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## Marginal fitnesses

In case you haven't already noticed, there's almost always more than one way to write an equation.10 They're all mathematically equivalent, but they emphasize different things. In this case, it can be instructive to look at the difference in allele frequencies from one generation to the next, :

where is the marginal fitness of allele . To explain why it's called a marginal fitness, I'd have to teach you some probability theory that you probably don't want to learn.11 Fortunately, all you really need to know is that it corresponds to the probability that a randomly chosen allele in a newly formed zygote will survive into a reproductive adult.

Why do we care? Because it provides some (obvious) intuition on how allele frequencies will change from one generation to the next. If , i.e., if the chances of a zygote carrying an allele of surviving to make an adult are greater than the chances of a randomly chosen zygote, then will increase in frequency. If , will decrease in frequency. Only if , , or will the allele frequency not change from one generation to the next.

Next: Patterns of natural selection Up: Genotype and allele frequencies Previous: Selection acts on relative
Kent Holsinger 2012-09-15