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Part 2
Natural Selection and
Natural Selection shapes a population, making it
adapted to its current environment. This
happens over a relatively short period of time.
Most scientists agree that natural selection,
acting over very long periods of time, leads to
speciation. (“Adding branches to the tree.”)
There are two patterns of evolution described
by scientists based on observations of living and
extinct organisms.
Patterns of Evolution
Divergent evolution: the process of two or
more related species becoming more and more
These species will share homologous structures
Convergent evolution: the process by which
distantly related species become more similar as
they adapt to the same kind of environment
These species appear similar on the outside, but do
not share homologous structures. Their similar
structures are considered analogous.
Example of Divergent Evolution
Each of these mantid species is exquisitely adapted to a very specific environment. The
“body plan” of each species is homologous, but adaptation by natural selection has made
each look different.
Examples of
What do these 2 examples
have in common?
Hawaiian Honeycreepers
Divergent Evolution of Animals
Example of Convergent Evolution
The placental
mouse has more
features with the
wolf than with
the marsupial
The similarities
between the
placental mouse
and marsupial
mouse are
Analogous Features
Homology vs. Analogy
More on Patterns of Evolution
Both divergent and convergent evolution involve
speciation, the formation of new species.
Isolated populations become more different over time
as natural selection acts to adapt them to a certain
environment, or a unique niche within the
If enough time passes and enough genetic change
occurs, the isolated populations become different
Species Definitions
Historically, a species has been defined by its structural differences compared
to similar species. However, structure is not adequate, by itself, in defining a
Morphological species concept: defines a
species as a group of structurally unique
organisms that differ from other described
 Biological species concept: defines a species
as a group of organisms able to interbreed to
produce viable offspring in nature.
Isolating Mechanisms are needed for
new species to arise
A key of isolating mechanisms is that they act as
barriers that keep two populations from
Geographic isolation: If a population splits into
2 due to a physical barrier (i.e.: mountain range
forms, a lake dries up into small ponds, a
subpopulation crosses to a different island, etc.), the
two populations become different due to natural
selection. Over time the populations may become
different enough that they can no longer
Geographic Isolation & Speciation
Examples of Geographic Isolation
Isolating Mechanisms, continued
Reproductive Isolation: type of isolation that
prevents interbreeding, even when two populations live
in the same geographic region.
Prezygotic barriers: these barriers prevent fertilization
between gametes of two different species. (ex: males of
one population use a different mating call to attract females
than a related species.)
Postzygotic barriers: these barriers prevent hybrid
offspring from either surviving or producing offspring.
(ex: an embryo fails to develop and dies or the resulting
offspring are sterile like the mule that results from the mating
of a donkey and a horse)
Reproductive Isolating
Natural Selection is not the only
mechanism of evolution
If a population is changing genetically, then
a biologist can investigate if it may be due
to one of the following:
Natural selection
 Nonrandom mating (selective breeding)
 Mutation
 genetic drift
 gene flow
Closer Look: Nonrandom Mating
Farmers and animal breeders have long known
that the traits of a population could be changed
by nonrandom mating.
Example: Oranges with smaller and smaller seeds
were bred until “seedless” oranges were created
 In this case, farmers did not allow “nature to take its
course”. They selected a trait they wanted and only
bred plants that had that form of the trait.
Closer Look: Genetic Drift
Genetic drift: the process by which random
evolutionary changes occur in small populations
due to chance or random events.
The evolutionary changes are not necessarily
adaptive, unlike evolution by natural selection.
Genetic Drift can decrease genetic
Bottleneck effect: occurs when a population
decreases rapidly due to natural disaster, disease,
etc. The small remaining population may not have
allele frequencies that are representative of the original
Founder effect: occurs when one or a few
individuals leave a large population and colonize a
new location. The founders only bring with them a
small sample of the alleles of the original population,
so the new population is likely to be significantly
different genetically.