Has evolution of great apes, including men, been influenced by sexual selection?
Sexual selection is a theory first proposed by Charles Darwin. He hypothesized that particular evolutionary traits are explained by intraspecific competition (a competition between members of the same species). According to Darwin it is a battle among individuals of one sex for the possession of the other sex. Nowadays scientists make a distinction between intrasexual selection- competition between males and intersexual selection- preference of mate choice.
Choosiness of mates is generally based on impressive morphology features (ornaments). These can be maintained due to preference for some features of mate morphology which may be increased by genetic drift, producing, in due course, selection for mates with the suitable ornaments. This runaway process has highlighted selection for aesthetic displays- also known as ‘sexy son' selection.
On the other hand, genes responsible for development of exaggerated morphological features or fighting capabilities can show better disease resistance or a more proficient metabolism. This idea emphasizes selection for genetic indicators and is known as good genes selection. In a case of female mammals the cost of internal fertilization, development and long-time lactation are very high, influencing variations among male competition and female preference, which is also why females are generally the ones who choose their mating partners.
The consequence of trade-offs of sexual selection as well as consequences of trade-offs between mating and parental efforts have led to sexual dimorphism. The most ancient and reproductively fundamental sexual dimorphism is males possessing tentacles and females possessing ova. Developed in later stages mating traits are female mammals with breasts and males retaining nipples.
Sexual selection can be observed in primate groups, which are usually formed of many males and females. It is intense because the social background of mating is complex and dynamic. Both sexes compete, select, and form dominance relationships and coalitions where sexual relations are being developed throughout weeks and years rather than minutes. Under these social circumstances, reproductive success became responsible for maintaining mental capacities for chimpanzee politics, social intelligence, friendships, and creative mating rather than plain physical ornaments and short-term mating processes as in most other animals.
It is, however, difficult to rebuild sexual selection outlines from extinct individuals since mate choosiness and mating behaviours do not fossilize. Nonetheless, it can be presumed that intense sexual selection within social packs of primates remained in hominid ancestors even though they formed bigger groups, and more complex relations and sexual strategies. Humans are the consequences of this primate inheritance refracted via a distinctive hominid sequence of habitation shared with the unpredictable outcomes of runaway social competition for social intelligence and runaway sexual selection for diverse mating behaviours.
Consequently, as in primates, social life in hominids was based on female groups with their offspring and the males trying to intimate with them. Under these circumstances, the crucial mating problem for males was to find mature, fertile females, whereas for females was to find good sperm and genes and possibly security from a few males. Hence, male hominids had to be rather secondary characters in human evolution, excluding only the ones that hold traits sexually-selected by females for their happiness or helpfulness.
Hominids probably did not live in isolated tribes. Instead, societies were presumably more complex as it is in other primates where mates could have been selected from larger assemblies. Social and sexual relationships were possibly as complex as they are nowadays, with a lot of polygamy, serial monogamy and disloyalty (Fisher 1992). With no marriages, mortgages, or money, why stay with only one mate through a lifetime? With this social structure each sex possibly developed plenty of approaches to pursue their mates, which can be compared to current societies.
A recent individual's approach may be influenced by personal qualities such as age, health or attractiveness, the social network status, or ecological and demographical circumstances.
Yet another reason to believe sexual selection influenced human evolution is K-selection. It typically decreases the energetic burden of reproduction on the female and almost removes the requirement for male help. Consequently, slow development increases female investment over a longer time, and small litters of well grown offspring are easier to take care of. Primates and particularly hominids belong to K-selected species. For this reason, they have slower gestation, bigger bodies, less offspring, greater survival rates, and longer lifespan than other taxa such as insects, fish, or rodents. With time hominids were becoming more K-selected, and sexual selection turned out to be more important and improved in comparison to natural selection.
Further, the genetic and non-genetic benefits from mating are influenced by the vulnerability and cost of infants. Selecting males for protection simplifies the energetic demands of maternity, but selecting males for their genetic quality and aesthetic displays decreases the risk of giving birth to unhealthy, unattractive offspring that might never reproduce.
Another aspect is the economic and geographic requirements of agriculture. These twisted human mate preference patterns, since agriculture needs long-term investment in maintaining the land and, hence, decreases the physical and social possibility to move which make choosing of sexual mates more difficult.
From a morphological point of view, human size dimorphism seems to be influenced by hominids evolving under a polygynous mating system where competition among males was greater than among females. Further, most of human physical traits such as face, breast, buttocks, penis and clitoris have most likely evolved under power of sexual selection and are possibly genetic indicators and aesthetic displays.
When considering development of human mentality, novelty and neophilia are the aspects that come to mind. Human nature continuously purses innovate ideas and emotions. For this reason, yet again, sexual selection alongside with mate choosiness might have influenced runaway sexual selection to form more complex and creative behavioural mating displays.
All in all, in species sexually reproducing, all genes must be transmitted via sex gateway, and mate preference is the warden of that gateway. Consequently, sexual reproduction was most likely fundamental part of human evolution, and is fundamental in human present days.
Further, sexual selection is, hence, another way of shaping organisms throughout the course of evolution. It may be helpful as well as harmful but nevertheless alongside with natural selection it is crucial to preserve life on Earth and make sure the Tree of Life is sustained well.
1. Fisher, H. E. (1992) Anatomy of love: The natural history of monogamy, adultery, and divorce. New York: Norton