Sexual Selection

Sexual selection is considered the second most important theory suggested by Charles Darwin in 1871. Sexual selection explains the selection of traits that are not adaptive, that is they do not enhance the survival chances of the individual but are yet selected. These non-adaptive, sexual traits can only be selected sexually and it can benefit the bearer by increasing the individual's reproductive success, even at the expense of their survival. 

This concept of selection of such sexually evolved traits might seem contradictory to the theory of natural selection but explains the reason for the selection of traits like the feather of peacock, which do not provide any help in survival are selected and evolved during reproduction. This article focuses on the understanding of sexual selection, types of sexual selection, intrasexual selection intersexual selection, defining the role of parental care and, the operation of sexual selection.

What is Sexual Selection?

It is important to define sexual selection according to Charles Darwin, sexual selection definition is “the advantage which certain individuals have over other individuals of the same sex because of the reproductive trait”. In simpler words according to sexual selection definition, it is a phenomenon of selection of traits that is not directly involved in survival of the organism and usually reduces the survival fitness. But are selected because of sexual reproduction. Traits that are selected by sexual selection decrease the overall survival fitness of the organism but increase reproductive success. These traits are nonadaptive, sexual traits.  Sexual selection operates by two mechanisms which are intersexual selection and intrasexual selection. Darwin sexual selection provides insight into female choice and intrasexual selection competition.

Recognition of Sexual Selection

The sexual selection definition came into recognition to explain the selection of nonadaptive sexually favoured traits. Darwin suggested that this works by two mechanisms one being intrasexual competition, which can be defined as competition between the same sex, to gain access to the mating opportunity. The second mechanism being the female choice or intersexual competition. The initial idea of female choice was not well supported in the scientific community with the claims that organisms showing female choice other than humans were less evolved. It was only after 80 years in 1991 this theory got accepted after the extensive work of Cronin. 

Examples of Sexually Selected Traits

There are the following examples of sexually selected traits, it is important to understand that these traits evolve due to intrasexual selection, the evolution of male armaments that provide individuals with an advantage when fighting off potential competitors are the best-suited examples of such trait. Some of the examples of such ornamental traits are as follows-

1. Antlers in deers

2. Feathers of peacock

3. Comb of chicken

4. High body weight of male

5. Fighting capability among males.

Sexual Selection in Male and Females of the Same Species

The role of the sexes can be defined on the basis of the gamete type they produce, female produce large nonmotile and singular gamete whereas the male gamete is large in number hand have high mobility. This difference between the number of gametes produced by both species leads to the asymmetry as only one egg and one sperm is required for successful fertilization. This concept of asymmetry was explained by Bateman's principle.

According to it the limitation of access to resources for nourishing these large female gametes leads to the limitation in female reproduction. The male counterpart is abundant in number but there is a relatively low abundance of female gametes thus creating a reproduction limitation for the male. 

The effects of high sexual selection among sexes can be defined by the variance in the traits The higher the reproductive variance, leads to the stronger sexual selection on that sex. Variance can be explained by an example, if there is a disproportionate share of reproduction among males, the male with highest reproductive success will have the highest variance, thus this trait is strongly sexually selected compared to other sex. 

As for females due to their limited number of gametes, the reproductive success of that female will have low variance thus low sexual selection. It is important to note that high sexual selection according to the sexual selection definition, will lead to exaggerated dimorphic traits in the sex with the highest variance. 

Role of Parental Care

The variance in a given population is not only determined by the availability of the gametes, but also by the parental care provided by the organism. For example, in the majority of the population, the female parent provides the nurturing to the offspring, during this time they can not mate thus reducing their reproductive success which in turn will reduce their variance. As described earlier the degree of sexual selection in the population is directly proportional to the variance. 

In a population that follows biparental care, both the parents are involved in the parental care of the offspring thus innately resulting in low reproductive success during this period, which in turn will lead to low variance. It is important to note that such a population results in the monomorphic phenotype between both males and females of the population. In this kind of population sexual selection is equal in both sexes.

A rare condition is also found in nature where the female parent produces less parental care to the offspring than the male. This condition also results in the sexually dimorphic population with variance among the female sex. In this condition, males might display higher intersexual selection.

Types of Sexual Selection

Types of sexual selection can be categorized into - intrasexual selection and intersexual selection. To understand these terms it is important to have an understanding of the sexual selection definition of biology, according to which traits that compromise survival fitness but enhance reproductive fitness are selected. Sexual selection works by the types of sexual selection that prevailed in nature. 

Intrasexual selection- According to the intrasexual definition it refers to the meaning of in between the same sex. Intrasexual competition is the competition between the same sex, it is seen usually among the males of the population. The competition might be to gain the following, status, tangible resources, or direct access to a mate. This might also be done to ensure the dimorphism of the trait. Intrasexual competitions result in the evolution of traits like antlers in deers, feathers of the peacock, a comb of chicken, the high body weight of males, fighting capability among males.

Intersexual competition- This is also known as the female mate choice. It the competition between the sexes to achieve reproductive success. In this type of selection, females choose their sexual partners. The choice is based on phenotypic traits like ornaments and weight. Since females are considered as limiting sex, as they invest more time in parental care, they create an asymmetry in the population that leads to intersexual selection. It generally tends to the selection of males who have strong ornaments.

How Does Darwin Sexual Selection Act?

It acts in two ways, first in the pre-copulatory stage and then in post copulation. During pre copulation sexual selection acts as by acquiring mating opportunities, excluding competitors, attracting, selecting, and/or retaining mates. These actions will inevitably lead to the reproductive success of the organism. Recent studies have shown that sexual selection also acts in the post-copulatory phase by sperm competition. Female exerts this choice based on the motility, biochemical nature of the sperm.