37 Introduction to Evolution of Sex

Our goal for this chapter is for you to begin to appreciate that sexual reproduction is costly from an evolutionary perspective, and that there are hypotheses to explain why sexual reproduction exists in spite of these costs.

Specifically, by the end of your reading, you will be able to:

  1. Define the following terms:
    • asexual reproduction
    • parthenogenesis
    • Muller’s Ratchet
    • The Red Queen
    • major histocompatibility complex (MHC)
    • polygenic
    • polymorphic
    • codominant
  2. Identify three hypotheses for the persistence of sexual reproduction.
  3. Where possible, list testable predictions associated with these hypotheses.
  4. Identify evidence in support of specific hypotheses for the existence of sex.

Asexual, lesbian lizards

It is difficult to mistake whiptail lizard copulation for anything else (Figure 1). A lizard mounting another lizard from behind certainly suggests sex. However, whiptail lizards of the genus Apidoscelis are all females and reproduce asexually.  Eggs are produced through meiotic division, but then undergo a doubling of chromosomes to return to a full set of chromosome pairs.  This type of asexual reproduction, in which embryos develop from unfertilized eggs, is called parthenogenesis (from the Greek, meaning “virgin birth”), and is seen in many plants and animals.

These lizards are doubly intriguing because they may exhibit a stereotypical mating behavior; in fact, this behavior has been shown to increase the number of offspring the lizards produce (even though there are no sperm involved!).

Species like the whiptail lizards highlight the major problem with sex.  There are many asexual organisms such as these lizards (figure 1), and they appear to be doing just fine without sexual reproduction. This ability to thrive without sex demands an answer to the question: why does sex exist?  Why would any organism incur all the costs associated with sexual reproduction, if asexual reproduction has so many obvious advantages?

Figure 1: Female – female copulation in whiptail lizards (Apidoscelis uniparens). Note the scientific name: “uniparens” means “one parent.” Which female assumes the dominant “male” role and which female assumes a “female” role depends on which female is ovulating.

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