Sunday, March 7, 2010

How Sex Drives Evolution

In our behavior unit, we learned about how animals had different types of courtship rituals to attract a mate. Are there other ways in which organisms attract mates of the opposite sex? Jerry Coyne mentions in page 144 of Why Evolution is True that sometimes species have adaptations that seem pointless in survival. For example, the male peacock, which has an extravagant display of tail plumage--which is detrimental in its ability to camouflage from predators as well as flight--is an example of a trait that clearly hurts the individuals survival. Despite this, creatures like peacocks and stag beetles continue to develop these disadvantageous traits. Give examples of organisms who exhibit sexual dimorphisms (differences between males and females of a species) and explain how these differences can give an evolutionary advantage. How does evolution explain how these seemingly disabling and detrimental traits actually help in survival of the species as a whole?


  1. While some of the sexual dimorphisms may seem only disadvantageous, they may help increase an individuals sexual selection, as Coyne defines as "...selection that increases an individuals chance of getting a mate." (148)According to Coyne, "Having a fancy tail or a seductive song doesn't help you survive, but it may increase your chances of having offspring-and that's how these flamboyant traits and behaviors arose." (148) So while one might speculate that traits such as the peacocks vibrant colors may be maladaptive, they actually increase the chances of reproduction by making themselves more attractive to females, thus increasing the chance of reproduction and passing along his genes. Another way that sexual dimorphisms can increase the chances of reproduction is during the direct competition between males. For example, on page 148 Coyne gives the example of male deer. "Deer with bigger antlers might struggle to survive under a metabolic burden, but perhaps they win jousting contests more frequently, thereby siring more offspring." So while these large antlers may seem disadvantageous, only consuming energy, they actually increase the chances of reproduction, and this may be why we continue to see large antlers in deer.

  2. First off, it is important to define sexual dimorphisms and speculate as to their role in evolution. As Coyne defines it, sexual dimorphisms are “traits that differ between males and females of a species” (146). Sexual dimorphisms come as a result of the differences in gender roles when it comes to reproduction. The assertion Coyne makes is that these sexual dimorphisms not only mark males and females as distinct from one another, but may be a waste of time and energy, thus reducing survival (146). He summarizes Darwin's findings with regard to sexual dimorphisms, noting that their “elaborate behaviors, structures, and ornaments are nearly always restricted to males” (148). The most obvious exception comes when the roles of males and females are switched (such as when males are the ones to house the developing embryo rather than females). Examples of this include seahorses and pipefish – the males in some of these species become pregnant and therefore invest more in reproduction than females. However, the basic principle remains unchanged – sexual dimorphisms still exist, allowing for the more promiscuous of the two sexes to develop seemingly useless features.

    This brings me to the main part of the idea of sex in evolution: sexual selection. The more promiscuous of the two sexes will develop certain features that may increase their probability of being selected by the other sex. This is perhaps one of the most unique and crucial forms of natural selection. Coyne describes sexual selection as having two parts: direct competition between males for access to females and female choosiness among possible mates (148). These are called intrasexual and intersexual selection, respectively, in our textbook. (Of course, this all assumes the standard gender roles of picky females and promiscuous males.) These two forms of sexual selection describe the sometimes overly flamboyant sexual dimorphisms, which, as our textbook states, the degree of which correlate with sexual selection. That is, in trying to win over the females so that their genes will be passed onto the next generation, males will develop features that may seem useless, yet somehow are appealing to the females, and are perceived by the females to be correlated with better overall genes in the male that they select. For females, reproduction is much more costly, given that they produce the egg and many house the developing embryo within their bodies in internally developing organisms, whereas males produce sperm that is intended to fuse with the egg inside the females in internally fertilizing organisms. Since they invest more than males, they do not develop these features (sexual dimorphisms) since they are not the ones to be competing; there are more male sperm to go around to a limited number of female eggs.

  3. (cont'd)
    Perhaps one of the most ingenious adaptations that result from sexual selection may be post-mating competition. Once the male has been selected, many tend to remain with the female for a while after copulation to ensure that other males do not interfere with their paternity. Coyne cites many examples of this, including male dragonflies who stay with their mates to physically block other male dragonflies from attempting to mate with the female, and the male Central American milllipede who rides the female after fertilization to prevent any competitors from mating. Males have also developed offensive weapons in trying to steal away potential mates to promote their genes being passed on to the next generation; when damselflies mate with a female that has just mated, backward pointing spines on the male's penis scoops out the sperm of the previous male mate. It is speculated that the human penis is shaped the way it is due to a similar kind of adaptation in an ancestral species.

    A tradeoff between survival and reproduction must be met; that is, deer antlers cannot continue to grow indefinitely. If bigger antlers are indicative of a better male, or one that a female would be more likely to choose, there is still a point where the antlers become so large as to prevent the deer from even moving its head and surviving to get to the point where he may reproduce. Finally, I would like to emphasize that evolution does not necessarily help a species to survive, since it acts on the individual level. Natural selection acts to allow the fittest organisms to reproduce. Adaptations that are better for survival or selected by the females in mating are the ones that are passed onto the next generation. Perhaps these do tend to translate to benefits for the species as a whole, but sexual dimorphisms do not necessarily help a species evolve as a whole, at least, not as much as natural selection based evolution does to promote species.