Introduction
Heredity refers to the passing of characteristics from one generation to the
next. Evolution is defined as the gradual process by which a simple life form leads to the
development of complex organisms over a period of time, spanning several
generations.
Heredity
The transfer of traits from one generation to the next is termed heredity. Genes are the
functional units of heredity that transfer characteristics from parents to offspring. Genes
are short stretches of DNA that code for a specific protein or RNA.
Genetics is the branch of biology that deals with the study of genes, heredity and
variations.
Sexual Reproduction
The mode of reproduction involves two individuals; one male and one female.
They produce sex cells or gametes, which fuse to form a new organism.
Genes
Gene is the functional unit of heredity.
Every gene controls one or several particular characteristic features in living
organisms.
Heredity
The process by which the features of an organism are passed on from one generation to
another is called heredity.
The process is done by genes, which define the characters in the organism.
Mendel’s Work
Gregor Johann Mendel, known as the ‘Father of Genetics’, was an Austrian Monk
who worked on pea plants to understand the concept of heredity.
His work laid the foundation of modern genetics.
He made three basic laws of inheritance – The Law of Dominance, The Law of
Segregation and The Law of Independent Assortment.
Dominant Traits
The traits that express themselves in an organism in every possible combination and
can be seen are called Dominant traits.
In Mendel’s experiment, we see that the tall trait in pea plants tends to express
more than the short trait.
Therefore, the tall trait of the plant is said to be dominant over the short trait.
Recessive Traits
A trait which is not expressed in the presence of a dominant allele is known as
recessive.
So, a recessive character/trait is present in an organism but cannot be seen if a
dominant allele exists.
Monohybrid Cross
When only one character is considered while crossing two organisms, then such a
cross is known as a monohybrid cross.
The ratio of characters arising out of this cross at F2 generation is called the
monohybrid ratio.
E.g., If a tall plant (TT) is crossed with a dwarf plant (tt), we get 3 tall:1 short
plant at the end of the F2 generation.
So, 3:1 is a monohybrid ratio.
Here, the height of the plant is considered at a time.
Below is an example of a monohybrid cross between a true-breeding pea plant with
green pods (GG) and yellow pods (gg). Here, the green colour of the pod is the
dominant trait. Hence, in the F1 generation, all plants contain green pea pods.
Dihybrid cross
When two characters are considered while crossing two organisms, then such a
cross is known as a dihybrid cross.
The ratio of characters arising out of this cross at F2 generation is called the
dihybrid ratio.
E.g., If a plant with round and green pea is crossed with a plant with wrinkled and
yellow pea,
The first-generation plants would all have round and green peas.
On crossing the same for an F2 generation, we would observe four combinations
of characters in the ratio of 9:3:3:1.
Thus, 9:3:3:1 is the dihybrid ratio.
Inheritance
In Biology, inheritance pertains to the transfer of traits from one generation to another.
Laws of Mendel
Law of Dominance says that a gene has two contrasting alleles and one always
expresses itself in the organism.
It is called the dominant gene, and it expresses in any possible combination.
Law of Segregation says that traits get segregated completely during the formation of
gametes without any mixing of alleles.
Law of Independent Assortment says that the traits can segregate independently of
different characters during gamete formation.
Sex Determination
The process of determining the sex of an individual based on the composition of
the genetic material is called sex determination.
In different animals, the sex of an embryo is determined by different factors.
In humans, sex determination happens on the basis of the presence or absence
of the Y chromosome.
XX is female, and XY is male
An ovum always contains an X chromosome.
An ovum, upon fusion with the Y-containing sperm, gives rise to a male child and
upon fusion with the X-containing sperm, gives rise to a girl child.
Traits
Traits are characteristic features of an organism, manifested in a physical form that is
visible or in a physiological aspect of the organism.
Acquired Characters
The traits that are acquired by an organism over the period of its lifetime are
termed acquired characteristics.
These characteristics that are not passed on to the DNA of germ cells do not get
transferred to the next generation. E.g. loss of muscles and less weight due to
starvation, loss of limb or tails due to injury, etc.
Inherited Characters
The traits that are inherited from the parents are called inherited characters.
These traits always get transferred to the next generation but depending on the
dominance or recessiveness, they may or may not be expressed.
Examples are height, skin colour and eye colour.
Variation
Variation is the measure of the difference between individuals of the same species.
Offspring is not identical to parents, there exist some variations. Each individual in a
population differs from the others. Recombination and mutation are the main causes of
variations.
Sexually reproducing organisms show great variation among individuals of a species
and the long-term accumulation of variations plays a significant role in evolution. The
selection of variants by environmental factors is one of the driving factors of
evolutionary processes.
Genetic Variations
The differences in the DNA sequences among every organism leading to the diverse
gene pool are called genetic variations. These differences lead to different/varied
physical characters or biochemical pathways.
Natural Selection
It is the phenomenon by which a favourable trait in a population of a species is
selected.
Changing natural conditions exert equal pressure on all the existing species.
The species/organisms which are better adapted to the changing conditions
survive and reproduce i.e. selected by nature and species/organisms which
cannot adapt perish i.e. rejected by nature.
Speciation
Genetic Drift
Natural selection can play an important role in deciding the traits that survive in a
population. However, random fluctuations in gene variants are seen on many occasions.
This phenomenon is known as genetic drift. Thus, genetic drift is a change in the
frequency of an existing allele in a small population.
Genetic drift may cause a gene variant to disappear from the population and thus
reduce genetic variation.
Speciation
It is the process of formation of a new species from existing ones due to several
evolutionary forces like genetic drift, isolation of populations, natural selection, etc.
Speciation leads to diversity in the ecosystem and the diversity and diversity lead to
evolution.
Gene Flow
Gene flow is the transfer of genes from one population to the next. This occurs due to
migration or the introduction of organisms to a new population. This results in the
change in gene frequencies of a population.
Population
A population is a community or a group of animals, plants or any living organism that
can reproduce with each other and have fertile, viable offspring.
Charles Darwin
Charles Darwin, also called the “Father of Evolution”, was an English
Naturalist and Biologist.
Five years of the expedition in a ship called HMS Beagle to Galapagos Island
helped him write his theory of evolution.
In 1859 he published a book called Origin of Species, in which he put his theory
of evolution in detail.
Evolution and Fossils
Evolution
Evolution is a tangible change in the heritable characteristics of a population over
several generations. These changes can give rise to a new species or the species might
change themselves to become better adapted to the surrounding environment.
Origin of Species
After a successful expedition on HMS Beagle, Charles Darwin wrote a book on
what he observed on the Galapagos Islands.
In the book named ‘The Origin of Species, he wrote a detailed theory of evolution
which was mostly based on Natural Selection.
Origin of Life – Haldane’s Theory
JBS Haldane was a British Scientist who theorized that life originated from
organic and lifeless matter.
His theory was proved to be correct by Urey and Miller’s experiment.
It was called the theory of abiogenesis.
Evolutionary Evidence – Fossils
There are plenty of pieces of evidence to support the theory of evolution.
Fossils happen to be the biggest of them.
Fossils are the preserved remains of ancient animals or plants that died millions
of years ago.
The fossils help us understand the anatomy and even physiology of these
organisms and understand how evolution worked and led to the formation of
organisms that we see today.
Formation of Fossils
Fossils are important pieces of evolutionary evidence and are formed by the following
steps:
Organisms die, and they get buried in mud and silt.
The soft tissues of the body get quickly leave behind the hard bones or shells
Over time sediments build over it and harden into rock
As the bones decay, minerals seep in to replace the contents cell by cell, a
process called petrification
If bones decay completely, it leaves behind the cast of the animal.
Evolutionary Relationships
Evolutionary relationships of animals can be deduced by studying the homologous
organs and analogous organs.
Homologous organs are those which have a similar structure but different
function
Wings of birds and forelimbs of mammals: have similar structures but are
modified to suit different functions.
A tendril of the pea plant and spine of the barberry plant: both are modified
leaves but perform different functions.
Analogous organs are those which have a similar function but a different
structure and origin too
Wings of bats, birds and wings of insects: both are used for flying, but
structurally are very different.
Leaves of opuntia and peepal: both perform photosynthesis, but leaves of
Opuntia are modified stems, whereas peepal leaves are normal leaves.
Evolution by Stage
Evolution is a slow process and does not happen overnight.
There are several stages in the evolution of almost every animal that we see
today.
Complexities do not evolve suddenly but evolve bit by bit and may have limited
use at certain stages.
This gradual evolutionary process is called evolution by stages.
Artificial Selection
Sometimes a single species can evolve into several different species due to
artificial selection.
E.g. the cabbage family. A single ancestor in the cabbage family gave rise to
several different species due to the selection of different traits.
Molecular Phylogeny
The evolutionary relationship among different biological species is called
phylogeny.
It gives rise to an evolutionary tree.
In molecular phylogeny, these relationships are studied at the hereditary
molecular level, mainly using DNA sequences.
It involves the analysis of DNA composition and gene comparison between
different species.
Human Evolution
Humans are known to belong to the primate family.
Humans today have a very close genetic connection to chimps and other
primates.
While the complete evolutionary process of Humans from Primates is still a
mystery, a larger picture of human evolution has been formed.
Some of the ancestors of Humans include Dryopithecus, Ramapithecus,
Australopithecus, Homo erectus, Homo sapiens neanderthalensis, Cro-magnon
man, and finally, us, the Homo sapiens.
Human evolution traces back to Africa. Then they migrated all over the
world.
