For sexually reproducing species, it's not enough just to find food and dodge predators. You also have to get a mate, because the prime directive, evolution-arily speaking, is to pass on your genes. How you go about choosing the best candidate to help you do just that is the topic of sexual selection.
Sexual selection refers to choosing certain characteristics over others when looking for a mate, and it's a subcategory of natural selection — the process by which heritable traits that promote survival become more common.
Sexual selection can lead to the evolution of some traits that, on the face of it, don't make much sense. It may favor a trait that increases an individual's ability to find a mate but decreases its ability to survive, for example. Although this
SjABCfl arrangement seems strange at first, it's perfectly reasonable if the trade-off is a good one — that is, if the fitness increase from increased mating is greater than the fitness loss from earlier death.
Sexual selection has two components: choice and competition. For the vast majority of cases, this means female choice and male-male competition. These are not mutually exclusive; you can see both in the same species and sometimes you even see the reverse (male choice and female-female competition). Also remember that sexual selection is found in lots of animal species from insects to mammals.
Some evolutionary biologists argue that sexual selection should be considered separately from natural selection, as the evolutionary changes produced are of a slightly different nature (flashy feathers as opposed to anything that's actually helpful for surviving, finding food, and so on.) But for our purposes here, I treat sexual selection as a subcategory of natural selection.
The Peacock's Tail: Sexual Selection and Female Choice
One kind of sexual selection system is one in which females choose their partners based on some outward characteristic. In these cases, the females' choice results in the males having some increasingly elaborate trait, even when that trait doesn't offer an obvious advantage in the male's survival. At face value, it's the classic example of choosing style over substance. This phenomenon has been puzzling evolutionary biologists since Darwin, because although it's obvious that females choose showy males, it's not entirely clear why.
In these species, evolutionary biologists figure that something else must be at play — something that makes it advantageous for the male (with the showy trait that serves no other purpose) and the female (whose choice of mate means that her offspring will end up with a trait that may actually be a disadvantage) to get together.
To understand this system, consider peacocks. If you've had the opportunity to see a peacock, you've no doubt been impressed by the size and beauty of his tail. The huge fan of iridescent green and blue feathers, which he displays so proudly, is unquestionably one of the most magnificent displays in the bird family.
That said, from a survival point of view, the fancy tail doesn't appear to have a lot of advantages. In fact, the tail seems to increase the chance that the peacock will be caught and eaten by predators. Peacocks don't fly especially gracefully, and the tail certainly makes it hard to hide. In addition, although the tail can't possibly make finding food any easier, the energy required to produce such an elaborate tail means that the peacock needs to find more.
In short, a showy tail doesn't seem to have any obvious fitness advantages, but it turns out that there is one, and it's a big one: Peahens like it! In fact, peahens like flashy tails so much that they preferentially choose to mate with males that have showier tails. Therefore, males with showier tails are much more likely to pass on their genes to the next generation than are males with less-showy tales. So having a fancy tail does indeed make a peacock more fit.
But what about the peahen? How does mating with the showy male increase her fitness? After all, she's deliberately choosing to mate with a very showy but otherwise less capable male. Although she's passing on her genes to the next generation (good from a fitness perspective), she's combining them with some genes that appear to make it more likely that her offspring will get eaten or starve (bad from a fitness perspective).
Two major ideas explain the curious choices that the females seem to make (and shallowness isn't one of them): the runaway-selection hypothesis and the good-genes hypothesis. The different mechanisms may be important in different systems.
It's not always the females that do the choosing. In some cases, the females display, and the males choose the females. These examples are very rare, however, so throughout this section, I concentrate on examples in which the female does the choosing. Just remember that in some cases, exactly the same phenomenon occurs, but with the roles of the sexes reversed.
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