Mate choice, or intersexual competition, is the non-random selection of sexual partners of one sex by the other sex. In most vertebrates, this is expressed as females choosing which males to mate with. Many mechanisms have been proposed for how and why mate choice might evolve. These mechanisms can generally divided into five categories, which are direct phenotypic effects, sensory bias, Fisherian, indicator mechanisms, and genetic compatibility mechanisms. Some of these mechanisms give direct fitness benefits to the chooser (direct phenotypic effects and sensory bias), others give indirect benefits to the chooser by benefiting their young (Fisherian and indicator mechanisms), and others may give other genetic benefits (genetic compatibility mechanisms). These mechanisms are not mutually exclusive. In fact, it is likely that several operate simultaneously in any give system. Each mechanism is described in more detail below.
Direct Phenotypic Effects are when females display a preference for a male ornament that gives the female some benefit. This benefit can be in the form of the male holding a superior territory, providing more or better food, increased protection from predators, or higher contribution to parental care of offspring. All these benefits are good for the female herself, directly. As such, signals that males can display that indicate that the male will provide these benefits can be preferred and selected for.
Sensory Bias is when mate choices are made on the basis of a preference that evolved for some other reason. For example, many animals have been shown to prefer specific colors. These colors are often associated with common or preferred food types. If a male develops the same color on his skin, fur, feathers, etc. females may tend to prefer him over other males that have not developed that color display. Biases could exist for other reasons (associated with habitat selection, predator avoidance, etc.), and also using other senses (scent, sound, etc.). The point is that the females sensory system already existed and was set up with a bias for certain displays that the males happened on and it then proved successful.
Fisherian (or Fisharian Runaway or Sexy Sons) is a process by which there is a genetic link between the display of a male and the preference for that display in the female. Once this link occurs (and it could occur through selection or genetic drift), females will be more likely to choose males that have that trait. They will then have sons that posses that trait and daughters that prefer it. In this way, the mate choice process becomes self reinforcing. This positive feedback between preferences and traits can drive the male trait to become quite extreme. The tail of a male Peacock is thought to be an example of Fisherian selection. This explanation of mate choice evolution was first developed by R. A. Fisher, a very famous evolutionary biologist, hence the name.
Indicator Mechanisms (or Good Genes or the Handicap Principle) is when a display trait in one sex has a selective disadvantage to the individual who has that trait. In this way, having that trait is a cost, and males who possesses such a trait are indicating that they are able to survive despite the added cost of the display trait. In other words, that they can succeed even with a handicap. These traits are also condition-dependent, meaning that the condition of the display trait is dependent on the condition of male himself. Females use these traits to determine which males have broadly high genetic quality. An important aspect of this mechanism of mate choice evolution is that it can favor males with a variety of genes because different males will be able to succeed in different way (some will be fast, some will strong, etc.). This idea of maintaining genetic variation is sometimes referred to as the genetic capture hypothesis.
Genetic Compatibility Mechanisms are ways in which one individual may choose a mate based on how the genes of that mate will interact with their own. For example, an individuals’ immune systems has been shown to mount a better defense against infections when the individual has a large amount of genetic diversity. This diversity generally comes in the form of being heterozygotic (having two different versions, or alleles, of a gene on each chromosome) at many loci. Females have been shown to prefer to mate with males who have fewer similar immune systems genes. Since the female and male immune system genes are so dissimilar they will tend to compliment each other, and this will increase the heterozygosity of their offspring.
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