For human beings at least, sex is fun, but it's hard work.  A study of evening primroses has found out why species do it - because it increases resistance to disease.  Species that simply clone themselves cannot adapt as quickly as species that have sex to reproduce, mixing characteristics from two individuals.

Erika Hersch-Green tested the idea on 32 different species of evening primroses.  These native North American wild flowers are unusual in that a number of species reproduce asexually, essentially through cloning themselves.  That allowed Hersch-Green and her colleagues to compare 16 species that reproduce sexually with an equal number that function asexually.

It was found that the sexual plants have an increased ability for adaptive, positive evolution.  This is in line with many of the theories of evolution of sex.

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.

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