Dimorphism (the difference between the sexes) arises because of intrasexual selection. This is because of intrasexual competition. If males have to compete, they need to be able to fight and this tends to result in larger males who have antlers for fighting or large tail feathers to attract females.
Alpha males and sneak copulation are evidence of intrasexual selection. Nonhuman animal hierarchies support the fact that males compete for dominance, which shows that less dominant males engage in secret copulation when the alpha male is unlikely to detect it.
Sperm competition theory and the testicular effect also support that there is an evolutionary explanation of human reproductive behaviour. Short (1991) proposes that testicle size is related to the level of sperm competition. Comparing the testicle size of chimps, humans, and gorillas supported this. It was found that gorillas has the smallest testicles, followed by humans and then chimps. This was consistent with Short’s theory as gorillas are polygynous, where the alpha male heads a harem and so have little competition, whereas chimps are promiscuous and so experience greater sperm competition. This supports intrasexual selection because it is evidence of competition between males.
Intersexual selection is when the females do the choosing and their selection is accounted for by 2 hypotheses; Fisher’s runaway hypothesis, and Zahavi’s handicap hypothesis.
Fisher (1930) proposed that females are attracted to those features of males that have survival value e.g. a bird who has a fairy long tail maybe better at flying and so at finding food. A female will prefer a male with a long tail because her offspring are likely to inherit this characteristic. As this carries on, the characteristic becomes more and more exaggerated and this is then referred to as the ‘runaway hypothesis’, also known as the ‘sexy sons’ hypothesis because the female’s choice is based on what will benefit her offspring and their survival. A significant problem with this theory however, is that the evolutionary advantage of characteristics being enhanced comes into question when the characteristic becomes so exaggerated that it starts to put the animal at a disadvantage, i.e. a stag’s antlers being too big and too heavy for the stag to be able to function normally.
On the other hand, contradictory to the above point, a male with a handicap can be seen as another evolutionary advantage. A male adornment such as a long tail has developed because females find it attractive. The handicap hypothesis states that many of these male adornments are a handicap to survival, but as they still survive, they must be fitter (good genes theory). Support for this theory has been found with Hamilton & Zuk (1982) who believe that males only acquire the adornment (and survive) if they are in good health. Those of poor health would find it an extra drain and not survive.
Further evidence to support the runaway hypothesis is the study by Moller (1990) on swallows. Moller found long-tailed birds are favoured because they are resistant to mites which means the female swallow’s offspring will be more likely to be resistant to mites as well which Is better for the species.
The female may also base selection on material resources as opposed to genetic resources. This also makes sense in terms of eliciting care for her offspring but such characteristics are not inheritable e.g. in bullfrogs the strongest males are the ones that get the best pond sites, and the females choose the males with the best pond sites.
Buss (1989) conducted a self-report study across 37 different cultures and found that males rated youth and physical attractiveness (indicators of health and child bearing potential) as more important in a potential mate than females did and that females rated resources more highly than males did. This provides some support for evolutionary explanations as the stability and universality of the mate choice preferences may be evidence of a genetic basis.
However, Buss’s research can be criticised, as it is subject to the researcher and participant effects that are weaknesses of self-report. Thus, Ps may not answer truthfully due to evaluation apprehension or due to the social desirability effect. Furthermore, reported preferences are not necessarily representative of behaviour and so the validity of the research is questionable.
Nonetheless, Personal advertisements support Buss’s findings as Davis (1990) found gender differences in the qualities emphasised in the ads that were consistent with parental investment and sexual selection explanations which puts strength behind it. The result of Davis’s study led him to conclude that women were looking for ‘success objects’ and men were looking for ‘sex objects’.
Clark & Hatfield (1989) also supports intersexual selection as it provides evidence that females are more discerning than males. Furthermore, Females’ promiscuity also supports Zahavi’s hypothesis of ‘good sense’. According to Brown (2000) women are genetically programmed to seek out variety as this increases reproductive potential.
On the other hand, Strassberg’s (1999) research on Internet dating contradicts the traditional sexual stereotypes. Fictitious ads were placed which advertised a woman as ‘very attractive’, ‘passionate and sensitive’, or ‘financially successful and ambitious’. The latter received more replies than the physically attractive condition and this challenges evolutionary explanations- if mate choices were coded in the genes then they should be relatively stable and universal, but this shows that they’re not.
Trivers’ parental investment theory (1972) claimed that differences in parental investment determine who will be the more discerning or ‘choosey’ sex when selecting a partner. Parental investment takes many forms: nutrients contained in the eggs, retaining the eggs in the body, food via the placenta, shelter, feeding, defending etc. In the majority, females invest more e.g. in female elephant seals are pregnant for several months before giving birth to a pup that may weigh up to 8 stone. However, in some cases, the males invest more e.g. in three spined stickleback fish. Trivers’ key notion was that the sex having more parental investment would tend to be more selective when mating than would the other sex. In humans, joint parental investment is necessary because of the long time it takes to pass through childhood.
Evolutionary theories lack empirical support, as they are post hoc i.e. proposed after the event, we do not know if they are true and so all explanations are inferences only. Also the evolutionary concept of the gene is criticised by geneticists, as complex behaviours are not coded onto our genes in the way suggested by evolutionists. The Genome project has shown that traits are a consequence of many interlinked genes, which may be inconsistent with evolutionary accounts of human reproductive behaviour.
An undeniable weakness of all the evidence for evolutionary explanations is that as most of the research is based on animals, not only is extrapolation difficult, but also basing the control of behaviour solely on genes the theories completely ignore the free will of the individual.
