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Why sex? Biologists find new explanations

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https://www.quantamagazine.org/why-sex-b...-20200423/

EXCERPT: Why did sex evolve? Theories usually focus on the diversity of future generations, but some researchers find compelling explanations in the immediate benefits to individuals. Sex might be biology’s most difficult enigma. The downsides of relying on sex to reproduce are undeniable [...] Asexual reproduction, or self-cloning, has none of these disadvantages. ... Yet despite all its benefits, asexual reproduction is the exception, not the norm, among organisms that have compartmentalized cells (eukaryotes). ... Among animals, only one out of every thousand known species is exclusively asexual. For centuries, biologists have pondered this apparent paradox.

In 1932, the geneticist Hermann Muller [...] went on to explain, “Sexuality, through recombination, is a means for making the fullest use of the possibilities of gene mutations.” In other words, the purpose of sex is simple: It increases genetic diversity in the pool of offspring. ... But perhaps it shouldn’t be. “The focus of sexual selection and sexual hypotheses tend to really focus on the future generations ... Research has often overlooked the potential for direct benefits to the individual” from having sex...

[...] researchers are now reexamining how sex and its associated cellular and physiological processes affect individuals. Their results suggest that the reason biologists have struggled to find a truly unifying explanation for sex is that there isn’t one. Instead, there’s a veritable smorgasbord of potential benefits from sex, and organisms may engage in it for whichever ones help them the most.

In some ways, sex is universal — almost every eukaryotic organism has sex. But it is also a unique experience for each species. What sex looks like is different if you’re talking about a plant, a single-celled protozoan, a fruit fly or a human being. [...] The multicellular Volvox algae ... belong to some of the oldest lineages of eukaryotes ... “the ancestral role of sex was not for reproduction.” Instead, “sex appears to have evolved as a means to respond  adaptively to stress.”

[...] it’s possible that benefits of sex not directly tied to reproduction, such as DNA repair, occur in fungi, plants or animals, too. And even if sex is an animal’s or plant’s only means of reproducing, these indirect benefits of sex may influence why, how, when and how often that happens.

[...] Much of the fruit fly literature suggests ... that there is a cost to mating. But [...for...] fruit flies in the wild ... the opposite [...has been found....]. Females that had mated lived longer than ones that had not. [...It is suspected...] that’s because the females benefit in more ways than one from receiving a male’s ejaculate.

[...] Neuroscientists have found there’s much more to the story by looking at male animals. ... male rats get an immune boost in their brains after having sex. This might mean that sex helps protect them from infection. Sex may also alter how well their brains work. Other scientists found that rats perform better on certain cognitive tests after mating, and that mating regularly can slow the decline in brain function associated with age.

[...] our preconceptions of what sex should look like and the reasons why an individual should or shouldn’t have it have biased our understanding of animal behavior. ... research on same-sex behaviors in animals as a prime example of this. ... scientific discourse surrounding same-sex behaviors involves a lot of weak or baseless assumptions — for example, that engaging in sexual acts is inherently costly, so same-sex sexual interactions must provide some overwhelming benefit, such as a large increase in lifetime reproductive output, for the behavior to arise and stick around through natural selection. But “in many situations, it isn’t costly, and it may have some kind of benefit that we don’t understand

[...] Instead of asking why same-sex behaviors would evolve [...researchers...] “flipped the question on its head” and asked why the behaviors wouldn’t. When they did this, they realized it’s possible and maybe even likely that same-sex behaviors have occurred all along; they just aren’t costly enough to be selected against... (MORE - details)
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