SEXUAL SELECTION: CHARACTERISTICS, TYPES AND EXAMPLES - BIOLOGY - 2025

The sexual selection is the differential reproductive success resulting from variations in mating success. It is usually considered as a component of natural selection.

There are secondary sexual characteristics in some animals that seem to challenge the survival of the carrier. For example, don't the colorful - and energetically expensive - feathers of birds make it more visible to potential predators?

Source: J. Gould, via Wikimedia Commons

The concept of sexual selection managed to explain sexual dimorphism - differences in the morphologies and ethology of the sexes, a phenomenon widely distributed in animals.

The argument for sexual selection is as follows: If there are heritable variations for a trait that has an effect on the ability to mate, then successful variants will increase in frequency.

In Darwin's time, sexual selection was considered a weak force, since its result does not lead to immediate death, but to a decrease in the number of descendants. Today sexual selection is considered relatively strong, because it affects the success of copulation and fertilization.

Sexual dimorphism

A key concept to understand sexual selection is sexual dimorphism. In nature, males and females do not look identical - in terms of secondary sexual characteristics.

In birds this phenomenon is very marked: males generally display bright colors and elaborate plumage, while females are opaque. They also differ in terms of behavior, the males emit melodious songs and the females are silent.

We can extrapolate this phenomenon to humans. In our populations, males and females differ substantially. Body size, fat distribution patterns, tone of voice, among other characteristics, are some of the differences between the sexes.

Evolutionary biologists are constantly analyzing the traits of organisms, in order to conclude if they are adaptive characteristics or not, and how they could increase the reproduction of their carrier.

In the case of sexual dimorphism, the theory of evolution by natural selection is apparently difficult to apply because several questions arise.

If a characteristic can increase the reproduction or survival of an individual, why is it unique to males? And why invest so much energy in features that increase visibility to predators?

characteristics

Charles Darwin

British naturalist Charles Darwin was the first to answer these two questions. Darwin recognized that individuals differ in their success in convincing members of the opposite sex to mate, and I call this sexual selection.

Today, most evolutionary biologists do not believe it necessary to separate the concepts of natural selection and sexual selection. In fact, they regard sexual selection as a component of natural selection.

The theory of sexual selection can be summarized as follows: if there are heritable variations in a trait that affect the ability to find a mate, then the variants that contribute to the success of this event will increase their frequency in the population.

One of the important characteristics in reproduction is the asymmetry of the process. There is no question that eggs and pregnancy are much more expensive than mere sperm production and ejaculation. In this way, in terms of time and energy, the contribution of the female is much greater.

For this reason, we can predict that males should be competitive and fight for mating opportunities. In contrast, females must be selective.

Types

In sexual selection we must take into account two factors: competition between males to find a partner of the opposite sex, willing to reproduce with them, and the female's selective capacity. Thus, we can distinguish two forms or types of sexual selection: intrasexual and intersexual.

Intrasexual selection

Intrasexual selection refers to the competition between individuals of the same sex to monopolize access to the opposite sex. Generally this fact involves fighting or fighting, and selective sex is paired with the winner of the battle.

The most intuitive way to access selective sex is combat. For this reason, the selection will favor the existence of morphologies that allow defense, call it horns, horns, etc.

Competition can go beyond copulation: sperm competition. Heavy ejaculations with a large number of sperm can be linked to victory. The most extreme cases of infanticide, as in the pride of lions, where males kill the cubs of other males.

Inverse pattern of intrasexual selection

In most species, it is the males that struggle to gain access to the female. However, there are many cases where males contribute to parental care and the investment in offspring can be equal to or greater than that of the female. For example, fish, frogs, and most birds show this pattern.

In the latter case where the pattern is inverted, the limiting factor in reproduction will be the male and the females will have to compete with each other to gain access to the males. It is also possible to predict that the selective sex will be the male.

Intersex selection

In certain species, males will not seek to control or monopolize access to the female. In contrast, they will look for ways to get their attention through exhibitions, which may involve singing, dancing, among others. At the end of the exhibition, the female chooses the "best" among her candidates.

It is the selectivity of the female that leads to the evolution of such courtship acts, which consume a great deal of time and energy, being an important selection mechanism.

There are several hypotheses that seek to explain the selectivity of the female. They may choose the "best male" for these genes to pass on to their offspring. In other words, they look for good genes. It is also possible that by choosing a good quality male, they are looking for direct benefits to the offspring, such as food.

Example: birds of paradise

The birds of paradise are a group of species belonging to the Paradisaeidae family and distributed throughout Oceania, the majority in New Guinea. They are characterized by being extremely diverse, in terms of morphologies and colorations. In addition to their complex courtship dances.

The ancestor of the current forms was similar to a raven. It took several million years of females choosing elaborate characters to arrive at current morphologies.

As the name implies, these birds live in "paradise" - literally. Their habitat is considered rich in food and predators are practically non-existent.

With this attenuation of these selective pressures, sexual selection acts to a greater extent, leading to the appearance of exaggerated features, in terms of coloration and courtship.

References

1. Arnqvist, G., & Rowe, L. (2013). Sexual conflict. Princeton University Press.
2. Clutton-Brock, T. (2007). Sexual selection in males and females. Science, 318 (5858), 1882-1885.
3. Darwin, C. (1888). The descent of man and selection in relation to sex. Murray.
4. Freeman, S., & Herron, JC (2002). Evolutionary analysis. Prentice Hall.
5. Irestedt, M., Jønsson, KA, Fjeldså, J., Christidis, L., & Ericson, PG (2009). An unexpectedly long history of sexual selection in birds-of-paradise. BMC Evolutionary Biology, 9 (1), 235.
6. Trivers, R. (1972). Parental investment and sexual selection. Cambridge: Biological Laboratories, Harvard University.