SEXUAL SELECTION AND MATE CHOICE
Is there any evidence that females choose on the basis of “good genes”? (Exam Question 1993)
The “good gene” hypothesis is one of the main hypotheses regarding sexual selection by female mate choice. Sexual selection was first defined by Darwin as “the advantage which certain individuals have over others of the same sex and species, in exclusive relation to reproduction”. He devised sexual selection theory to account for the evolution of survival-decreasing traits, which nevertheless increase individual reproductive success by enabling individuals to acquire mates in competition with others. Sexual selection consists of two components, one arising when the members of one sex compete for mates (often called intrasexual selection) and the other occurring when members of the choosier sex determine which members of the other sex will have a chance to mate (often called intersexual selection). The good gene hypothesis is concerned with the latter or rather with the evolution of female preferences for male traits. First, I am going to outline the main components of the good gene hypothesis and then I am going to explore whether there is any evidence that females choose on the basis of “good genes”.
The idea that conspicuous, costly male traits become targets of female choice because such traits indicate high heritable viability and that after mating with a highly ornamented male, a female would then bear offspring that survive well, was suggested by Fisher (1915) as well as by Williams (1966). Nevertheless neither developed the idea any further and Williams even came to doubt whether the heritability of fitness is high enough for such a process to work, but their versions of this idea gave rise to what has been termed “good genes”, “handicap” or “indicator” models. Zahavani’s (1975) handicap principle popularised the notion that females prefer to mate with males who have demonstrated their superior genetic quality for survivorship. He suggested that females can evaluate a male’s survaival ability by assessing the magnitude of the handicap with which he is able to survive. There is no disagreement that many sexually dimorphic traits can increase male mortality, and Zahavani suggested that these handicaps to survival evolve as honest signals, allowing females to assess male genetic quality; thus, the term female choice for “good genes”. There are several variations of Zahavani’s original handicap proposal. In the original model, the handicap is fixed and it acts as a filter to survival. If a male has a large handicap and has survived, he is likely to have high intrinsic survival ability. Males without handicaps cannot be judged and thus are assumed to be ignored by females. A more plausible variant of the original model is the condition-dependent handicap. Here, the investment in the handicap varies with the male’s condition such that he is optimising the trade-off between mate attraction and survivorship to maximise his reproductive success. The good gene hypothesis relies on a genetic correlation between the preference and the male’s good genes, which is taken to mean some heritable component for viability. This actually is one of the main problems of the hypothesis as the heritability of total fitness seems not to have been measured accurately for any organism, neither in the lab nor in the wild. The heritability of traits related to fitness have, however, been studied in some laboratory and domestic species. Two conclusions are of particular interest in this case. First, although several components of fitness show considerable heritability, there may be negative genetic correlations between such components. Total fitness therefore need not necessarily show much heritability even if some fitness components do so. Second, the highest heritability is found in traits with probably little effect on fitness, such as tail length in mice. Life history traits more closely related to fitness, such as fecundity, have relatively lower heritability. Thus it is not clear whether fitness has high enough heritability for genetic handicap mechanisms to play an important role in sexual selection and will require further empirical studies.