If you think about it rationally, sex may be fun but it's too much work and, from a reproduction standpoint,  the payoff is uncertain. Scientists have speculated for a long time on why all living things don't simply make like amoebas and split.

Scientists believe that sexual reproduction offers two big advantages: It can sweep bad mutations out of the gene pool more quickly. Also, by shuffling parents' genetic material each generation, it increases the likelihood that new genetic combinations will arise that help organisms adapt to their environment.

A new paper also says sexual reproduction strengthens an organism's ability to adapt; specifically, it may lead to stronger disease resistance. The researchers
 tested 32 different species of evening primroses, native North American wildflowers that are unusual in that only some species reproduce asexually, essentially through cloning themselves. That allowed them to compare 16 species that reproduce sexually with an equal number that function asexually.

By comparing sexual and asexual species of evening primroses, researchers demonstrated that sexual reproduction offers a significant advantage. Photo: MTU

"We found that the sexual plants have an increased ability for adaptive, positive evolution,"  
said Erika Hersch-Green, biologist at Michigan Technological University. "That's in line with many of the theories of evolution of sex."

To find evidence of those good genetic recombinations, they sequenced the gene chitinase A in each of the 32 species. All plants have the gene, which makes the enzyme chitinase and helps them identify and fight off diseases such as powdery mildew. However, the genes are not exactly the same; there are some differences in their building blocks, called nucleotides, which means that there are slight variations in chitinase from species to species.

Once they knew the sequence of nucleotides in the gene for each species, the researchers compared them using well-established mathematical models. They found that the chitinase A genes in the evening primrose species that reproduce sexually were different in four places from the ancestral gene, while the genes in the asexual species had not changed. In addition, the genes in the sexual plants had a higher expression level, meaning they produced more of the chitinase enzyme used to fight off disease.

They also looked at how well 12 different genotypes of the same asexual evening primrose species weathered attacks by powdery mildew. Those that sustained the most damage from the mildew also were less fit, meaning they produced fewer fruits. "This suggests that common pathogens of evening primrose plants are an important selective agent for these plants," Hersch-Green said. "Furthermore, molecular changes that increase chitinase expression and reduce disease damage are likely to be very good for the plants."

"This is in line with the theory that sex provides an evolutionary advantage to organisms," Hersch-Green said. "Until now, there had been no empirical evidence to support this advantage of new genetic combinations."

Citation:  E. Hersch-Green, H. Myburg and M. T. J. Johnson, 'Adaptive Molecular Evolution of a Defence Gene in Sexual but not Functionally Asexual Evening Primroses', Journal of Evolutionary Biology