Components of selection

Depending on which of those three assumptions is violated and how it's violated we recognize that selection may happen in different ways and at different life-cycle stages.³

Assumption #3:

Meiosis is fair. There are at least two ways in which this assumption may be violated.

• Segregation distortion: The two alleles are not equally frequent in gametes produced by heterozygotes. The $t$-allele in house mice, for example, is found in 95% of fertile sperm produced by heterozygous males.

• Gamete competition: Gametes may be produced in equal frequency in heterozygotes, but there may be competition among them to produce fertilized zygotes, e.g., sperm competition in animals, pollen competition in seed plants.

Assumption #6:

All matings produce the same number of progeny.

• Fertility selection: The number of offspring produced may depend on maternal genotype (fecundity selection), paternal genotype (virility selection), or on both.

Assumption #8:

Survival does not depend on genotype.

• Viability selection: The probability of survival from zygote to adult may depend on genotype, and it may differ between sexes.

At this point you're probably thinking that I've covered all the possibilities. But by now you should also know me well enough to guess from the way I wrote that last sentence that if that's what you were thinking, you'd be wrong. There's one more way in which selection can happen that corresponds to violating

Asssumption #2:

Individuals mate at random.

• Sexual selection: Some individuals may be more successful at finding mates than others. Since females are typically the limiting sex (Bateman's principle), the differences typically arise either as a result of male-male competition or female choice.

• Disassortative mating: When individuals preferentially choose mates different from themselves, rare genotypes are favored relative to common genotypes. This leads to a form a frequency-dependent selection.