Population Genetics and Natural Selection
Chapter 8
Microsatellite alleles for three loci depict genetic variation within
a Great Tit population.
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Definitions
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Evolution – a change in gene frequencies in
a population; may lead to speciation.
Natural Selection – differential reproduction
and survival of individuals in a pop. due the
environmental influences on the pop.; the
environment affects fitness.
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Offspring like parents
Variation among individuals is heritable.
More offspring than environment can support
Some individuals have higher fitness under
particular environmental conditions.
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Gene – a discrete unit of hereditary
information consisting of a specific
nucleotide sequence in DNA
 Alleles – alternative forms of a gene
Genotype – the genetic makeup of an
individual
Phenotype – the physical and physiological
traits of an organism
Campbell and Reese 2002
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Variation Within Populations
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Phenotype is environmental and genetic
Variation in Plant Populations
 Many plant species differ dramatically in
form from one elevation to another (e.g. )
Potentilla glandulosa.
 Distinctive ecotypes
locally adapted and genetically
distinctive populations within a species.
 Cross breeds produce viable seeds that
are reproductive – not separate species.

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Hypothesis: Morphology differences partly due to genetics.
Null Hypothesis: No genetic difference; all environmental.
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Hardy Weinberg
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Hardy Weinberg principle states that in a population mating
at random in the absence of evolutionary forces (natural
selection), allele frequencies will remain constant.
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The H-W Model allows for predicting genetic change due to
environmental forces.
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Hypothesis: environmental factor influences change in
allele frequency change.
Null Hypothesis: allele frequency remains unchanged.
1 = (p+q)2 = p2+2pq+q2
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Conditions Necessary for Hardy Weinberg
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Random Mating
No Mutations
Large Population Size
No Immigration
Equitable Fitness Between All Genotypes
 Likely, at least one of these will not be met
and allele frequencies will change.
 Potential for evolutionary change in
natural populations is very great.
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Natural Selection
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Some individuals in a population, because of
their phenotypic characteristics, produce
more offspring that themselves live to
reproduce.
 Natural selection can favor, disfavor, or
conserve the genetic make-up of a
population.
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Stabilizing Selection
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Stabilizing selection acts to impede changes
in a population by acting against extreme
phenotypes and favoring average
phenotypes.
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Directional Selection
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Directional selection leads to changes in
phenotypes by favoring an extreme
phenotype over other phenotypes in the
population.
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Disruptive Selection
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Disruptive selection creates bimodal
distributions by favoring two or more extreme
phenotypes over the average phenotype in a
population.
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Change Due To Chance
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Random processes can change gene
frequencies in populations, especially in
small populations. Called genetic drift.
Major concern of habitat fragmentation is
reducing habitat availability to the point
where genetic drift will reduce genetic
diversity within natural populations.
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Genetic Variation In Island Populations
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In general, genetic variation is lower in
isolated and generally smaller, island
populations.
 Reduced genetic variation indicates a
lower potential for a population to evolve.
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Microsatellite Analysis
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Whose your Daddy?
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Linkage to social function
(Young and Hammock, 2005)
http://www.nsf.gov/news/news_images.jsp?cntn_id=104238&org=NSF
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