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Evolution of Populations
Chapter 23
AP BIOLOGY
Populations are the Units of
Evolution
• Population=group of individual organisms
living in the same place at the same time
• Species=a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring in nature
• Evolution is measured as the change in
frequency of a given characteristic within a
population over a succession of
generations
Modern Synthesis
• In 1920s population genetics combined
with Darwinian principles
– Integrates discoveries and ideas from
many different fields—paleontology,
taxonomy, biogeography, and
population genetics
– Within a geographical area there will be
a concentration of certain alleles in the
gene pool
• Microevolution—when relative allele
frequencies in a population change
over a number of generations
Five Conditions Required for HardyWeinberg Equilibrium
(No Evolution)
1. Population is large
2. Population is isolated; no migration in/out
3. Mutations (changes in genes) do not alter
gene pool
4. Mating is random
5. All individuals are equal in reproductive
success; natural selection does not happen
Conditions rarely, if ever, met in nature
Hardy-Weinberg Equilibrium
• Allele frequencies will remain constant
in a populationsexual reproduction
does not lead to microevolution
• 500 iguanas
– 320 have WW
– 160 have Ww
– 20 have ww
W = non-webbed feet
w = webbed feet
p2 + 2pq + q2 = 1
Causes of Microevolution
• Genetic drift—change in gene pool of a
small population due to chance
– Loss/gain of individuals
Causes of Microevolution
• Bottleneck effect—
results from an
event/disaster that
drastically reduces
population size
(elephant seals after
being hunted in
1890s)
http://www.cambriarealty.com/images/seal_pic1.jpg
Causes of Microevolution
• Founder effect—random change in gene pool
that occurs in a small colony
• A few individuals start a new population
http://www.blackwellpublishing.com/korfgenetics/jpg/300_96dpi/Fig7-8.jpg
Causes of Microevolution
• Gene flow—gain/loss of allele from a
population
– Tends to reduce genetic differences
between populations
Causes of Microevolution
• Mutations—change in an organism’s
DNA
– Rare events
– Ultimate source of the genetic variation
that initiates evolution
– New mutation that is transmitted in
gametes can immediately change the
gene pool of a population by substituting
one allele for another
Causes of Microevolution
• Nonrandom mating
– Inbreeding—mating between closely
related partners
– Assortative mating—individuals select
partners that are like themselves in
certain phenotypic characteristics
Causes of Microevolution
• Natural selection
– Differential success in reproduction
– Results in alleles being passed along to
the next generation in numbers different
from the original population
Mechanisms of Adaptive Evolution:
Modes of Natural Selection
• Original population demonstrates the
continuum of shell color (light to dark)
• Stabilizing
Selection
– favors
intermediate
variants
– Stable
environments
where conditions
reduce
phenotypic
variation
• Directional
Selection
– Shifts overall
makeup of
population
– Acts against
individuals of
one of the
phenotypic
extremes
•
The horse has evolved
from a small-bodied
ancestor built for
moving through
woodlands and
thickets to its longlegged descendent
built for speed on the
open grassland. This
evolution has involved
well- documented
changes in teeth, leg
length, and toe
structure.
• Diversifying
Selection
– Favors both
extremes of a
phenotypic range
over
intermediate
individuals