Sex is a puzzle for evolutionary biologists.  It seems to be a waste of reproductive output.  A population of a fixed size which requires two members to produce offspring reproduces, and therefore grows, at half the rate of the same sized asexsual population (which requires only one member to produce one offspring.)

So to explain the prevalence of sexual reproduction in nature we need to find some advantage to offset this so-called two-fold cost of sex.  There are two prominent theories.  The first is that sexual reproduction allows a species to shed disadvantageous mutations.  Sexual reproduction thus ensures that offspring loses any harmful mutation with probability 1/2 (we are assuming that the parents do not have mutations of the same gene, a good approximation when there are many genes.)  But with asexual reproduction, these mutations just accumulate.

Another theory is that sexual reproduction, by mixing around genes, ensures genetic diversity which enables a species to survive changes in the environment.

Not Exactly Rocket Science reports on an experiment designed to test these theories.

Like humans, C.elegans has two sexes but unlike us, they are males and hermaphrodites (with males making up just one in every two thousand individuals).  Equipped with both sets of genitals, hermaphrodites worms can fertilise themselves without male help – far from being rude, telling C.elegans to go &$&! itself is a feasible lifestyle suggestion. Hermaphrodites could also mate with males, but they do that on less than one in 20 occasions.

The biologists manipulated the genetics of a population of these worms so that half would always mate with themselves and the others would always mate sexually.  Next, they exposed the worms to a chemical that raised their rate of mutations.  As the theory predicts, the sexually reproducing worms were more successful.

Next, they exposed the worms to a deadly bacterium.  Consistent with the second theory, the sexually reproducing worms also fared better in this experiment.

Now the big puzzle.  If sexual reproduction is beneficial, why do all sexually reproducing species in nature do it in pairs?  This paper by economists Motty Perry, Phil Reny, and Arthur Robson proves that, at least with respect to the harmful mutation theory, a particular form of tri-parental sex dominates bi-parental sex.   In the Perry-Reny-Robson world, reproduction requires two males and one female.  The offspring receives genes with half-probability from the mother and 1/4-probability from each of the fathers.

(With this particular menagerie, in every reproductive cycle each female gets two partners per encounter but each male gets two encounters.  Not only does this ensure that the “cost of sex” is again two-fold and not three-fold, but it also maintains equity in the gettin’ busy department.  Only fair.)