The utility of sex, according to an intriguing new theory of evolutionary biology, may be its ability to promote genes that play well with many other partners rather than those that shine with just one specific set of genes.
This idea of genetic mixability, described in the journal Proceedings of the National Academy of Sciences the week of Nov. 24, hits on the difficulty evolutionary biologists have had in understanding sex, specifically its role in population genetics and Darwin's survival-of-the-fittest mantra.
"It's the generalist winning over the specialist," said Christos Papadimitriou, professor of computer sciences at the University of California, Berkeley, and co-author of the paper.
"During the past century, it has often been assumed that sexual reproduction should somehow facilitate the increase in fitness under natural selection, leading to the 'best' combinations of genes," said lead author Adi Livnat, a Miller Institute post-doctoral fellow based at UC Berkeley's Department of Electrical Engineering and Computer Sciences. "But no agreement has been reached on whether and how this could really work. One might think, for example, that by bringing together genes from different individuals, sexual reproduction could create a very successful combination of genes. But just as sexual reproduction will create that very successful combination of genes, it could also break it down in the next generation."
That sex can actually impede the increase in the fitness of the population raises the question of how it can remain the dominant form of reproduction across all manner of species. Sex - at its core the merging of genes from different individuals to create genetically unique offspring - is the reproductive method of choice from humans to plants to many fungi. This form of reproduction must be doing something right, in terms of evolution.
Veering from the assumption that sexual reproduction increases the average fitness of a population, the researchers came up with a new measure they call "mixability" to represent a gene's ability to perform well across many different combinations. They tested the mixability measure in a number of scenarios within a well-established population-genetic framework.
They found that if the goal is to maximize fitness by finding a particularly good combination of genes, asexual reproduction - which increases a population's numbers at a much faster rate than sexual reproduction - works very well.
In contrast, sexual reproduction, through the process of recombination and segregation of chromosomes, strongly favors genes that work well in many different variations rather than any one good combination. In that view, the authors wrote, alleles of the same gene compete with each other based upon how well they perform on average rather than how well they perform in any one specific combination.
"It's important to note that during the process of evolution, the mixability value increases, though it doesn't increase all the time," said the paper's co-author Marcus Feldman, professor of biology at Stanford University and a world-renowned theorist in evolutionary biology. "The approach we take is different from usual because we're interested in evolutionary transience, and in the long run, our mixability value may actually decrease because too much variability is lost from the population."
Even so, sexual reproduction has a great advantage for mixability compared with asexual reproduction, according to the models in the paper.
In what may appear at first to be an unconventional comparison, sexual reproduction has something in common with the rebalancing of a stock portfolio at regular intervals, explained Livnat.