Table of Contents Sexual Selection Darwin considered traits selected according to their role in mating to be separate from those acted on by natural selection.
The sciences of evolutionary psychologyhuman behavioural ecologyand sociobiology study the influence of sexual selection in humans. Hamiltonholds that the fact that the male is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is taken by the female to be a testament to his overall fitness.
Such handicaps might prove he is either free of or resistant to diseaseor that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait. InHamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency.
InMichael Ryan and A. Rand, working with the tungara frogproposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist.
Rice have been added. In the late s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.
These include cuckoldrynuptial gifts, sperm competitioninfanticide especially in primatesphysical beautymating by subterfuge, species isolation mechanisms, male parental care, ambiparental care, mate location, polygamy, and homosexual rape in certain male animals.
Sexual conflict leads to an antagonistic co-evolution in which one sex tends to control the other, resulting in a tug of war. Besides, the sexual propaganda theory only argued that mates were opportunistically lead, on the basis of various factors determining the choice such as phenotypic characteristics, apparent vigour of individuals, strength of mate signals, trophic resources, territoriality etc.
One possible explanation for the apparent lack of costs is that "compensatory traits" have evolved in concert with the sexually selected traits. Geoffrey Miller proposes that sexual selection might have contributed by creating evolutionary modules such as Archaeopteryx feathers as sexual ornaments, at first.
Some have suggested that the feathers served as insulation, helping females incubate their eggs. But perhaps the feathers served as the kinds of sexual ornaments still common in most bird species, and especially in birds such as peacocks and birds-of-paradise today.
If proto-bird courtship displays combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed.
November Main article: Sexual dimorphism Sex differences directly related to reproduction and serving no direct purpose in courtship are called primary sexual characteristics. Traits amenable to sexual selection, which give an organism an advantage over its rivals such as in courtship without being directly involved in reproductionare called secondary sex characteristics.
The rhinoceros beetle is a classic case of sexual dimorphism. Also, unlike a female, a male except in monogamous species has some uncertainty about whether or not he is the true parent of a child, and so is less interested in spending his energy helping to raise offspring that may or may not be related to him.
As a result of these factors, males are typically more willing to mate than females, and so females are typically the ones doing the choosing except in cases of forced copulationswhich can occur in certain species of primatesducksand others.
The effects of sexual selection are thus held to typically be more pronounced in males than in females. Differences in secondary sexual characteristics between males and females of a species are referred to as sexual dimorphisms.
These can be as subtle as a size difference sexual size dimorphism, often abbreviated as SSD or as extreme as horns and colour patterns. Sexual dimorphisms abound in nature.
Examples include the possession of antlers by only male deerthe brighter coloration of many male birds in comparison with females of the same species, or even more distinct differences in basic morphology, such as the drastically increased eye-span of the male stalk-eyed fly.
The peacockwith its elaborate and colourful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism.
Male and female black-throated blue warblers and Guianan cock-of-the-rocks also differ radically in their plumage. Early naturalists even believed the females to be a separate species. The largest sexual size dimorphism in vertebrates is the shell dwelling cichlid fish Neolamprologus callipterus in which males are up to 30 times the size of females.
Many other fish such as guppies also exhibit sexual dimorphism. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders and birds of prey.
In different taxa[ edit ] SEM image of lateral view of a love dart of the land snail Monachoides vicinus. Sexual selection in birds - mammals - humans - scaled reptiles - amphibians - insects - spiders - major histocompatibility complex Human spermatozoa can reach million in a single ejaculation Sexual selection has been observed to occur in plants, animals and fungi.
Today, biologists say that certain evolutionary traits can be explained by intraspecific competition - competition between members of the same species - distinguishing between competition before or after sexual intercourse.
Illustration from The Descent of Man showing the tufted coquette Lophornis ornatus: Also, intersexual selection, or mate choiceoccurs when females choose between male mates. Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to an animal, thereby lowering its chances of survival.
Bright colourations and showy ornamenations, such as those seen in many male birds, in addition to capturing the eyes of females, also attract the attention of predators.In contrast to features that result from natural selection, a structure produced by sexual selection results in an advantage in the competition for mates.
Learn More in these related Britannica articles. Jul 28, · Let's find out what Sexual Selection is all about in this brief but comprehensive educational video. First, we'll discover how Sexual Selection differs from Natural Selection. The Concepts of Natural Selection and Sexual Selection in Evolution Darwin wasn't the first to come up with the idea that evolution took place; many others had thought that was the case, but had no reasonable explanation for how it could happen.
Natural selection. Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift. Darwin's grand idea of evolution by natural selection is relatively simple but often misunderstood.
We tend to think of natural selection—”survival of the fittest”—but sexual selection works the same way and can be just as strong in shaping how species look and act.
Filmed and photographed by Tim Laman. 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.
Selection makes many organisms go to extreme lengths for sex: peacocks (top left) maintain elaborate tails.