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). In plants, for example — which are somewhat known for their genetic flexibility — less than 1% of species are thought to reproduce asexually often. 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, whose work on radiation-induced mutations would eventually garner a Nobel Prize, believed he had the answer. “Genetics has finally solved the age-old problem of the reason for the existence (i.e., the function) of sexuality and sex,” he boasted in The American Naturalist. He 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. That diversity could then raise the fitness of future generations by making them stronger, faster, more resistant to parasites or otherwise more adaptable. Muller wasn’t the first biologist to suggest this, but his influence was so great that his name has been forever tied to the idea, which remains prevalent today.

But perhaps it shouldn’t be. After all, the diversity justification doesn’t explain how or why single-celled organisms evolved an essential component of sex: meiosis, the process of halving the genome that enables the production of egg and sperm cells.

“The focus of sexual selection and sexual hypotheses tend to really focus on the future generations,” said Caitlin McDonough, who studies the evolution of reproductive systems at Syracuse University. McDonough’s work on the evolution of sexual behaviors has found evidence that theories centering exclusively on ideas about what’s good for offspring or the species as a whole are incomplete. “Research has often overlooked the potential for direct benefits to the individual” from having sex, they said.

McDonough and other 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.

Everybody Does It

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.

Even the idea that sex is for reproduction doesn’t hold across the entire swath of eukaryotes. For the algae studied by Aurora Nedelcu, a biologist at the University of New Brunswick in Canada, sex isn’t about making more offspring. “They reproduce better asexually,” she said. The Volvox species she works with are facultatively sexual, meaning that they choose whether to clone themselves or have sex. When they opt for sex, it’s to improve their odds of survival.

For most of their lives, these algae live with what would be considered half a genome by human standards: They have only one copy of each chromosome, and so are haploid. In this state, they can undergo mitosis, the process of cloning themselves that all cells perform. First they make a copy of each of their chromosomes, then those copies line up down the centerline of the cell and are pulled apart into two new daughter cells identical to their parent.