• Species - Ernst Mayer’s definition: “Species are groups of interbreeding natural

populations that are reproductively isolated from other such groups.”

- Is a closely related organism that are very similar and capable of producing fertile offspring.

• Reproductive Isolating Mechanism

- The mechanisms of reproductive isolation are a collection of evolutionary mechanisms,
behaviors and physiological processes critical for speciation. They prevent members of
different species from producing offspring, or ensure that any offspring are sterile. These
barriers maintain the integrity of a species by reducing gene flow between related species.

1. Pre-zygotic isolation mechanisms

- prevent fertilization and zygote formation.
- happens before fertilization occurs between gametes.

1.1 Geographic or ecological or habitat isolation

- occurs when two species that could interbreed do not because the species live in different
areas. The two species live in different habitats and will not encounter one another: each is
isolated from the other species.

1.2 Temporal or seasonal isolation

- different groups may not be reproductively mature. For example, two populations of plants
may produce flowers in different seasons, making mating between the populations
impossible

1.3 Behavioral isolation

- patterns of courtship is different. For example, eastern & western meadowlark songs differ.

1.4 Mechanical isolation

- differences in reproductive organs prevent successful interbreeding. Mechanical isolation
occurs when mating is physically impossible.

1.5 Gametic isolation

- incompatibilities between egg and sperm prevent fertilization. Often this occurs because
the female immune system recognizes sperm as foreign and attacks it.

2. Post-zygotic isolation mechanisms

- allow fertilization but nonviable or weak or sterile hybrids are formed. In these cases, the
zygote formed is called a hybrid. However, even after a hybrid zygote forms, reproduction
may still not be successful

2.1 Hybrid inviability

- fertilized egg fails to develop past the early embryonic stages. For example, when tigers
and leopards are crossed, the zygote begins to develop but the pregnancy ends in
miscarriage or stillborn

2.2 Hybrid sterility

-their hybrids are sterile because gonads develop abnormally or there is abnormal
segregation of chromosomes during meiosis. A horse and a donkey may produce a hybrid
offspring, a mule. Mules are sterile

2.3 Hybrid breakdown

- F1 hybrids are normal, vigorous and viable, but F2 contains many weak or sterile
individuals

• Speciation - is the evolutionary process by which populations evolve to become distinct

species. It is the process by which new species develop from existing species.

• Mode of Speciation
A. Allopatric Speciation (allo – other, patric – place; ‘other place’)
- Allopatric speciation or geographic speciation occurs when som members of a population
become geographically separated from the other members thereby preventing gene flow.
Examples of geographic barriers are bodies of water and mountain ranges.

B. Sympatric Speciation (sym – same, patric – place; ‘same place’)

- occurs when members of a population that initially occupy the same habitat within the
same range diverge into two or more different species. It involves abrupt genetic changes
that quickly lead to the reproductive isolation of a group of individuals. Example is change in
chromosome number (polyploidization).

C. Parapatric Speciation (para – beside, patric – place; ‘beside each other’)

- occurs when the groups that evolved to be separate species are geographic neighbors.
Gene flow occurs but with great distances is reduced. There is also abrupt change in the
environment over a geographic border and strong disruptive selection must also happen.
CAROLUS LINNAEUS
● Swedish naturalist/explorer (first to frame principles for defining natural genera and
species of organisms)
● created a uniform system for naming them “binomial nomenclature”

THOMAS MALTHUS
● An Essay on the Principle of Population (1798)
● food production improved the well being of the populace, but the improvement was
temporary because it led to population growth, which in turn restored the original per
capita production level.

GEORGE CUVER
● Father of Paleontology
● Theory of Catastrophism = boundaries represent floods, droughts, etc. that destroyed
many species living at that time
● fossils are remains of extinct life forms

JAMES HUTTON
● Theory of Gradualism = Profound changes can result from cumulative effect of slow
but continuous processes.
● Proposed that the Earth was shaped by geological forces occurring over very long
periods of time, MILLIONS not THOUSANDS of years old

CHARLES LYELL
● Principles of Geology = formation of Earth's crust took place through countless small
changes occurring over vast periods of time (natural laws)
● "uniformitarian" proposal = molding the planet today have operated continuously
throughout its history.
● The principle of uniformitarianism says that the present is the key to the past.

JEAN BAPTISTE-DE LAMARCK

● recognize that living things changed over time and that all species were descended
from other species.
● Lamarckism - characteristics that an animal acquired during its lifetime could be
passed on to its offspring
● 1809- Published his ideas about “Inheritance of Acquired Characteristics”

CHARLES DARWIN
● Evolution of Darwin’s Theory
● His voyage and his observations led him to write ‘The Origin of Species
● In 1831, 22-year old Charles Darwin left England as naturalist aboard the HMS
Beagle for 5 year voyage around the world. His mission is to chart the South
American coastline
● plants and animals were different from those in Europe
● He wrote thousands of pages of observations and collected vast number of
Specimens
● (month observation) Galapagos Islands=each island has different rainfall and
vegetation and its own unique assortment of plant and animal species.
 ● collected 14 species of finches, hypothesized=Galapagos had be colonized by
organisms from the mainland that had then diversified on the various.
● 1859 , his book On the Origin of Species by Means of Natural Selection was
published. It presented evidence and proposed a mechanism for evolution that he
called NATURAL SELECTION.
● 1.Like Lamarck, Darwin assumed that species can change over time. The fossils he
found helped convince him of that.
● 2. From Lyell, Darwin saw that Earth and its life were very old. Thus, there had been
enough time for evolution to produce the great diversity of life Darwin had observed.
● 3. From Malthus, Darwin knew that populations could grow faster than their
resources. This “overproduction of offspring” led to a “struggle for existence,” in
Darwin’s words.
● 4. From artificial selection, Darwin knew that some offspring have variations that
occur by chance, and that can be inherited. In nature, offspring with certain variations
might be more likely to survive the “struggle for existence” and reproduce. If so, they
would pass their favorable variations to their offspring

A. Fossils
- Fossils are preserved remnants of once living organisms (trapped in rocks, tar pits,
frozen in ice or embedded in amber).
- common fossils; bones, shells and seeds or pollen grains.
- Most fossils are formed in sedimentary rock.
- Palaeontologists use the fossils found in rocks to track the evolutionary history of
many organisms.
- Age of the rocks=age of the fossil.
● Relative dating rocks; dated by their position with respect to one another; rocks in
deeper strata are generally older.
● Absolute dating rocks; dated by measuring the degree of decay of certain
radioisotopes contained in the rock; the older the rock, the more its isotopes have
decayed (isotopes in a rock act as an internal clock, determining the time after the
rock was formed)

B. Embryology
● An embryo is an organism in its initial phases of development.
● embryology=study of the development of the anatomy of an organism to its adult
form
● (Scientists) during the development process, embryos of many different animals
appeared so similar that it was difficult to tell them apart.
● During development (at some time); all vertebrates have a supporting dorsal rod,
called a notochord, and demonstrate paired pharyngeal pouches.
● organism passes through some of the embryonic stages that its ancestors passed
through. Then numerous variations occur in ways appropriate to an organism’s final
form.

C. Anatomical Evidence
● comparing the anatomy and the development of organisms=shows a unity of plan
among those that are closely related.
 ● The more body structures that two species have in common=the more closely they
are related.
● “descent from a common ancestor”
- homologous structures; Similar structures in different species irrespective of their
functions are called. ex;The limb skeletons of vertebrates are homologous structures.
- Analogous structures; structures which are different in appearance but have the
similar function. Analogy does not indicate common ancestry
- Vestigial structures; are anatomical features that are usually reduced and have no
- function in many organisms. give more evidence for evolutionary change.
- These are organs that were previously functional in the ancestors of the species but
are only remnants in the present-day species. Ex; appendix

D. Biochemical
● Living organisms; shares related biochemical molecules (DNA, ATP, amino acids,
and enzymes).
● The more closely linked organisms are the more related is their biochemical genetic
makeup.
1. DNA/Nucleic acids
- (Genes are located in the chromosomes) The more closely related two living
organisms are=the more similar the sequence of their DNA molecules will be.
2. Proteins
- Proteins; molecules that are used to build up and repair body parts.
- more similar the structure of protein molecules=more related they are/recent
existence
- Unrelated species have different amino acids.

E. Biogeography
- Biogeography is the study of the geographic distributions of organisms.
- Darwin’s trip around South America allowed him to observe the diversity of
organisms in different areas and the resemblance of such species of birds and
tortoises in an island to nearby mainland.
- Darwin believed that the group of organisms in each island is adapted to a distinct
way of life.
- The common ancestors of these organisms had come from one place, expanding out
into other accessible regions.