Chapter 23 The Evolution of Populations
Population Genetics
• It is the population, not its individual, that evolve.
• Evolution on the scale of populations, called microevolution, is defined as a change in
the allele frequencies in a population.
A population’s gene pool is defined by its allele frequencies
• Population- a localized group of individuals that belong to the same species.
• Species- a group of populations whose individuals have the potential to
interbreed and produce fertile offspring in a nature.
• Members of a population are far more likely to breed with members of the same
population than with members of other populations
Gene pool- the total genes in a population at any one time is called the population
The Hardy-Weinberg Theorem describes a nonevolving population
• Hardy-Weinberg theorem describes the gene pool of a nonevolving population.
• The frequencies of alleles and genotypes in a population’s gene pool will remain
constant over generations
• unless acted upon by agents other than Mendelian segregation and
recombination of alleles.
• Populations at Hardy-Weinberg equilibrium must satisfy five conditions.
1) Very large population size.
2) No migrations
3) No net mutations
4) Random mating
5) No natural selection.
Hardy-Weinburg Equation
p2 + 2pq + q2 = 1
• p2 = Frequency of homozygous dominant genotype
• 2pq = Frequency of both heterozygous genotypes
• q2 = Frequency of homozygous recessive genotype
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In a population of 500 plants
20 have white flowers (rr). 480 have red flowers (Rr or RR), (320 are RR, 160 are Rr.)
• Genotype frequency- % of population that have that particular genotype
• Allele frequency- % of a particular allele that is circulating in the population
• p + q = 1, If q2 equal to .04 (for rr) than q = 0.2
• If q =0.2, than p = 0.8
In Drosophilia, the allele for normal length wings is dominant over the allele for vestigial wings.
In a population of 1,000 individuals, 160 show the recessive phenotype.
• 160 (nn) / 1000 (population) = 0.16 for genotype frequency for nn
• q2 = 0.16, then q = 0.4,
p + q = 1,
p + 0.4 = 1, then p = 0.6,
q = 0.4, p = 0.6
Allele frequency
• 60% N, 40% n,
q = 0.4, p = 0.6,
p2 + 2pq + q2 = 1,
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• (0.6) + 2(0.6*0.4) + (0.4) = 1,
0.36 + 0.48 + .16 = 1
• Genotype frequency= NN 36%, Nn 48%, nn 16%
• Verify. We know 160 are recessive out of 1000!
The allele for the hair pattern called "widow's peak" is dominant over the allele for no "widow's
peak." In a population of 1,000 individuals, 360 show the dominant phenotype.
• WW + Ww = 360,
Therefore ww = 640
• 640 (ww) / 1000 (population) = 0.64 ww
Genotype frequency = 0.64 for ww
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• If q = 0.64, then q= 0.8,
p + q = 1,
If q= 0.8, then p = 0.2
• (0.2) 2 + 2(0.2*0.8) + (0.8) 2= 1,
0.04 + 0.32 + .64 = 1
Genotype frequency: 4% for WW, 32% for Ww , 64% for ww
Verify: 360 show dominant phenotype, WW (4%) + Ww (32%) = 36%, 36% of 1000= 360
Microevolution
• Microevolution- generation-to-generation change in a population’s frequencies of
alleles.
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The two main causes of microevolution are drift and natural selection
Genetic drift -chance events that occur when populations are finite in size.
• Bottleneck effect- occurs when the numbers of individuals in a larger population are
drastically reduced by a disaster.
• Founder effect- occurs when a new population is started by only a few individuals that
do not represent the gene pool of the larger source population.
• Natural selection- in a population with variable individuals some individuals to leave
more offspring than others.
• Natural selection accumulates and maintains favorable genotypes in a population.
• Gene flow- genetic exchange due to migration of fertile individuals or gametes between
populations.
• Mutation- a change in an organism’s DNA.
Genetic variation occurs within and between populations
• Quantitative characters- vary along a continuum within a population. Usually due to
polygenic inheritance. Ex: height of humans
• Discrete characters- Usually determined by a single locus with different alleles with
distinct impacts on the phenotype. Ex: flower color
• Polymorphism occurs when two or more discrete characters are present and noticeable
in a population. Ex: Freckles, blood types
• Gene diversity- measures the average percent of gene loci that are heterozygous.
• Nucleotide diversity- measures the level of difference in nucleotide sequences (base
pair differences) among individuals in a population.
• Geographic variation- differences in genetic structure either between populations that
inhabit different areas.
Mutation and sexual recombination generate genetic variation
• New alleles originate only by mutation.
• Most point mutations, those affecting a single base of DNA, are probably harmless.
• Mutations that alter the structure of a protein enough to impact its function are more
likely to be harmful than beneficial.
Diploidy and balanced polymorphism preserve variation
• Diploidy in eukaryotes prevents the elimination of recessive alleles via selection because
they do not impact the phenotype in heterozygotes.
• Even recessive alleles that are unfavorable can persist in a population through
their propagation by heterozygous individuals.
• Heterozygote protection maintains a huge pool of alleles that may not be
suitable under the present conditions but that could be beneficial when the
environment changes.
• Heterozygote advantage- heterozygous individuals have an advantage and are able to
reproduce more than homozygotes.
• Ex: sickle-cell disease
• Homozygous recessive causes the disease
• Homozygous dominant are vulnerable to malaria
• Heterzygous are resistant to malaria
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Directional selection- shifts the frequency curve for a phenotypic character in one direction by
favoring what had been rare individuals.
Diversifying selection- when environmental conditions favor individuals at both extremes of
the phenotypic range over intermediate phenotypes.
Stabilizing selection favors intermediate variants and acts against extreme phenotypes.
Ex: birth weight
Sexual selection may lead to pronounced secondary differences between the sexes
Sexual dimorphism- differences between the sexes: size, coloration, enlarged or
exaggerated features, or other adornments.
• Males are usually the larger and showier sex, at least among vertebrates.
Intrasexual selection is direct competition among individuals of one sex (usually males)
for mates of the opposite sex.
Intersexual selection or mate choice occurs when members of one sex (usually females)
are choosy in selecting among individuals of the other sex.
Natural selection cannot fashion perfect organisms
1. Evolution is limited by historical constraints.
• Evolution co-opts existing structures and adapts them to new situations.
2. Adaptations are often compromises.
• Organisms are often faced with conflicting situations that prevent an organism
from perfecting any one feature for a particular situation.
3. Not all evolution is adaptive.
• Chance affects the genetic structure of
• Founders of new populations may not necessarily be the best individuals,
but rather those individuals that enter the open habitat by chance.
4. Selection can only edit existing variations.
• Selection favors only the fittest variations from those phenotypes that are
available.
• New alleles do not arise on demand.
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