There is a cool paper in Evolution this month that demonstrates the power of natural selection. By Kleven, et. al. “Intraspecific Variation in Sperm Length is Negatively Related to Sperm Competition in Passerine Birds“

Abstract:

Spermatozoa are among the most diversified cells in the animal kingdom, but the underlying evolutionary forces affecting intraspecific variation in sperm morphology are poorly understood. It has been hypothesized that sperm competition is a potent selection pressure on sperm variation within species. Here, we examine intraspecific variation in total sperm length of 22 wild passerine bird species (21 genera, 11 families) in relation to the risk of sperm competition, as expressed by the frequency of extrapair paternity and relative testis size. We demonstrate, by using phylogenetic comparative methods, that between-male variation in sperm length within species is closely and negatively linked to the risk of sperm competition. This relationship was even stronger when only considering species in which data on sperm length and extrapair paternity originated from the same populations. Intramale variation in sperm length within species was also negatively, although nonsignificantly, related to sperm competition risk. Our findings suggest that postcopulatory sexual selection is a powerful evolutionary force reducing the intraspecific phenotypic variation in sperm-size traits, potentially driving the diversification of sperm morphology across populations and species.

So here you have a trait (sperm length) that is thought to be involved in sperm competition- now the title of this post is misleading because the test is on the variance in sperm length. The authors make no claim about the specific size of sperm and sperm competition.. So the test is to compare variation in sperm length to a measure of sperm competition- in this case they use the rate of extra pair copulation. They show that in species where there are al lot of EPC’s (high sperm competition), then there is less variance in sperm length. This is thought to be the result of stabalizing selection.

Pretty cool. I will definitely use it as an example when teaching the “forms” of natural selection.