Gen Bio 2 Lesson 3

Evolution and Origin of
Biodiversity: Patterns of Descent

with Modification
Biological evolution is a
powerful and important process. It
is a process which, over billions of
years, gradually selects the
organisms that are better adapted
to their environment to
continuously change life and
make all living organisms in our
world the way they are today.
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.
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.
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
B.Sympatric Speciation
C.Parapatric Speciation
A. Allopatric Speciation (allo
other, patric place; 'other
place’)
–occurs when some 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).
1. Pre-zygotic isolation mechanisms
-prevent fertilization and zygote formation.
-happens before fertilization occurs
between gametes.
•Habitat Isolation
•Temporal Isolation
•Behavioral Isolation
•Mechanical Isolation
•Gametic Isolation
•Habitat isolation
–The two species live in
different habitats and will not
encounter one another: each is
isolated from the other species.
•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.
•Behavioral isolation
–patterns of courtship is different. For
example, eastern & western meadowlark
songs differ.
•Mechanical isolation
–differences in reproductive organs prevent
successful interbreeding. Mechanical isolation
occurs when mating is physically impossible.
•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.
•Hybrid inviability
•Hybrid sterility
•Hybrid Breakdown
•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.
•Hybrid sterility
-their hybrids are sterile because gonads develop
abnormally or there isabnormal segregation of
chromosomes during meiosis. A horse and a
donkey may produce a hybrid offspring, a mule.
Mules are sterile
•Hybrid breakdown
–F1 hybrids are normal, vigorous and viable,
but F2 contains many weak or sterile individuals
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.

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Gen Bio 2 Lesson 3

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Evolution and Origin of

Biodiversity: Patterns of Descent

•Temporal or seasonal isolation

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