Evolution by Sexual Selection
Aspect of Evolutionary Theory May Account for “Useless” Adornments
Jun 15, 2009
While most are familiar with the theory of evolution by natural selection, co-discovered by Charles Darwin and Alfred Russel Wallace, and first described by Darwin in his 1859 Origin of Species, fewer are aware of a subcategory of natural selection that Darwin also expended a great deal of ink upon: sexual selection, which was a topic of his 1871 book The Descent of Man. This aspect of natural selection refers to the evolution of traits that are particularly (and sometimes exclusively) utilized in attracting and competing for mates. Sexual selection goes a long way toward explaining biologically expensive features that are apparently useless or sometimes even detrimental to the animal bearing them.
Male Adornments as Sexual Attractants
Everywhere in the animal kingdom, competition for mates is fierce, and in the majority of species it is the males who must vie with one another to attract the attention of females. Methods for achieving this vary, but some of the best-known cases involve various types of birds. Many diverse bird species, including peacocks, cardinals, and birds of paradise, have plain females but boast males of extraordinary beauty, with brightly colored or patterned plumage, decorative arrangements of feathers, and so forth. Other species, in addition to or in place of bright colors, employ less visual markers like complicated songs, the building of bowers, and ritualized movements and dances. These traits and behaviors seem to have no purpose other than impressing females, and some of them can be downright hazardous; gaily colored feathers and loud birdsong not only attract females, but also predators. It appears, however, that the advantages outweigh the dangers.
Sexy Son Hypothesis or Genetic Fitness
There are two hypotheses concerning this type of selection, where females select males on the strength of their aesthetic qualities, though these hypotheses are not mutually exclusive. The so-called "sexy sons" hypothesis posits that females arbitrarily "decide" that certain traits are more attractive than others; they mate more with males bearing those traits, then pass on to their female offspring the same preference; male offspring inherit from their fathers the very traits that were so attractive in the first place. In this way, an arbitrary "fashion" becomes self-reinforcing, and in some cases can lead to truly spectacular follies, like the famed tail of the peacock, which is beautiful to the peahen (as well as to humans), but is generally burdensome to the male bird himself.
A second hypothesis states that the female's choice is not arbitrary at all, but caused by a recognition that flashy adaptations are a signal of genetic superiority; since many of these traits are biologically costly to produce and maintain, it is argued, males are in essence advertising that they are so robust and healthy that they can afford to "waste" precious resources on something that carries no significant survival value.
Male-On-Male Combat for Mates
A more violent side of sexual selection comes in the form of physical intimidation or actual combat between males in order to have access to mating opportunities. This aspect of sexual selection is less subtle than that described above; it is far more obvious to us that the moose with the biggest antlers, for example, will be able to intimidate or defeat rivals in large numbers, allowing him to win more couplings with females, and hence pass on more of his genes. Huge antlers are of course a burden to their bearers - they are heavy and may become entangled in tree branches, for example - but the seemingly maladaptive nature of the trait does not seem to preclude its being disseminated throughout a population, as the net increase in number of offspring more than makes up for any inconveniences caused by the features.
War Between the Sexes, or Cryptic Female Choice
A rather more insidious aspect of sexual selection, known as cryptic female choice, occurs when females mate with certain males, but then later dispose of their sperm without their knowledge. This strategy may allow the female to get rid of genetic material that is less than optimal, while still keeping males in the dark about which offspring they have sired. This trickery may either encourage male participation in rearing the young, or alternately prevent males from killing offspring they know are not their own.
Sources:
Ridley, Matt (2003). The Red Queen: Sex and the Evolution of Human Nature. Harper Perennial. ISBN 0060556579.
Judson, Olivia (2003). Dr. Tatiana's Sex Advice to All Creation. Holt Paperbacks. ISBN 0805063323.
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