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The Evolution of Populations
Ch 11
Genetic Variation within
Populations
 Genetic variations in a population increases
the chance some individuals will survive
 Gene pool- the combined alleles of all of the
individuals in a population.
 Allele Frequency- measure of how common
a certain allele is in the population
Gg
gg
How many G
alleles are there?
How many g
alleles are there?
GG
GG
gg
GG
Gg
gg
GG
GG
gg
GG
How many G
alleles are there?
=7
Allele frequency
How many g
alleles are there?
G = 5/12 x 100 = 41.7%
=5
G = 7/12 x 100 = 58.3%
What causes genetic
variation?
 Mutations- random change in DNA can
form new alleles
 Recombination- when sex cells are
made, parents’ alleles are arranged in
new ways (crossing over)
Natural Selection at Work
 Distribution where the frequency is highest near
the mean value = normal distribution (bell
shaped curve)
 Phenotypes in the middle are the most common
and extremes are less common
Directional Selection
 Type of selection that favors
phenotypes at one extreme of a trait’s
range
Ex: Rise of drug resistant
bacteria
Before the 40’s, drug
resistance in bacteria existed
but didn’t give bacteria an
advantage. One antibiotics
developed, resistant bacteria
had an advantage and
reproduced in high numbers
Stabilizing Selection
 Phenotype in the middle becomes more
common
Disruptive Selection
 When both extreme phenotypes are
favored and those in the middle are
selected against
Male Lazuli Buntings:
Bright blue and bluish brown
males are seen as threat
Dull birds are left alone and win
mates and the brightest blue
attract mates regardless because
of their color. Those in the middle
are too blue to be left alone and
too dull to win a mate on color
alone
Gene Flow
 Movement of alleles from one
populating to another
Genetic Drift
 Changes in an allele frequency due to
chance (causes a loss in genetic
diversity)
Genetic Drift
 Bottleneck effect
 Occurs after an event that
greatly reduces the size of a
population
 Founder effect
 Occurs after a small number
of individual s colonize a
new area.
 These gene pools are often
different from those of the
larger population
Sexual Selection
 Certain traits increase mating success




Competition among males to win a mate
Males display certain traits that attract females
http://www.youtube.com/watch?v=VjE0Kdfos4Y
http://www.youtube.com/watch?v=L54bxmZy_NE
Hardy - Weinberg Equilibrium
 G. Hardy and W. Weinberg  1908 showed that genotype frequencies in
a population stay the same over time as
log as certain conditions are met. These
frequencies can be predicted.
Conditions for Equilibrium
(otherwise evolution occurs!)





1. A very large population (no genetic drift)
2. No emigration or immigration (no gene flow)
3. No mutations
4. Random mating (no sexual selection)
5. No natural selection (all traits equally aid
survival)
Hardy-Weinberg Equation
 p2+ 2pq + q2 = 1
 p+q=1


p = frequency of dominant allele
q = frequency of recessive allele
 p2 = frequency of homozygous dominant genotype
(Ex: TT)
 q2 = frequency of homozygous recessive genotype
(Ex : tt)
 2 pq = frequency of heterozygous genotype (
(Ex: Tt)
Example: Calculate predicted
genotype frequencies
 In a population of 1000 fish, 640 have
forked tails and 360 have smooth tails.
Tail fin shape is determined by 2 alleles:
 T = forked t = smooth
 Frequency of homozygous recessive (tt)
 q2 = 360/1000 = .36
 √. 36 = q = 0.60
 p + q = 1 , therefore p = .40
 Calculate the predicted genotype
frequencies from the allele frequencies:
 p2 = (.4)2 = 0.16 or 16% TT
 2pq = 2 x (.4)(.6) = 0.48 or 48% Tt
 q2 = (.6)2 = 0.36 or 36% tt
Speciation through isolation
 Reproductive isolation- members of
different populations can no longer mate
successfully
 Offspring don’t survive or are infertile
 Speciation- rise of 2 or more species from an
existing species
 Geographic isolation- physical barriers divide
population
Types of Evolution
 Convergent = different species adapt to
similar environments
 Analogous structures
 Divergent = closely related species evolve in
different directions
 Co-evolution = process by which 2 or more
species evolve in response to changes in each
other.
Speciation Patterns
 Punctuated Equilibrium =
Speciation occurs suddenly in
geologic time and followed by
long periods of little evolution

 Adaptive radiation =
diversification of one ancestral
species into many
descendants