Sexual selection theory has been revived over last two decades through combined efforts of researchers. Such revival involves theoretical population genetics, experimental behavioural biology, primatology, evolutionary anthropology and evolutionary psychology.
Presently natural selection theory serves as conceptual and rhetorical foundation for evolutionary psychology (vide Tooby and Cosmides, 1992). On the other hand, sexual selection theory seems to guide more actual day-to-day research.
Darwin distinguished between male competition for female mates, (which typically gives rise to weapons) and female choice of male mates (which typically gives rise to gifts and ornaments) in sexual selection process.
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He recognized that female choice and male competition are often two sides of same coin, because mate choice by one sex, usually implies competition by other sex, either through direct “interference competition” (e.g., physical fights over opposite sex) or through indirect “exploitation competition” (e.g., scrambles to find and seduce the opposite sex before someone else does).
Darwin had no real explanation of why males usually compete harder for mates than females do, why male’s court, and female choose, though he offered a staggering amount of evidence that this pattern holds from insects through human. After Darwin, sexual selection received such a frosty reception from Wallace and others that it was virtually forgotten (Cronin, 1991).
Modern synthesis of Mendelian genetics and Darwinism in 1930s, viewed male competition as a sub-class of natural selection, while continuing to reject female choice. Sexual ornaments were assumed to intimidate other males or were “species recognition weakness” to help animals avoid cross species mating (Cott, 1940).
Perhaps the best recent reviews of sexual selection have done by Andersson (1994) and Cronin (1991). Accessible introductions to human sexual selection include works of a number of scientists. Champions of Zahavi’s (1975) handicap principle have emphasized selection for genetic indicators also called “good genes”, “good sense” or “healthy offspring” selection.
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Champion of R.A. Fisher’s (1930) runaway process has emphasized selection for aesthetic displays also called “good taste” or “sexy son” selection. These different mate choice criteria are often considered competing models of how sexual selection works but there is now sufficient evidence for each (vide Andersson, 1994) that they can be considered well established, often complementary selective forces.
Mate choice favours many other important qualities including parental ability and resources, fertility (vide Singh, 1993), optimal genetic distance and similarity in appearance, behaviour and personality (vide Rushton, 1989).
Origin of Mate Choice Mechanisms:
Mate choice is the behavioural outcome of mate preferences. Such preferences are usually “mental adaptations” implemented as complex neural circuits and constructed through interaction between genes and environment. Mostly, such systems function without conscious awareness, deliberation or complex aesthetic feelings.
Mate choice mechanism operates by rejecting some potential mates and accepting or soliciting others. Although sexual harassments of females are common in nature, “Successful rape” seems fairly rare, being reported in only flucks, squid, dolphins, orangutans and humans (Thornhill and Thornhill, 1992).
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Mate choice evolves based on rationale that random mating is stupid mating. Such choice also attempts to select the genetic quality of male. Ugly, unhealthy mates yield ugly, unhealthy offspring. By forming a joint genetic venture with an attractive, high quality mate, one’s genes are much more likely to be passed on.
Even modern women who deny “role of genes in human behaviour” tend to choose their sperm donors quite carefully (vide Scheib, 1994). Mate choice mechanisms may evolve through direct selection for mate choice efficiency (i.e. better preferences lead to more or better offspring) and through linkage with another trait that is undergoing natural selection or genetic drill.
The last three processes generally produce harmful changes in mate choice mechanisms. Unpredictability of such processes is important in explaining diversity of sexually selected ornaments across similar closely related species (Miller and Todd, 1995).
Selection for Indicators:
Probably the most fundamental form of sexual selection is mate choice for various “indicators” of viability and fertility. Health, nutritional status, size, strength, social status, disease resistance or overall vigour can function as an indicator.
Three processes can maintain heritable variation viz., temporal variation in selection, spatial variation in selection and mutation pressure. Temporally varying selection may result from co-evolution between ecological competitors, between predators and prey and perhaps most importantly between hosts and parasites (Low, 1990). Spatially varying selection in various geographic areas, combined with migration, can maintain heritable variation in a population.
Mutation pressure also plays role in maintaining heritable fitness variation because most mutations are harmful and give rise to an excess of low-fitness individuals (Rice, 1988). Studies suggest that sexually selected traits have much higher heritabilities and genetic variances than naturally selected traits, despite strong directional selection (Pomiankowski, 1995).
Significance of heritable fitness variation is also confirmed by experiments in which females that are allowed to choose their mates have offspring with higher phenotypic quality than females not allowed choosing (Reynolds and Gross, 1992).
Selection for Sperm Competition:
Gonads and genitals are clear expressions of sexual selection. They are most directly related to fertilization but serve no survival function. That primary sexual characters, such as penises are necessary for breeding and hence are favoured by natural selection is misleading (Andersson, 1994).
Females of many species mate with more than one male. In such cases, males evolve larger testicles, larger ejaculates, and faster swimming sperm (Smith, 1984). Male North Atlantic right whale has 2000 pound testicles to pump out gallon of semen. In primates, testicle size increases with intensity of sperm competition across species (Harcourt and Harvey, 1984).
Female chimpanzees are highly promiscuous. So, male chimpanzees have evolved large 4-ounce testicles. Male genitals often function as “internal courtship devices” to stimulate females into accepting sperm from copulating male. Human female orgasm may function partially to suck sperm into uterus (Baker and Bellis, 1995).
Sex Differences and Sexual Selection:
Trivers (1972) pointed out that females invest more matter and energy into producing each egg than males invest in producing each sperm. Eggs from a limiting resource for males than sperm do for females.
Thus, male should compete more intensively to fertilize eggs than female do to acquire sperm, while females should be choosier than males. Males compete for quantify of females and females compete for quality of males. In short, males court and female choose (vide Trivers, 1995). A man’s reproductive success generally increases with his number of sexual partners in absence of contraception.
A woman reaches her reproductive limit rather quickly as her number of sexual partner’s increases. Women under ancestral conditions probably used abortion and infanticide to avoid maternal investment during difficult times (vide Hausfater and Hrdy, 1984) but they could not induce another woman to bear a child for them. Maternal investment was obligatory in hominids, paternal investment was not.
Sexual Selection in Primates:
Sexual selection in multi-male, multi-female primate groups is intense because social context of mating is complex and dynamic. Sexes compete, both sexes are choosy; both sexes have dominant relations and both sexes form alliances. Sexual relationships develop over weeks and years rather than minutes.
Under these relentlessly social conditions, reproductive success came to depend on mental capacity for “Chimpanzee politics” (De waal, 1989), “Machiavellian intelligence” (Byrne and Whiten, 1988), “special friendships” (Smuts, 1985) and creative courtship (Miller, 1993) rather than simple physical ornaments and short term courtship behaviours as in most other animals.
Primates and especially hominids are extremely “K-selected taxa” having slower development, larger bodies, fewer offspring, higher survival rates and longer lifespans than more “r-selected taxa” such as insects, fish or rodents (Harvey, Martian and Glutton- Brock, 1996). The more K-selected the species, the more important sexual selection usually becomes compared to natural selection (Miller and Todd, 1995).
K- Selection usually reduces relative energetic demand of reproduction on female and almost eliminates need for male help, because slow gestation spreads maternal investment over a longer period and small litters of large, well-developed offspring are easier to care for.