ZOOL 304, Class Notes

Chapter 7, Feb. 28 - March 3.

Text reading:  Chapter 7.

This chapter introduces what Graham Bell has called "the queen of problems in evolutionary biology".  Why is sex so common?  What conditions of selection led to the emergence of sexual reproduction, and what conditions of selection maintain sex in so many populations?

Sex poses a problem for one profound reason and one mundane reason.

1. The profound reason is huge.  Sex comes with an intrinsic 50% cost.  

   This is sometimes referred to as a two-fold advantage for asexual reproduction.  Whether this is called a 50% cost or a two-fold advantage, the numbers are the same.  One is just the reciprocal of the other.  The difference is just a matter of whether we speak of cost or advantage.  (Fifty cents is 50% less than a dollar.  A dollar is two-fold greater than fifty cents.)

   Reference to a "two-fold cost" (as in our text) is somewhat unfortunate, not following the best usage.  By analogy, when referring to height one might say "twice as short" when one really means "half as tall".  Both phrases are usually understood to mean the same thing, but only the latter represents proper usage.

   There are two ways to imagine this cost of sex (or advantage of asex), as potential rate of increase, or as genetic contribution.  These are in fact equivalent, but they appear rather different.

   • Females who reproduce sexually can produce, on average, only half as many female offspring as asexually reproducing females who do not expend any of their reproductive capacity producing males.  Thus, for a given reproductive investment, a sexual female's potential rate of increase could be doubled by reproducing asexually.
      
   • Each offspring of a parent carries only half of that parent's genes.  Thus, for a given reproductive investment, a parent's genetic contribution could be doubled without sex.
      
2. The mundane reason is simply the logistics of sex.  
   • Sex is complex -- sex requires specialized body parts and behaviors.
   • Sex is risky -- sex involves behaviors that expose the participants to danger.
   • Sex is difficult -- sex requires effort and energy to find and win a mate.

There are several potential advantages for sex, discussed in the text.  However, none of the proposed advantages appears to be universal enough, or potent enough, to outweigh the disadvantages of sex, especially that huge 50% cost.  Thus there is an outstanding problem -- a big outstanding problem.

Sex is a case where optimality modelling powerfully demonstrates that, in the words of John Maynard Smith, "some crucial aspect of the problem has been overlooked."  The measurable cost for sex is huge.  The benefits are difficult to measure.  This makes it difficult to decide whether benefits are much greater than they appear or whether sex is protected by genetic constraints which prevent or slow down its loss.

Further discussion is posted on a separate page, Sex and Evolution.

Also see Chapter 9 discussion on for sexual selection.

• Outside link:  Evolution of Sex (BIOL597 course notes from Purdue University)
  • Lecture Notes
  • Discussion Questions

Notes for chapter 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17

304 index page

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