As we discuss in chapter 7, genetic diversity is the ultimate evolutionary defense. The more diversity in one’s offspring, the more likely some of these offspring can survive challenges such as plague or famine. But how do we get that diversity? Do we just acquire mutations in our DNA? It’s rare for a mutation to be helpful, and mutation is not the major way in which diversity is created in sexually reproducing organisms.
Instead, measurable genetic diversity is achieved in a safer way–a way of rearranging genes, which have already proven to be helpful, into innovative combinations. This is basically what sex is, and where sex comes into the evolutionary equation. Here we get a ‘tried and tested’ collection of genes from two parents who were, at a minimum, biologically successful enough to make it to adulthood, acquire a mate, and have a child. Sex creates novel combinations of DNA while still maintaining many of the genes that were helpful for an individual’s parents. All the fascinating quirks of sexual biology that you read about in other chapters are really just strategies to help facilitate a single act: Fertilization.
Fertilization is the fusion of two sex cells (one sperm and one egg) into one cell, one that has the genetic potential to become a viable (or “functioning”) individual. In this chapter we will explore how this process works and how the creation of these cells determines genetic identity. In other chapters we’ll discuss the connection between copulation (or sex as an act) and fertilization, plus what happens after fertilization—gestation and birth.