There are two consequences of anisogamy, intrasexual selection and intersexual selection. Intrasexual selection refers to the suggestion that males will compete amongst each other to mate with females. This results in some males proving their fitness and health over others, and therefore those males that emerge as winners of this intrasexual competition are more likely to be selected by females for mating. Size dimorphism is evidence of intrasecual selection and refers to the difference in size between male and females.
Dimorphism is evidence of intrasexual selection and polygyny as it suggests that a makes fight for access to females. Dimorphism is not pronounced, which means polygyny is limited. Alpha males and sneak copulation are also evidence of intrasexual selection. The fact that males compete for dominance is supported by nonhuman animal hierarchies, which show that less dominant males engage in secretive copulation when the alpha male is unlikely to know about it. Sperm competition theory and the “testicular effect” proposed by Short who says that testicle size is related to the level of sperm competition.
After comparing testicle size of gorillas, chimps and humans, it was found that gorillas had the smallest testicles followed by humans and chimps. This supports Short’s theory as gorillas are polygynous creatures, as the alpha male heads a harem and so have little sperm competition, compared to chimps who are promiscuous and so experience greater sperm competition. This supports intrasexual selection, as it is evidence of competition between males. Intersexual selection refers to what males and females find attractive in the opposite sex.
There are a number of specific predictions follow illustrating intersexual selection. The female evolutionary strategy is to prefer males with signa of quality, commitment and resources, there show desirable traits to pass on in the genes. The male evolutionary strategy is to find females who are young and physically attractive as this shows good health and infertility. Men will place a high value on chastity or celibacy in their partner as females can be sure any child they produce is theirs and men lack this certainty.
This makes the males feel a need to control female sexuality. Such control increases the possibility that nay child the man rears is carrying his genes. Following on from this, males should protect their women by showing jealousy and should show greater concern about sexual infidelity since it reduces paternal certainty. Females should be more concerned emotional infidelity. Evidence for intersexual selection can be seen in studies by Buss, Davis and Clark and Hatfield.
Buss did a survey across 37 cultures and found that males rated youth and physical attractiveness, and that females rated resources. Davis’ study supports Buss’ as Davis found gender differences in qualities emphasized in personal adverrtisements. Women tended to emphasise their physical attrativeness and desire for a high-status man, whereas men tented to indicate their resources and their desire for a younger physically attractive partner. This led Davis to conclude that women were looking for “success objects” and men were looking for “sex objects”.