I photographed these 2 eastern tailed blues (Everes comyntas) last September in Germantown, Maryland. They were mating on a grassy strip next to a busy road, completely oblivious to the traffic and to me.
The question of why sex evolved is one of the perennial questions in evolutionary biology. An article by Paland and Lynch1 and an accompanying essay by Nielsen2, both in last week's Science, offer some explanations.
Sexual reproduction allows genetic recombination, that is, the exchange of genetic material between parental chromosomes. In this case, the chromosomes of the male butterfly will recombine with the chromosomes of the female and their offspring will have new sets of chromosomes.
Genetic recombination provides a mechanism for genetic variants that have arisen in different individuals to be combined and tested for fitness in one individual. When a lucky butterfly has several beneficial mutations in its genes, it is more likely to survive and produce its own offspring. As Nielsen summarizes: "genetic recombination is advantageous because it allows natural Darwinian selection to work more efficiently."
The results by Paland and Lynch suggest that asexual populations of the water flea Daphnia pules accumulate deleterious mutations. They conclude: “our results indicate that sexual reproduction enhances the efficiency of purifying selection, supporting the theory that deleterious-mutation accumulation is a leading evolutionary force contributing to the short longevity of asexual lineages.”
Yes, but, what about bdelloid rotifers? They are the largest group of animals that appear to reproduce exclusively by parthenogenesis (they are all females) and they may have been around for quite some time.
Perhaps a more appropriate, but certainly less attention grabbing, title for this post would have been "Recombination on the grass".
1. Susanne Paland & Michael Lynch. 2006. Transitions to Asexuality Result in Excess Amino Acid Substitutions. Science 311:990-992.
2. Rasmus Nielsen. 2006. Why Sex? Science 311:960-961.