Fertility selection and sexual selection

So far we've been talking about natural selection that occurs as a result of differences in the probability of survival. There are, of course, other ways in which natural selection could operate.

• Heterozygotes may produce gametes in unequal frequencies, or gametes may differ in their ability to participate in fertilization--gametic selection.
• Some genotypes may be more successful in finding mates than others--sexual selection.
• The number of offspring may differ among mated pairs-- fertility selection.

In fact, in most studies where several components of selection have been identified, differences in fertility among genotypes are far larger than differences in viability. Thus, fertility selection is a very important component of fitness differences among individuals in most plants and animals. As we'll see in just a moment, sexual selection is mathematically equivalent to a particular type of fertility selection. But before I show you that, let's take a careful look at the mechanics of fertility selection.