Selection and Genetic Variation1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are homozygous consider Dawson’s flour beetles: started with population of all heterozygotes, + / l

l / l is lethal, but + / l is same as wildtype +/+

Selection and Genetic Variation1) selection against recessive alleles 

Although selection initially removed the l allele from population at a rapid rate, with each generation the frequency of l declined more slowly

Selection and Genetic Variation

2) selection against homozygotes This population was started with 100% heterozygotes for a viable allele V, and an allele L that is lethal when homozygous

although selection rapidly caused the V allele to increase in frequency, the L allele never disappeared

in fact, the frequency of L stabilized at 0.21

 

Selection and Genetic Variation

2) selection against homozygotes 1/5th of the population carried the lethal allele at equilibrium (the point where the population ceased to evolve)

Why?

 

Selection and Genetic Variation3) selection against heterozygotes

  consider the case of flies with compound chromosomes

normal pair of homologouschromosomes

compound chromosomes: arms swapped - one ends up with both left halves - other ends up with both right halves

when these flies make sperm/eggs, meiosis gets screwed up... they make 4 kinds of gametes

Selection and Genetic Variation3) selection against heterozygotes

 

- Flies can be homozygous for C (compound) or N (normal) allele

- two N/N flies can reproduce; all zygotes are viable (fitness =1)

- two C/C flies can reproduce; 1/4th of zygotes viable (fitness = 0.25)

- C/N flies don’t exist; they never develop (fitness = 0)

C and N flies can’t make viable zygotes together

Selection and Genetic Variation3) selection against heterozygotes  one or the other allele quickly becomes fixed in a mixed population

 

Selection and Genetic Variation3) selection against heterozygotes  one or the other allele quickly becomes fixed in a mixed population

- why? if there are few N/N flies, the odds of 2 mating are low- most N/N flies will not produce viable offspring- the allele will vanish

- if there are many N/N flies, they quickly out-breed C/C flies, due to their 4-fold advantage in producing viable offspring

this is underdominance:

Models of heterozygote superiority and inferiority

- in overdominance (heterozygote fitness > homozygote fitness),

population fitness is maximized at its stable internal equilibrium,

the point to which the population naturally returns

Models of heterozygote superiority and inferiority

- in underdominance (homozygote fitness > heterozygote fitness),

the population fitness is minimized at the unstable internal

equilibrium, the point from which the population naturally diverges

Frequency-dependent selection

scale-eating fish of Lake Tanganyika

Attack other fish by sneaking up, rushing them, biting off a mouthful of scales - Those with mouths that curve to the right attack the left side of victims, and vice-versa - Handedness of mouth is determined by a single locus with 2 alleles (simplest case!) - Right-handedness is dominant

Frequency-dependent selection

- victims come to expect attacks from the direction that the majority of the scale-eaters attack from, at that particular time

- when right-handed fish are more common, victims pay less attention to their right side (where few attacks come from); this gives left-handed fish the edge

- as left-handers get more food, they survive and reproduce better

- then, when left-handed offspring are the majority, the situation reverses

Frequency-dependent selection

- squares = proportion of successful breeding adults

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Frequency-dependent selection

The equilibrium point should be 50/50 of each phenotype…

…so what are the expected allele & genotype frequencies? Alleles: R LAllele frequencies 0.3 0.7 Possible genotypes: RR RL LL Hardy-Weinberg predicts: R2 + 2RL + L2

 Genotype frequencies: 0.09 0.42 0.49

Frequency-dependent selection 2Another case: pea aphid Acyrthosiphon pisum occurs in green and red color morphs

- what maintains polymorphism, the occurrence of both phenotypes in the population?

Differential vulnerability to predation versus parasitism, depending on color

- green aphids are more parasitized by wasps that lay their eggs inside aphids

- red aphids get eaten more by ladybugs (they’re more obvious sitting there on green plants)

Mutation as an evolutionary force

Mutation is ultimately responsible for creating new alleles and genes, but.. - can mutation also represent an evolutionary force, by changing allele frequencies?

- can mutation affect the predictions of Hardy-Weinberg equilibrium? 

Mutation as an evolutionary force

Consider a population where allele frequencies are: 

A a (a recessive, loss-of-function allele) 0.9 0.1

 In the ordinary Hardy-Weinberg state, adult genotypes will be: 

AA Aa aa0.81 0.18 0.01

 

Mutation as an evolutionary force

Now assume A mutates to a at a rate of 1 per 10,000 genes each generation  due to mutation, the allelic makeup of gametes will be: 

A a 0.9 – (0.9)(0.0001) 0.1 + (0.9)(0.0001)= 0.899991 = 0.10009

 

Mutation as an evolutionary forceWhen gametes randomly fuse to form zygotes, the genotype frequencies will be: 

AA Aa aa0.80998 0.18016 0.01002

Hardly any change; mutation had little effect over one generation Over thousands of generations, mutation can affect allele frequencies