D (https://u
i.adsabs.harvard.edu/abs/2017CBio...27E3752D). doi:10.1016/j.cub.2017.10.040 (https://doi.org/1
0.1016%2Fj.cub.2017.10.040). PMID 29153320 (https://pubmed.ncbi.nlm.nih.gov/29153320).
S2CID 43864355 (https://api.semanticscholar.org/CorpusID:43864355). Archived (https://web.archiv
e.org/web/20210308145807/https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pdf?t=pghuw
e) (PDF) from the original on 2021-03-08. Retrieved 2021-05-14.
124. Pace, N. R. (May 2006). "Time for a change" (https://doi.org/10.1038%2F441289a). Nature. 441
(7091): 289. Bibcode:2006Natur.441..289P (https://ui.adsabs.harvard.edu/abs/2006Natur.441..289
P). doi:10.1038/441289a (https://doi.org/10.1038%2F441289a). PMID 16710401 (https://pubmed.nc
bi.nlm.nih.gov/16710401). S2CID 4431143 (https://api.semanticscholar.org/CorpusID:4431143).
125. Stoeckenius, W. (October 1981). "Walsby's square bacterium: fine structure of an orthogonal
procaryote" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC216199). Journal of Bacteriology. 148
(1): 352–60. doi:10.1128/JB.148.1.352-360.1981 (https://doi.org/10.1128%2FJB.148.1.352-360.198
1). PMC 216199 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC216199). PMID 7287626 (https://pu
bmed.ncbi.nlm.nih.gov/7287626).
126. "Archaea Basic Biology" (https://basicbiology.net/micro/microorganisms/archaea). March 2018.
Archived (https://web.archive.org/web/20210428221114/https://basicbiology.net/micro/microorganis
ms/archaea) from the original on 2021-04-28. Retrieved 2021-05-14.
127. Bang, C.; Schmitz, R. A. (September 2015). "Archaea associated with human surfaces: not to be
underestimated" (https://doi.org/10.1093%2Ffemsre%2Ffuv010). FEMS Microbiology Reviews. 39
(5): 631–48. doi:10.1093/femsre/fuv010 (https://doi.org/10.1093%2Ffemsre%2Ffuv010).
PMID 25907112 (https://pubmed.ncbi.nlm.nih.gov/25907112).
128. Moissl-Eichinger. C.; Pausan, M.; Taffner, J.; Berg, G.; Bang, C.; Schmitz, R. A. (January 2018).
"Archaea Are Interactive Components of Complex Microbiomes". Trends in Microbiology. 26 (1): 70–
85. doi:10.1016/j.tim.2017.07.004 (https://doi.org/10.1016%2Fj.tim.2017.07.004). PMID 28826642 (h
ttps://pubmed.ncbi.nlm.nih.gov/28826642).
129. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The origin and
diversification of eukaryotes". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates.
pp. 402–419. ISBN 978-1464175121.
130. O'Malley, Maureen A.; Leger, Michelle M.; Wideman, Jeremy G.; Ruiz-Trillo, Iñaki (2019-02-18).
"Concepts of the last eukaryotic common ancestor". Nature Ecology & Evolution. 3 (3). Springer
Science and Business Media LLC: 338–344. Bibcode:2019NatEE...3..338O (https://ui.adsabs.harvar
d.edu/abs/2019NatEE...3..338O). doi:10.1038/s41559-019-0796-3 (https://doi.org/10.1038%2Fs415
59-019-0796-3). hdl:10261/201794 (https://hdl.handle.net/10261%2F201794). PMID 30778187 (http
s://pubmed.ncbi.nlm.nih.gov/30778187). S2CID 67790751 (https://api.semanticscholar.org/CorpusI
D:67790751).
131. Taylor, F. J. R. 'M. (2003-11-01). "The collapse of the two-kingdom system, the rise of protistology
and the founding of the International Society for Evolutionary Protistology (ISEP)" (https://doi.org/1
0.1099%2Fijs.0.02587-0). International Journal of Systematic and Evolutionary Microbiology. 53 (6).
Microbiology Society: 1707–1714. doi:10.1099/ijs.0.02587-0 (https://doi.org/10.1099%2Fijs.0.0258
7-0). PMID 14657097 (https://pubmed.ncbi.nlm.nih.gov/14657097).
132. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The evolution of plants".
Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 420–449.
ISBN 978-1464175121.
133. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The evolution and diversity
of fungi". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 451–468.
ISBN 978-1464175121.
134. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Animal origins and
diversity". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 469–519.
ISBN 978-1464175121.
135. Wu, K. J. (15 April 2020). "There are more viruses than stars in the universe. Why do only some
infect us? – More than a quadrillion quadrillion individual viruses exist on Earth, but most are not
poised to hop into humans. Can we find the ones that are?" (https://web.archive.org/web/202005281
54701/https://www.nationalgeographic.com/science/2020/04/factors-allow-viruses-infect-humans-cor
onavirus/). National Geographic Society. Archived from the original (https://www.nationalgeographi
c.com/science/2020/04/factors-allow-viruses-infect-humans-coronavirus/) on 28 May 2020.
Retrieved 18 May 2020.
136. Koonin, E. V.; Senkevich, T. G.; Dolja, V. V. (September 2006). "The ancient Virus World and
evolution of cells" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1594570). Biology Direct. 1 (1):
29. doi:10.1186/1745-6150-1-29 (https://doi.org/10.1186%2F1745-6150-1-29). PMC 1594570 (http
s://www.ncbi.nlm.nih.gov/pmc/articles/PMC1594570). PMID 16984643 (https://pubmed.ncbi.nlm.ni
h.gov/16984643).
137. Zimmer, C. (26 February 2021). "The Secret Life of a Coronavirus - An oily, 100-nanometer-wide
bubble of genes has killed more than two million people and reshaped the world. Scientists don't
quite know what to make of it" (https://ghostarchive.org/archive/20211228/https://www.nytimes.co
m/2021/02/26/opinion/sunday/coronavirus-alive-dead.html). The New York Times. Archived from the
original (https://www.nytimes.com/2021/02/26/opinion/sunday/coronavirus-alive-dead.html) on
2021-12-28. Retrieved 28 February 2021.
138. "Virus Taxonomy: 2019 Release" (https://ictv.global/taxonomy). talk.ictvonline.org. International
Committee on Taxonomy of Viruses. Archived (https://web.archive.org/web/20200320103754/https://
talk.ictvonline.org/taxonomy) from the original on 20 March 2020. Retrieved 25 April 2020.
139. Lawrence C. M.; Menon S.; Eilers, B. J. (May 2009). "Structural and functional studies of archaeal
viruses" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2675988). The Journal of Biological
Chemistry. 284 (19): 12599–603. doi:10.1074/jbc.R800078200 (https://doi.org/10.1074%2Fjbc.R800
078200). PMC 2675988 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2675988). PMID 19158076
(https://pubmed.ncbi.nlm.nih.gov/19158076).
140. Edwards, R.A.; Rohwer, F. (June 2005). "Viral metagenomics". Nature Reviews. Microbiology. 3 (6):
504–10. doi:10.1038/nrmicro1163 (https://doi.org/10.1038%2Fnrmicro1163). PMID 15886693 (http
s://pubmed.ncbi.nlm.nih.gov/15886693). S2CID 8059643 (https://api.semanticscholar.org/CorpusI
D:8059643).
141. Canchaya, C.; Fournous, G.; Chibani-Chennoufi, S. (August 2003). "Phage as agents of lateral gene
transfer". Current Opinion in Microbiology. 6 (4): 417–24. doi:10.1016/S1369-5274(03)00086-9 (http
s://doi.org/10.1016%2FS1369-5274%2803%2900086-9). PMID 12941415 (https://pubmed.ncbi.nl
m.nih.gov/12941415).
142. Rybicki, E. P. (1990). "The classification of organisms at the edge of life, or problems with virus
systematics". South African Journal of Science. 86: 182–86.
143. Koonin, E. V.; Starokadomskyy, P. (October 2016). "Are viruses alive? The replicator paradigm
sheds decisive light on an old but misguided question" (https://www.ncbi.nlm.nih.gov/pmc/articles/P
MC5406846). Studies in History and Philosophy of Biological and Biomedical Sciences. 59: 125–
134. doi:10.1016/j.shpsc.2016.02.016 (https://doi.org/10.1016%2Fj.shpsc.2016.02.016).
PMC 5406846 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5406846). PMID 26965225 (https://p
ubmed.ncbi.nlm.nih.gov/26965225).
144. Begon, M; Townsend, CR; Harper, JL (2006). Ecology: From individuals to ecosystems (4th ed.).
Blackwell. ISBN 978-1-4051-1117-1.
145. Habitats of the world (https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238). New York:
Marshall Cavendish. 2004. p. 238. ISBN 978-0-7614-7523-1. Archived (https://web.archive.org/we
b/20210415113154/https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238) from the
original on 2021-04-15. Retrieved 2020-08-24.
146. Tansley (1934); Molles (1999), p. 482; Chapin et al. (2002), p. 380; Schulze et al. (2005); p. 400;
Gurevitch et al. (2006), p. 522; Smith & Smith 2012, p. G-5
147. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The distribution of Earth's
ecological systems". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 845–
863. ISBN 978-1464175121.
148. Odum, Eugene P (1971). Fundamentals of Ecology (https://archive.org/details/fundamentalsofec000
0odum) (3rd ed.). New York: Saunders. ISBN 978-0-534-42066-6.
149. Chapin III, F. Stuart; Matson, Pamela A.; Mooney, Harold A. (2002). "The ecosystem concept".
Principles of Terrestrial Ecosystem Ecology. New York: Springer. p. 10. ISBN 978-0-387-95443-1.
150. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Populations". Principles of
Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 864–897. ISBN 978-1464175121.
151. Urry, Lisa; Cain, Michael; Wasserman, Steven; Minorsky, Peter; Reece, Jane (2017). "Population
ecology". Campbell Biology (11th ed.). New York: Pearson. pp. 1188–1211. ISBN 978-0134093413.
152. "Population" (http://www.biology-online.org/dictionary/Population). Biology Online. Archived (https://
web.archive.org/web/20190413145351/https://www.biology-online.org/dictionary/Population) from
the original on 13 April 2019. Retrieved 5 December 2012.
153. "Definition of population (biology)" (https://web.archive.org/web/20160304104019/http://www.oxfordd
ictionaries.com/definition/english/population?q=population). Oxford Dictionaries. Oxford University
Press. Archived from the original (http://oxforddictionaries.com/definition/english/population?q=popul
ation) on 4 March 2016. Retrieved 5 December 2012. "a community of animals, plants, or humans
among whose members interbreeding occurs"
154. Hartl, Daniel (2007). Principles of Population Genetics. Sinauer Associates. p. 45.
ISBN 978-0-87893-308-2.
155. Chapman, Eric J.; Byron, Carrie J. (2018-01-01). "The flexible application of carrying capacity in
ecology" (https://doi.org/10.1016%2Fj.gecco.2017.e00365). Global Ecology and Conservation. 13:
e00365. Bibcode:2018GEcoC..1300365C (https://ui.adsabs.harvard.edu/abs/2018GEcoC..1300365
C). doi:10.1016/j.gecco.2017.e00365 (https://doi.org/10.1016%2Fj.gecco.2017.e00365).
156. Odum, E. P.; Barrett, G. W. (2005). Fundamentals of Ecology (https://web.archive.org/web/20110820
163059/http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=97805
34420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topi
cName=) (5th ed.). Brooks/Cole, a part of Cengage Learning. ISBN 978-0-534-42066-6. Archived
from the original (http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_is
bn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag
=true&topicName=) on 2011-08-20.
157. Wootton, JT; Emmerson, M (2005). "Measurement of Interaction Strength in Nature". Annual Review
of Ecology, Evolution, and Systematics. 36: 419–44. doi:10.1146/
annurev.ecolsys.36.091704.175535 (https://doi.org/10.1146%2Fannurev.ecolsys.36.091704.175535)
. JSTOR 30033811 (https://www.jstor.org/stable/30033811).
158. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Ecological and evolutionary
consequences within and among species". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer
Associates. pp. 882–897. ISBN 978-1464175121.
159. Smith, AL (1997). Oxford dictionary of biochemistry and molecular biology. Oxford [Oxfordshire]:
Oxford University Press. p. 508. ISBN 978-0-19-854768-6. "Photosynthesis – the synthesis by
organisms of organic chemical compounds, esp. carbohydrates, from carbon dioxide using energy
obtained from light rather than the oxidation of chemical compounds."
160. Edwards, Katrina. "Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically
Active Ridge Flank". Woods Hole Oceanographic Institution.
161. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "Ecological communities".
Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 898–915.
ISBN 978-1464175121.
162. Riebeek, Holli (16 June 2011). "The Carbon Cycle" (http://earthobservatory.nasa.gov/Features/Carb
onCycle/?src=eoa-features). Earth Observatory. NASA. Archived (https://web.archive.org/web/2016
0305010126/http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features) from the
original on 5 March 2016. Retrieved 5 April 2018.
163. Hillis, David M.; Sadava, David; Hill, Richard W.; Price, Mary V. (2014). "The distribution of Earth's
ecological systems". Principles of Life (2nd ed.). Sunderland, Mass.: Sinauer Associates. pp. 916–
934. ISBN 978-1464175121.
164. Sahney, S.; Benton, M. J (2008). "Recovery from the most profound mass extinction of all time" (http
s://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596898). Proceedings of the Royal Society B: Biological
Sciences. 275 (1636): 759–65. doi:10.1098/rspb.2007.1370 (https://doi.org/10.1098%2Frspb.2007.1
370). PMC 2596898 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596898). PMID 18198148 (htt
ps://pubmed.ncbi.nlm.nih.gov/18198148).
165. Soulé, Michael E.; Wilcox, Bruce A. (1980). Conservation biology: an evolutionary-ecological
perspective. Sunderland, Mass.: Sinauer Associates. ISBN 978-0-87893-800-1.
166. Soulé, Michael E. (1986). "What is Conservation Biology?" (https://web.archive.org/web/2019041208
5412/http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf) (PDF). BioScience. 35
(11). American Institute of Biological Sciences: 727–34. doi:10.2307/1310054 (https://doi.org/10.230
7%2F1310054). JSTOR 1310054 (https://www.jstor.org/stable/1310054). Archived from the original
(http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf) (PDF) on 2019-04-12.
Retrieved 2021-05-15.
167. Hunter, Malcolm L. (1996). Fundamentals of conservation biology (https://archive.org/details/fundam
entalsofco00hunt). Oxford: Blackwell Science. ISBN 978-0-86542-371-8.
168. Meffe, Gary K.; Martha J. Groom (2006). Principles of conservation biology (3rd ed.). Sunderland,
Mass.: Sinauer Associates. ISBN 978-0-87893-518-5.
169. Van Dyke, Fred (2008). Conservation biology: foundations, concepts, applications (https://books.goo
gle.com/books?id=Evh1UD3ZYWcC) (2nd ed.). New York: Springer-Verlag.
doi:10.1007/978-1-4020-6891-1 (https://doi.org/10.1007%2F978-1-4020-6891-1). hdl:11059/14777
(https://hdl.handle.net/11059%2F14777). ISBN 978-1402068904. OCLC 232001738 (https://www.wo
rldcat.org/oclc/232001738). Archived (https://web.archive.org/web/20200727115147/https://books.go
ogle.com/books?id=Evh1UD3ZYWcC) from the original on 2020-07-27. Retrieved 2021-05-15.
170. Sahney, S.; Benton, M. J.; Ferry, P. A. (2010). "Links between global taxonomic diversity, ecological
diversity and the expansion of vertebrates on land" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2
936204). Biology Letters. 6 (4): 544–7. doi:10.1098/rsbl.2009.1024 (https://doi.org/10.1098%2Frsb
l.2009.1024). PMC 2936204 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2936204).
PMID 20106856 (https://pubmed.ncbi.nlm.nih.gov/20106856).
171. Koh, Lian Pin; Dunn, Robert R.; Sodhi, Navjot S.; Colwell, Robert K.; Proctor, Heather C.; Smith,
Vincent S. (2004). "Species coextinctions and the biodiversity crisis". Science. 305 (5690): 1632–4.
Bibcode:2004Sci...305.1632K (https://ui.adsabs.harvard.edu/abs/2004Sci...305.1632K).
doi:10.1126/science.1101101 (https://doi.org/10.1126%2Fscience.1101101). PMID 15361627 (http
s://pubmed.ncbi.nlm.nih.gov/15361627). S2CID 30713492 (https://api.semanticscholar.org/CorpusI
D:30713492).
172. Millennium Ecosystem Assessment (2005). Ecosystems and Human Well-being: Biodiversity
Synthesis. World Resources Institute, Washington, D.C.[1] (http://www.millenniumassessment.org/d
ocuments/document.354.aspx.pdf) Archived (https://web.archive.org/web/20191014033601/http://w
ww.millenniumassessment.org/documents/document.354.aspx.pdf) 2019-10-14 at the Wayback
Machine
173. Jackson, J. B. C. (2008). "Ecological extinction and evolution in the brave new ocean" (https://www.n
cbi.nlm.nih.gov/pmc/articles/PMC2556419). Proceedings of the National Academy of Sciences. 105
(Suppl 1): 11458–65. Bibcode:2008PNAS..10511458J (https://ui.adsabs.harvard.edu/abs/2008PNA
S..10511458J). doi:10.1073/pnas.0802812105 (https://doi.org/10.1073%2Fpnas.0802812105).
PMC 2556419 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556419). PMID 18695220 (https://p
ubmed.ncbi.nlm.nih.gov/18695220).
174. Soule, Michael E. (1986). Conservation Biology: The Science of Scarcity and Diversity. Sinauer
Associates. p. 584. ISBN 978-0-87893-795-0.

Further reading
▪ Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. (2002). Molecular Biology of the
Cell (https://archive.org/details/molecularbiolog000wils) (4th ed.). Garland.
ISBN 978-0-8153-3218-3. OCLC 145080076 (https://www.worldcat.org/oclc/145080076).
▪ Begon, M.; Townsend, C. R.; Harper, J. L. (2005). Ecology: From Individuals to Ecosystems
(4th ed.). Blackwell Publishing Limited. ISBN 978-1-4051-1117-1. OCLC 57639896 (https://www.worl
dcat.org/oclc/57639896).
▪ Campbell, Neil (2004). Biology (7th ed.). Benjamin-Cummings Publishing Company.
ISBN 978-0-8053-7146-8. OCLC 71890442 (https://www.worldcat.org/oclc/71890442).
▪ Colinvaux, Paul (1979). Why Big Fierce Animals are Rare: An Ecologist's Perspective (https://archiv
e.org/details/whybigfierceanim00paul) (reissue ed.). Princeton University Press.
ISBN 978-0-691-02364-9. OCLC 10081738 (https://www.worldcat.org/oclc/10081738).
▪ Mayr, Ernst (1982). The Growth of Biological Thought: Diversity, Evolution, and Inheritance (https://b
ooks.google.com/books?id=pHThtE2R0UQC). Harvard University Press. ISBN 978-0-674-36446-2.
Archived (https://web.archive.org/web/20151003080726/https://books.google.com/books?id=pHThtE
2R0UQC) from the original on 2015-10-03. Retrieved 2015-06-27.
▪ Hoagland, Mahlon (2001). The Way Life Works. Jones and Bartlett Publishers inc.
ISBN 978-0-7637-1688-2. OCLC 223090105 (https://www.worldcat.org/oclc/223090105).
▪ Janovy, John (2004). On Becoming a Biologist (2nd ed.). Bison Books. ISBN 978-0-8032-7620-8.
OCLC 55138571 (https://www.worldcat.org/oclc/55138571).
▪ Johnson, George B. (2005). Biology, Visualizing Life (https://archive.org/details/holtbiologyvisua00jo
hn). Holt, Rinehart, and Winston. ISBN 978-0-03-016723-2. OCLC 36306648 (https://www.worldca
t.org/oclc/36306648).
▪ Tobin, Allan; Dusheck, Jennie (2005). Asking About Life (3rd ed.). Belmont, California: Wadsworth.
ISBN 978-0-534-40653-0.