At Quanta, the problem explained:
Sex might be biology’s most difficult enigma. The downsides of relying on sex to reproduce are undeniable: It takes two individuals, each of whom gets to pass on only part of their genome. Because these individuals generally have to get fairly intimate, they make themselves vulnerable to physical harm or infections from their partner. Asexual reproduction, or self-cloning, has none of these disadvantages. Clones can be made anywhere and anytime, and they receive the full complement of an individual’s genes.
Yet despite all its benefits, asexual reproduction is the exception, not the norm, among organisms that have compartmentalized cells (eukaryotes).
Christie Wilcox, “Why Sex? Biologists Find New Explanations” at Quanta
To count as “science,” the explanations offered must be accidental favorable developments, even if they involve contortions of thought or ridiculously unlikely probabilities. In any event, explanations have tended to center on increasing the diversity of the gene pool (Muller and others). Wilcox tells us that more recent explanations focus on the benefit to the individual from having sex. One example cited is algae, which reproduce sexually when unders stress, in which case a second chromosome is an asset.
During meiosis, the chromosomes from each haploid parent line up and may swap sections with each other, a phenomenon known as recombination. This step greatly increases genetic diversity, but it also gives chromosomes the ability to essentially copy and paste sections from the other haploid genome to repair any damage that may have befallen their own.
Christie Wilcox, “Why Sex? Biologists Find New Explanations” at Quanta
Then the payload:
It may even be that, if the fitness costs of the sexual act are low enough and the benefits are high enough, it’s not always worth searching for a suitable mate of another sex. Individuals might ultimately live longer and pass on more of their genes by having sex early and often with any member of their species they come across, or even by frequently engaging in masturbation. Such hypotheses have likely gone unexplored because our views on sex in other species are shaped by our views on sex in our own.
But as more research is conducted on how sex affects different organisms, scientists are shedding that bias and discovering that sex can have myriad positive effects, any of which might subtly shape how a species does it. “Absolutely anything that has even a modicum of benefit in terms of the number of offspring that can be produced or the quality of offspring that could be produced will be selected for by natural selection,” Worthington said.
Christie Wilcox, “Why Sex? Biologists Find New Explanations” at Quanta
It’s not clear how masturbation would increase the number or quality of offspring but let that be. In the age of Woke, Darwinian biology must be pressed into the service of whatever is Woke and there is no requirement that it make sense.
Something worth considering: In animals, generally speaking, sexual bonding resulting in co-operative parenting is most evident among the larger, more complex, and more intelligent species. If so, a microbe doesn’t do what a wolf pack or elephant herd does because the demands of pack or herd life may be beyond the microbe’s information resources. Perhaps only smarter species can manage the demands of sexual reproduction in a given environment. In that case, we can once again turn the question around. It’s not so much, “Why did sex evolve?” but “Under what circumstances could sex evolve?”
In that case, it’s not so much whether sexual reproduction offers an advantage but under what circumstances it might do so. That is, we need not assume, Darwin-like, that sex must convey an advantage everywhere to all. Perhaps it conveys an advantage but only if some basic resources already exist within the organism—one might be the ability to handle a more complex stream of information.
See also: Can sex explain evolution?