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Evolution in Populations
Single Gene Traits
Can lead to a shift in
allele frequencies.
This genetic selection
can then lead to the
evolution or a change
in the species.
Polygenic Traits
Can lead to a shift in
allele frequencies.
Changes that can
alter the phenotypic
make-up of a species.
Directional Selection
Stabilizing Selection
Disruptive Selection
Evolution in Populations
Single Traits
Evolution in Populations
Polygenic Traits
Evolution in Populations
Polygenic Traits
Evolution Caused by a
Genetic Change
Genetic Drift
A shift in allele
frequencies in a
population due to
chance.
Bottlenecks
An abrupt and severe reduction
in the number of individuals
during the history of a species,
resulting in the loss of
diversity from the gene pool.
The generations following the
bottleneck are more genetically
homogenous than would
otherwise be expected.
Bottlenecks often occur in
consequence of a catastrophic
event.
The American Heritage® Science Dictionary
Genetic Drift
Founders Effect
The frequency of the “a”
allele is low in the initial
population, but a small
subset, in which one
individual is Aa, is
removed from the large
population and founds a
new population. The
frequency of “a” is
markedly higher in this
new population, due to its
relatively high frequency
in the founders.
www.blackwellpublishing.com/korfgenetics/figu...
Temporal Isolation
Breed at differing times of the year.
Western Spotted
Skunk (Spilogale gracilis)
Breeds in late
summer
Eastern Spotted
Skunk (Spilogale putorius)
Breeds in late
winter
Behavioral Isolation
Two species that
are capable of
interbreeding,
but don’t because
of courtship
rituals or other
similar behaviors.
Mechanical Isolation
When structural
differences prevent
successful mating. For
example, in many closely
related species of plants,
there are certain
differences, such as
differences in color, that
help attract different
kinds of pollinators.
Gamete Isolation
(Lock and Key Isolation)
Fertilization
cannot take place
if the gametes
are not
compatible.
Geographical Isolation
Individuals of a
species that are
physically
separated by a
barrier may
form subspecies
such as these
spotted owls.