What is Sexual Selection?
Sexual selection is a “special case” of natural selection. Sexual selection acts on an organism’s ability to obtain (often by any means necessary!) or successfully copulate with a mate.
Sexual selection has shaped many extreme adaptations that help organisms find mates: peacocks (top left) maintain elaborate tails, elephant seals (top right) fight over territories, fruit flies perform dances, and some species deliver persuasive gifts.
After all, what female Mormon cricket (bottom right) could resist the gift of a juicy sperm-packet? Going to even more extreme lengths, the male redback spider (bottom left) literally flings itself into the jaws of death in order to mate successfully.
Sexual selection is even powerful enough to produce features that are harmful to the individual’s survival. For example, extravagant and colorful tail feathers or fins are likely to attract predators as well as interested members of the opposite sex.
Darwin’s Theory of Sexual Selection
Charles Darwin proposed that all living species were derived from common ancestors. The primary mechanism he proposed to explain this fact was natural selection: that is, that organisms better adapted to their environment would benefit from higher rates of survival than those less well equipped to do so.
However he noted that there were many examples of elaborate, and apparently non-adaptive, sexual traits that would clearly not aid in the survival of their bearers.
He suggested that such traits might evolve if they are sexually selected, that is if they increase the individual’s reproductive success, even at the expense of their survival (Darwin 1871).
Darwin noted that sexual selection depends on the struggle between males to access females. He recognized two mechanisms of sexual selection:
- Intrasexual selection, or competition between members of the same sex (usually males) for access to mates, and
- Intersexual selection, where members of one sex (usually females) choose members of the opposite sex.
The idea of cumbersome traits evolving to aid males in competition during aggressive encounters was readily accepted by scientists shortly after Darwin’s publication.
However, the idea of female mate choice was received with ridicule, and was not seriously reconsidered until nearly 80 years later (Cronin 1991). In the 40 years since, there has been much progress in our understanding of how sexual selection operates.
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.
1. 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.
2. Intersexual competition
Intersexual competition 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.
Examples of Sexual Selection
Intrasexual Selection
Lion’s Mane
Male lions are known for their majestic manes, which can vary in size and color. Male lions with impressive manes are often seen as more dominant and formidable by other males. They are more likely to be selected by females for mating.
Deer Antlers
In deer, male antlers serve as formidable weapons during intrasexual selection. These antlers are shed and regrown each year. During the mating season, male deer engage in physical combat to establish dominance. The males with the largest and most robust antlers often win these battles and gain access to females.
Elephant Seal
Male elephant seals engage in fierce physical battles with their fellow male members in an area to establish their supremacy over others. The successful male gets access to mating with the females residing there.
Bird Songs
Many male bird species engage in complex and melodic songs, particularly during the breeding season. Female birds are attracted to males with the most intricate and captivating melodies. These songs are a sign of physical fitness and indicate that the male can defend a territory and provide food for offspring.
Intersexual Selection
Peacock’s Tail
Male peacocks have incredibly ornate and colorful tail feathers displayed in intricate courtship dances. These displays are meant to attract females. Peahen females, conversely, are highly selective, favoring males with the most dazzling and symmetrical tails.
By choosing mates with impressive tails, female peafowls indirectly select for good genetic quality, as only healthy and well-nourished males can afford to grow such extravagant plumage.
Savanna Baboon’s Genitals
Female baboons exhibit conspicuous genital enlargements known as ‘sexual swellings.’ Male baboons prefer mating, with females displaying the most pronounced swellings, often seen as potential indicators of heightened fertility.
Courtship Displays in Birds of Paradise
Male birds of paradise, found in New Guinea, are renowned for their elaborate courtship displays. These displays involve intricate dances and the flaunting of vibrant plumage.
Females are highly selective and prefer males with the most extravagant displays. These displays indicate genetic fitness and the male’s ability to provide for offspring.
Fitness in Humans
In humans, sexual selection is not limited to the animal kingdom. People often choose mates based on physical attractiveness, including facial symmetry, body proportions, and clear skin.
Beyond physical traits, human chooses their mate based on personality characteristics, intelligence, sense of humor, and other behavioral traits.
Why Does Sexual Selection Occur
The following two hypotheses can explain the reason behind sexual selection in nature:
Good Genes Hypothesis suggests that individuals with attractive traits may have better genes, leading to healthier offspring. By choosing a mate with these attractive traits, the selecting individual increases the chances of having healthy and genetically fit offspring.
Fisherian Runaway Selection proposes that once a preference for a particular trait exists in a population, it can reinforce itself over time.
In this scenario, individuals with the desired quality become preferred mates simply because they possess the trait, leading to a self-perpetuating selection cycle.
Why is sexual selection so powerful?
It’s clear why sexual selection is so powerful when you consider what happens to the genes of an individual who lives to a ripe old age but never produced offspring: no offspring means no genes in the next generation, which means that all those genes for living to a ripe old age don’t get passed on to anyone! That individual’s fitness is zero.
Compare that to an individual who does not live very long, but leaves behind children.
Selection is a two-way street
Sexual selection usually works in two ways, although in some cases we do see sex role reversals:
Male competition
Males compete for access to females, the amount of time spent mating with females, and even whose sperm gets to fertilize her eggs. For example, male damselflies scrub rival sperm out of the female reproductive tract when mating.
Female choice
Females choose which males to mate with, how long to mate, and even whose sperm will fertilize her eggs. Some females can eject sperm from an undesirable mate.