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When all individuals in a population have an equal likelihood of surviving and producing surviving offspring, i.e. no agents of evolution operate on a population, alleles end up in gametes and, in turn the offspring generation in direct proportion to their frequencies in the parental generation. This causes a population's allele frequencies to be perpetuated unchanged from one generation to the next, a condition known as genetic equilibrium .
Does the same thing hold true for genotype frequencies? That is, when every individual has an equal probability of surviving and producing surviving offspring are the genotype frequencies of the parental generation necessarily replicated in the offspring generation?
To investigate the relationship between parental and offspring generation genotype frequencies when a population is not being influenced by an agent of evolution, let’s review how offspring genotypes are formed when every potential parent is equally likely to survive and reproduce.
Under these conditions, the allele contributed by one parent does not and is not influenced in anyway by the allele contributed by the second parent; there is no sexual selection. Nor is a parent's likelihood of contributing alleles to the next generation influenced by environmental circumstances that favor survival of one parental phenotype over another or by chance events as occur during genetic drift.
Thus, if survival and reproduction are truly random events, each parent's allelic contribution to fertilization is a statistically independent event. Put differently, the genotype of the resulting offspring is the result of two independent , chance events - one chance event per allele. Imagine drawing two parents at random from a population and randomly collecting one allele from each.
Test your understanding by answering the question below.
If female peacocks prefer to mate with males who have a larger number of eye-spots on their elaborate tail feathers over males with fewer eye-spots (and the number of eye-spots is heritable), is the likelihood that a male will contribute alleles to the next generation independent of, i.e. unaffected by, the female? Why or why not? Please explain.
No, male and female contributions to a fertilization event are not independent. Females preferentially mate with males with a larger number of tail feather eye-spots. This favors the likelihood that the alleles associated with this trait will end up in the next generation over alleles associated with fewer eye-spots. This is sexual selection, i.e. non-random mating.
When the independent events condition described above is met, a given genotype is expected to appear in the offspring generation with a likelihood equal to:
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