ZOOL 304
Class Notes
Chapter 9
Text reading: Chapter 9.
- No written assignment.
Discussion:
Understanding sexual selection involves addressing two more-or-less distinct questions.
- Why are there (usually) two distinct sexes, male and female, in which two sexes differ in the relative size, mobility, and resource supply of the gametes that each produces?
This question is answered, in principle, by the instability of isogamy with resultant disruptive frequency-dependent selection for anisogamy . See Chapter 7, pp. 142-143, for a basic discussion. So-called "primary" sexual characteristics are those sexually dimorphic traits which are directly related to the production of differentiated eggs or sperm. "Ecological" sex differences emerge if males and females adopt different niches (e.g., blood and nectar feeding by female and male mosquitos) suited to their different reproductive needs. These ecological sex differences are shaped by ordinary natural selection rather than sexual selection.
- How does the existence of distinct sexes lead to secondary sexual dimorphism, with often elaborate and exaggerated traits?
This question is the subject of this chapter. The ways in which selection acts to shape secondary sexual characteristics is called sexual selection.
Sexual selection involves two basic types of interaction (p. 179):
- Same-sex interactions, in the form of competition for access to mates.
- Opposite-sex interactions, in the form of mate choice.
Competition for access to mates can be fairly simple, without any meaningful choice by the opposite sex. Members of the competing sex (usually male) simply fight among themselves; the winners mate and losers don't. The typical effect is selection favoring larger size and larger "weapons", limited only by tradeoffs as increasing the size of these features interferes with other traits that are also needed for survival and reproductive success.
Mate choice can involve rather more subtle effects, especially if it occurs (as it often does) in combination with competition. Members of the "choosy" sex (usually female) should prefer mates who offer the prospect of higher quality (more fit) offspring.
Several ideas are associated with mate choice. Note that these ideas are not mutually exclusive. More than one may be involved, in some degree, in any example of mate choice.
- Direct benefit. In some species, males provide a resource (territory, food, care of offspring) that directly supports and improves female fitness, and female choice is rather simply based on the quality of this resource.
- Sensory bias. All sexually-reproducing species necessarily have some signalling mechanisms which enable individuals to find and identify opposite-sex members of the same species. An exaggerated sensory preference can then drive selection of an exaggerated signal.
In a wide variety of animals, the neural mechanisms for recognizing meaningful signals can often be "fooled" by exaggerated or "supranormal" signals. In fact, experimental manipulation has repeatedly demonstrated that females may prefer trait forms far more extreme that those presented by actual males. (This is unsurprising, given that evolved instincts simply need to work; they need not be intelligently farsighted). Such experiments demonstrate the basis for sensory bias that can lead to evolution of an exaggerated signal.
- "Sexy sons". If a sensory bias leads females to prefer mates with an exaggerated trait, then females who choose mates with the most exaggerated trait are more likely to have sons who will themselves be preferred and have greater success at mating.
This simple fact creates selection upon females to prefer the most exaggerated trait, thus exaggerating the initial sensory bias. And this, in turn, can drive further exaggeration of the preferred trait. The result, as Fisher realized, can be "runaway sexual selection" in which both trait and preference become more and more exaggerated until the process is halted by some counteracting tradeoff (or by extinction).
- "Good genes". Obviously, it would be advantageous for females (or, whichever sex is "choosy") to mate preferentially with individuals having the "best" (most fit) genes. The question, then, is how might such a choice be made? Two possibilities have been proposed. (We presume that direct assessment of "good genes" is not practical, but that mate choice must be based on some more-or-less simple trait that can more-or-less accurately represent overall genetic quality.)
"Handicap". A trait which imposes a substantial and visible handicap, one which is costly to grow and maintain, demonstrates (advertises) that the bearer is strong enough and capable of accumulating sufficient resources to do so (rather like a flashy car or a big roll of cash). The more extreme the trait (i.e., the bigger handicap), the higher quality is implied for all the functional genes which enable its growth and maintenance.
"Parasite indicator". Like handicaps, which may provide a reliable indicatory of overall genetic quality, any trait which is reliably and noticably altered by the presence of parasites can provide an indicator of genetic ability to resist parasitism.
Some additional discussion is posted on a separate page, Sex and Evolution.
Notes for chapter 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17
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