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Sexual Selection: Shaping Man and Society Every animal has the goal of finding a mate and reproducing. Humans are no exception to that general rule. Many animals, humans included, use sexual selection to determine their choice of mate. Sexual selection is, by definition, selecting for characteristics in the opposite sex, thereby bringing about modifications of those characteristics in that species (“Sexual selection”, 2005). Sexual selection is a form of natural selection, first proposed by Charles Darwin, and can bring about evolution of certain traits in a species. Sexual selection has affected human traits as well, bringing about dimorphic secondary sexual characteristics.

Intersexual Selection

Intersexual selection is the case where one sex (usually the female) chooses a mate (Herbers, 2014).Selection is driven most importantly by gamete size, which directly relates to investment in the offspring. Since females generally have the larger gamete they are the ones who are selecting for mates. Females generally have the most to lose if they select a wrong mate due to the fact that eggs are costly to the female (Trivers, 1972). Humans are a perfect example of how great the difference in parental investment is between males and females before the baby is even born. Women have to carry the baby for nine months, meaning they are unable to reproduce during that time, which reduces their overall fitness; while males are able to keep reproducing continuously. It is in the female’s best interest to be picky when selecting a mate (Trivers, 1972). For Intersexual selection the female choice hypothesis describes how females select a mate. There are four components to this hypothesis (none of which are mutually exclusive): direct benefits, good genes, runaway sexual selection, and sensory bias (Herbers, 2014).

Direct Benefits

Direct benefits involves the female selecting a male because he has benefited her in some way, such as he brought her food, built her a shelter, or provided her with protection (Herbers, 2014). These benefits can also include helping to raise the young, as it generally is with humans. Benefits of parental investment can be similar to the benefits the female receives, such as food and protection, but the male could also provide knowledge for the offspring and social status (Trivers, 1972).There are certain personality traits that females select for in a perspective mate when parental investment of both parents is high. Females will tend to select males that have predictable behaviors caring for offspring. With predictable behaviors the female can determine the amount of caring for the offspring the male is likely to contribute and adjust her effort accordingly. It would be very costly to the female if she had to constantly readjust her efforts to compensate for the male (Schuett & Tregenza, 2009).

Good Genes

There are certain phenotypic traits that translate to having good genes, for instance, the peacock’s eye spots translate to parasite resistance (Herbers, 2014). The major histocompatibility complex (MHC) plays an integral role in women being able to detect good genes in males. The MHC allows women to detect genetic diversity, which reduces inbreeding and increases immunity for the offspring (Penn & Potts, 1998). These MCH genes are detectable because they are fragrant genes, meaning they are able to bind peptides in the air and be translated though the process of olfaction (Milinski, 2006). This process is able to bypass conscious brain activity and be directly connected to the amygdala and hypothalamus, which are responsible for sexual behaviors (Penn, Potts, 1998). The sense of smell is a great indicator of gene quality because it is more truthful than physical appearance (Penn & Potts, 1998). Infection, for example, changes a person’s odor due to immunological or stress responses, and changes the composition of the person’s normal flora. People naturally secret odorants called copulins which attract mates. During infection the MHC will alter the coupulin chemical makeup so that it is no longer attractive (Penn & Potts, 1998). Infection will also cause immunological byproducts to be released which can increase the amount of “alarm” odors and decrease sex pheromones (Penn & Potts, 1998). Olfaction is a sexually dimorphic trait. Women are equipped with structures that allow for gene determination, while men are equipped with structures that allow for female detection. Though human olfaction is lacking compared to other animals, humans have more sent glands than any other primate. This suggests that chemical signals might have more of an effect on mate choice than previously thought (Penn & Potts, 1998).

Runaway Selection

Runaway selection is the case where females chose a trait for no apparent reason (i.e. there is no benefit of having the trait). There seem to be linked genes for having the trait and the preference of that triat (Herbers, 2014). Humans seem to have a runaway selection pattern of mate choice when selecting for physical and behavioral traits (Verweij et al., 2014). The study done by Verweij et al. was able to produce genetic correlations between a specific trait, such as hair color or religiousness, and a preference for that trait in the opposite sex (2014).  There have been recent studies conducted that have shown that people find specific traits attractive, and that these traits are under selective pressure. Since these attractive traits are being selected for they have the ability to shape our behavioral characteristics and physical appearance (Verweij et al., 2014). 

Intrasexual Selection

Intrasexual selection is all about keeping other males from the female or females of interest. Therefore it is reasonable to conclude that certain male characteristics would have developed through selection to help the males in procuring a mate through fending off other males.  Many male secondary sexual characteristics are thought to be derived from male-male competition for females. Secondary sexual characteristics are traits that are not directly linked to reproducing, such as vocal pitch. Men are thought to have acquired a deep voice by selection because it is a sign of dominance (Putts, 2010). Another male trait thought to be brought on by competition is facial hair. There is no other evolutionary advantage to facial hair other than the fact that it makes the jaw appear larger and more distinguished. This is a tactic used in male-male competition to show dominance (Putts, 2010).  Secondary sexual characteristics are not the only traits thought to be the byproduct of contests; body mass dimorphism is also thought to be evidence that male- male contests have shaped the physical form of people. Further evidence is the aggressiveness of men and boys compared to women.  While young girls are more likely to play with toys, young boys are interested in “play fighting” and rough housing. This type of behavior is seen across all cultures (Putts, 2010).

There is evidence of female- female intrasexual selection as well. However, females do not have the same type of intrasexual selection as males do. Males will use intimidation or violence to keep another male away from his mate, while females will make themselves more appealing to attract the attention of their mate from another female. The physical evidence that females obtained a mate through attraction rather than violence is that they have reduced body hair and high voices which resemble youth in the prehistoric age when these traits were evolving. The development of breasts and curvy hips also alludes to this attraction hypothesis. These fat stores are signals of fertility to men since they are needed for ovulation and a healthy pregnancy. These fat deposits are also developed during sexual maturity, furthering the hypothesis that they are ornamental to attract mates (Putts, 2010).

Culture

Sexual selection even plays a role in shaping human culture. There is evidence that male- male competition, which has molded aggressive behaviors in men, has influenced art, literature, and music. These media are now used as courtship display, instead of actually acting out the violence. Women, on the other hand, are benefitted more by not getting involved with large displays of courtship, as this could lead to unwanted attention. Sexual selection can account for some of the social dimorphism currently seen in societies (Miller, 1998).