Population Genetics and

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Chapter 8

• Population Genetics and
• Natural Selection

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Premises of Natural Selection

• Organisms beget like organisms
• Individual variations exist
• Overproduction of offspring occurs
• Unequal survival of offspring

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Variations an example
A middle-elevation herbaceous plant species
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Variation Elevation Experiment
Potentilla responded to its environment, growing
smaller in the alpine setting than in the lowland
setting. Therefore, the environment alone can
influence the morphology of plantsbut is there a
genetic difference?
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Variation Common Garden
Seeds from different populations sewn in the same
location
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Variation Common Garden Results
Plants grown from seed collected in different
areas showed differences in stem length and
flower length Therefore there is a genetic
basis to differences among populations Population
s that exhibit different traits based on location
are called ecotypes
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Variation on Animal Populations
Chuckwalla an herbivorous lizard of the
Southwest USA
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Variation with Rainfall
Body size in the chuckwalla is related to
rainfall.. more rain leads to longer
chuckwallas What about in a common location?
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Variation Animal Common Garden
A genetic difference is evident among populations
since animals obtained from higher elevations
(more rainfall) grew to larger sizes than animals
obtained from lower elevations (less
rainfall). Therefore ecotypes
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Variation Alleles
Spotting in the Asian lady beetle is controlled
by genes. One form of the gene (an allele) codes
for lots of spots and the other allele codes for
no spots. A heterozygote has a moderate number
of spots compared to the homozygotes.
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Variation Hardy-Weinberg Principle

• A population mating at random in the absence of
  evolutionary forces will maintain constant allele
  frequencies
• Large population
• Random mating
• No immigration or emigration
• No mutations
• Genotypes have equal fitness

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Variation Gene Frequencies

• p frequency of S allele in population
• q frequency of A allele in population
• pq 1
• p2 2pq q2 1

frequency of SS
frequency of SA
frequency of AA
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Variation Population Size
Genetic diversity increases with population
size. Therefore small populations are suspect
to random loss of genes (genetic drift)
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Variation Inbreeding Depression
The Glanville fritillary butterfly has a small
population size, which leads to inbreeding. This
process tends to homogenize the genetic make-up
of a population and leave it susceptible to
extinction.
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Selection Unequal Reproduction
Brown anoles face a trade-off in hind-leg length
between moving fast and staying mobile on small
branches. Therefore vegetation size can
influence hind-leg size. What happens if we
change the vegetation?
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Selection Unequal Reproduction
The greater the difference between the original
vegetation and the new vegetation, the greater
the change in hind-leg length in the lizard
populations. Therefore the more fit individuals
had more offspring and more successful offspring
than the less fit individuals and the population
evolved.
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Selection Unequal Reproduction
Specifically, hind-leg length was positively
correlated with perch diameter in the populations
after time had passed.
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Summary

• Variation exists within and among populations
• Hardy-Weinberg identifies forces that can affect
  gene frequency and therefore population
  characteristics
• Random processes can affect gene frequency,
  especially in small populations
• Natural selection can result in adaptation to
  changes in the environment