Lecture © Genaro R.
BIOL 606 Session, University of Alberta, March 8, 2001
Sexual selection has been defined as a special subset of natural selection (Brown, 1975), or as a type of natural selection in which reproductive success among individuals is determined by the way in which mating occurs (Valentine, 2001). Darwin was the first one to characterized sexual selection. In the "Descent of man and selection in relation sex" (Darwin, 1871), he distinguished between two processes involved in sexual selection: competition for mates among members of the same sex, and the sexual selective process or mate choice. Darwin suggested that sexual selection can favor the evolution of traits that increase the attractiveness of individuals to members of the other sex. According to him, sexual selection arises from differences in reproductive success caused by competition over mates. Females are the limiting sex, and females invest more in offspring than males, so that males tend to be competing for females. Sexual selection results in morphological distinction of the sexes, or sexual dimorphism. Favorable hereditary characters or traits are perpetuated only if reproduction is successful. However, Darwin never proposed how female preferences evolved.
Fisher (1915, 1930) proposed a new hypothesis, the "runaway sexual selection" hypothesis. This hypothesis proposes that females develop a preference for some male trait (without regard to fitness) and then mate with these males. The offspring of these matings will therefore have the genes for both the trait and the preference for the trait. An alternative to Fisher's hypothesis is known as the 'good genes hypothesis" (Zahavi, 1975). The "good genes" hypothesis states that the display indicates some component of male fitness. In this context, bright coloring or long tails in male birds indicates a lack of parasites, and these characters serve as a signal to females of some other important trait (ex. parasite load) indicating that this particular male has "good genes. In both of these hypotheses, which are not mutually exclusive, it is predicted that female mating preference will be correlated with the male trait. In the "runaway" hypothesis, the genes for the trait and preference for the trait are, or become, linked, while in the "good genes" hypothesis the trait is a signal for some other, underlying beneficial traits (Anderson, 1994).
Female mate choice has been assessed by functional and mechanistic studies (Krebs and Davies, 1997). Functional studies deal with behavioral interactions that cause mate choice and how this generates selection of male traits and female preferences. Mechanistic studies attempt to identify the physiological processes underlying mating preferences, such as neural mechanisms and/or cognitive processes that guide female preferences. These mechanistic studies also have an historical component. The historical approach acknowledges that extant traits and preferences have experienced a long history of selection and constraints that influence their current expression. The interaction of selection forces and constraints that influence the performance of the sensory system in different situations has been named sensory traps (West-Eberhard, 1979), or sensory bias (Endler, 1992). These sensory traps or bias recognize that the variety of factors that influence sensory perceptions implicated in mate choice, or in other functions, might be influenced by sensory biases that evolved in other contexts (Kirkpatrick, 1982). A specific hypothesis arose from this idea, where the males evolve traits to exploit such pre-existing biases or preferences (sensory exploitation; Ryan, 1990). The sensory exploitation hypothesis assumes that the evolution of sexually selected traits is influenced by these pre-existing biases. This assumption generates a prediction about the historical pattern of trait-preference evolution where preferences evolved prior to sexually selected traits. This approach is entirely different from that of the good gene and runaway hypotheses in which the preference and trait evolve in concert.
Class discussion focused on the significance of the runaway, good genes and sensory bias hypotheses and how they are related to female mate choice. Different points were brought up based on the preference for the white color and crests in Australian grassfinches (focal paper). Is the preference for crested males relevant in a group that lacks this character? Is this evidence of a pre-existing sensory bias? The focal paper tried to illustrate how a preference in two different species that lack it may be used as evidence of a preexisting bias towards crested males in grassfinches. However, the experimental design of the paper wasn't really strong enough to accept the conclusions and remarks of these authors. Therefore, discussion of the role of sensory bias was focused on the how pre-existing biases can occur and how they can actually evolve. Simultaneous evolution of traits and preferences for those traits (in the sense of the runaway hypothesis) seems to be obvious and intuitive. However, this hypothesis appears to be against preferences of females for unique or rare characters due either to preexisting biases or to "good gene" signals. Selection of unique characters may favor male fitness at a certain point (rare male preference) but for others selection of this type of character may be detrimental (selection against the rare). Other concerns such as the strength of the evidence for pre-existing biases and the value of them were mentioned but these topics remain controversial.
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