While natural selection has been accepted as a valid explanation for evolutionary changes, sexual selection explains how the behavior between different sexes of various species affects the behavior of the opposite sex. Sexual selection is the selective force that applies the “characteristics that provide individuals with advantages in gaining access to mates,” (Workman & Reader, 2021, p. 59). Men from different cultures find women with certain unique features, such as a given shape, attractive and hence suitable as mating partners. Competition for mates may take the intrasexual form or the intersexual form. Intrasexual selection occurs in the form of members of the same sex competing with each other for access to the opposite sex. Intersexual selection, on the other hand, consists of members of one sex attempting to impress members of the opposite sex to lure them to accept mating. In humans, females prefer men with resources and the ability to acquire resources since these would be necessary for their children’s survival (Buss, 2019, pp. 153–154). According to Møller et al. (1998), female swallows prefer males with longer tail feathers, thus proving the adornment selection theory. The two selection forms lead to different adaptations as members adopt those attributes that make them more competitive while the opposite sex seeks such features in their mating selection. This paper reviews relevant theories of sexual selection to make a holistic view of their concurrence and divergence.
Ronald Fisher (1930; cited by Workman & Reader, 2021) explained sexual selection from the perspective of the need for survival. According to Fisher (1930), females’ need for survival would explain the relationship between female choice and male adornment. Logically, females’ need for survival would drive them to look for males with well-maintained tail feathers, which would signify their ability to fly high and forage, all of which enhanced their survival abilities. Norberg (1994) found a functional explanation of the long tail streamer in swallows as it allowed them to navigate tunnels better than short tails, thus associating long tails with survival superiority. However, according to Fisher (1930), once selected for survival, the features would continue attracting females for mating, thus exaggerating their presence in the next generation. Offsprings of such genetically endowed breeders would have even more pronounced features of the desired qualities, and the cycle would continue until the cost of such features exceeded their benefits. An example of such a scenario is where conspicuous bright colors in male birds attracted predators so much that such bright colors lost their value due to their inability to serve their survival needs.
Parental investment theory elaborates on the female choice theory of sexual selection. Female choice theory posits that females choose the males with the desirable traits for mating. Nevertheless, why is it the females choosing? Parental investment theory explains this through asymmetry between the sexes in raising the offspring (Bjorklund & Shackelford, 1999). While males only need to donate their sperm to it, females bear the burden of carrying the offspring during the gestation period, which is more tasking.
Furthermore, upon delivery, females spend more time with the offspring and perform more tasks caring for the offspring than males, such as breastfeeding. Parental investment theory, therefore, associates this great effort by females with their strict choice of mating partners due to their great investment in their offspring compared to male investment. Therefore, females are more invested in choosing their mating partners who have the desired traits so that the investment meets their desires. Failure to exercise due diligence in selecting a suitable mate would cause the female more cost in the burden of raising a less-desirable offspring. Female choice theory, therefore, reinforces the intersexual theory that females select mating partners and the intrasexual theory that males will seek to develop attractive features to attract females for mating.
Zahavi (1975; cited by Workman & Reader, 2021) developed the handicap hypothesis to explain male adornment for sexual selection. According to this hypothesis, males developed ornaments to project an image of survival capabilities despite being handicapped by their conspicuous adornments. The hypothesis constructs that, for such males to survive, even when they are prone to attacks due to their ability to attract predators, they must be good at surviving. Females would, therefore, see this ability to survive and connect it to good genetics and select such males for mating. In line with intrasexual selection theory, the handicap hypothesis posits that males compete by developing adornments that will portray them as prone to predator attacks and, therefore, good at surviving. The hypothesis connects the intrasexual with the intersexual theory in that females would then select the males with adornments as they would perceive them as having superior survival genes.
Parasite theory argues that “male adornments developed to demonstrate to females that they are free from parasites,” (Workman & Reader, 2021, p. 65). According to this theory, parasites account for a larger proportion of fatalities than predators, and therefore, males with healthy genes that can defeat parasites should be chosen so that they pass on healthy genes to their offspring. Therefore, if males with the least parasites develop ornaments, and females choose such males for mating, then they should pass disease resistance to the offspring. While the handicap theory presents adornments as a handicap, the parasite theory presents the same as a display of male health. The parasite theory projects healthy genes as the source of ornaments in males, whereas Fisher’s theory associates male ornaments to a female selection of beauty, thus evolving to a generation of beautiful offspring.
The intrasexual and intersexual selection theories explain species adaptation to acquire desirable features for mating. Intrasexual selection theories attribute male competition and adoption of adornments to the desire to attract females. Intersexual selection theories posit that females make choices of whom to mate with and, therefore, look for superior genes in selecting mating partners. Intermediating theories explain the various adaptations males take to acquire and project an image of superiority to lure females to mating. Some common attributes associated with gene superiority are ornaments and large tail feathers, which males project as a marker of superiority. The continuous selection of such males for mating explains the evolution of birds like a peacock and swallows to adopt ornaments and long tail feathers as markers of superior genetic makeup, thus attracting inter-generation mating selection.
References
Bjorklund, D. F., & Shackelford, T. K. (1999). Differences in Parental Investment Contribute to Important Differences Between Men and Women. Current Directions in Psychological Science, 8(3), 86–89. https://doi.org/10.1111/1467-8721.00020
Buss, D. M. (2019). Evolutionary psychology : the new science of the mind (6th ed., pp. 153–154). Routledge.
Møller, A. P., Barbosa, A., Cuervo, J. J., Lope, F. de, Merino, S., & Saino, N. (1998). Sexual selection and tail streamers in the barn swallow. Proceedings of the Royal Society of London. Series B: Biological Sciences, 265(1394), 409–414. https://doi.org/10.1098/rspb.1998.0309
Norberg, R. Å. (1994). Swallow tail streamer is a mechanical device for self-deflection of tail leading edge, enhancing aerodynamic efficiency and flight manoeuvrability. Proceedings of the Royal Society of London. Series B: Biological Sciences, 257(1350), 227–233. https://doi.org/10.1098/rspb.1994.0119
Workman, L., & Reader, W. (2021). Evolutionary psychology an introduction (p. 59). Cambridge, United Kingdom New York, NY Cambridge University Press.
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