But new insights into the "costs and benefits" of sex are beginning to emerge from population genetic theory, lab experiments and field observations across the tree of life, from yeast to humans.
Some of these insights were presented on Thursday (Feb 17) at the American Association for the Advancement of Science (AAAS) annual meeting in Boston, where evolutionary biologists are presenting their latest research on what drives the evolution and maintenance of sexual reproduction.
One prevailing hypothesis for the evolution of sex is that it helps purge deleterious mutations from a population - a process called "Muller's ratchet".
With asexual reproduction, an organism inherits all of its genes from its parent, and any new deleterious mutations it accumulates through time cannot be weeded out, leading to a gradual accumulation of mutations and a decline in the population.
But sexual reproduction, by mixing genes from two parents, creates opportunities for these mutations to be purged as beneficial mutations can compensate for their harmful effects.
To test this hypothesis, researchers can perform simulations that model populations with asexual reproduction and sexual reproduction, and measure how quickly deleterious mutations accumulate in both cases.
For instance, in a recent study, researchers used computer simulations to model the long-term evolutionary consequences of sex and found that sexual lineages accumulated fewer deleterious mutations than asexual lineages.
Theoretical models also predict that under some conditions, sex can accelerate adaptation when the environment changes rapidly. For instance, sex can enable new combinations of alleles that can help a species adapt to new environmental conditions more quickly than asexual species.
In yeast, for example, when the environment fluctuates rapidly, sexual evolution allows beneficial alleles to combine more rapidly, allowing yeast to adapt more rapidly. However, when the environment is relatively stable, the beneficial effects of sex diminish.
Other benefits of sex include:
- The ability to continually generate genetic diversity, which may help a species adapt to changing environments.
- Resistance to parasites that specialize in attacking one sex over the other, meaning sexually reproducing species are less likely to be heavily burdened by parasites.
- The increased efficiency of DNA repair mechanisms in sexual organisms.
- The benefits of sexual selection, which allows organisms to choose their mates based on desirable traits, promoting the spread of these desirable traits through the population.
It's important to note, however, that the benefits and costs of sex can vary greatly depending on the specific species and environment.
Overall, the evolutionary dynamics of sex are complex, and while biologists are gaining a deeper understanding, there are still many mysteries to unravel about why sexual reproduction evolved and persists in the natural world.
