BIOC51H3 Lecture 11: BGYB51H3 Lecture 11 Notes Oct 15
11 Aug 2010
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BGYB51H3 Lecture 11 Notes
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-allele frequencies change due to selection (see figure 6.12)
-in populations with 2 alleles, sum of allele frequencies and sum of genotype frequencies must
add up to 1
-selection on mutations causes evolutionary change
-under frequency dependent selection, the phenotype fitness depends on its frequency
(increase in frequency, decrease in fitness)
-frequency-dependent selection maintains genetic variation
-in heterozygote superiority (overdominance), heterozygotes have highest fitness so that both
alleles are maintained in the population
-one allele favoured involves rate of change depending on initial frequencies of each allele and
its dominance
-heterozygote inferiority means that fixation of one allele is possible depending on initial
frequencies and relative fitness of homozygotes
-heterozygote inferiority maintains variation among populations and not within populations
-frequency-dependent selection involves negative frequency dependence
-phenotypes with higher frequency in the population have lower frequency of breeding adults
in frequency dependent selection
-some alleles that are lethal are maintained because alleles are hidden from selection by
heterozygosity
-homozygote recessive lethal alleles decrease dramatically
-at low frequencies of recessive, most copies are found in heterozygotes which will mate with
the dominant homozygotes
-deleterious alleles may persist under mutation-selection balance
-the higher the selection coefficient, the lower the fitness of the allele selected against
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Document Summary
Allele frequencies change due to selection (see figure 6. 12) In populations with 2 alleles, sum of allele frequencies and sum of genotype frequencies must add up to 1. Under frequency dependent selection, the phenotype fitness depends on its frequency (increase in frequency, decrease in fitness) In heterozygote superiority (overdominance), heterozygotes have highest fitness so that both alleles are maintained in the population. One allele favoured involves rate of change depending on initial frequencies of each allele and its dominance. Heterozygote inferiority means that fixation of one allele is possible depending on initial frequencies and relative fitness of homozygotes. Heterozygote inferiority maintains variation among populations and not within populations. Phenotypes with higher frequency in the population have lower frequency of breeding adults in frequency dependent selection. Some alleles that are lethal are maintained because alleles are hidden from selection by heterozygosity. At low frequencies of recessive, most copies are found in heterozygotes which will mate with the dominant homozygotes.
