Larger sperm are found in rhabditid nematode species that experience more sperm competition, and sperm size is the single most divergent character among the group. In C. elegans , larger sperm outcompete smaller sperm but are more costly to produce. This suggests that pressure from sperm competition drives the evolution of sperm size, but such data do not demonstrate cause and effect. We tested the effect of sperm competition on male sperm size evolution in three outcrossing lines of C. elegans. These lines were forced to outcross using a
spe-8 mutation (which renders hermaphrodites self-sterile, but has little effect on male fertility), and genetic variation was introduced from wild isolate strains CB4855, DR1345, DR1350, and AB1. Sperm volume increased nearly 20% in the outcrossing lines over the course of 60 generations. No such sperm size evolution occurred in selfing control lines. These data demonstrate that increased sperm competition causes larger sperm to evolve. Sperm morphology is extremely variable both within and among animal taxa, while ova show much less variability. Perhaps the pressures of sperm competition explain some of this morphological variation not only in nematodes but in other animals as well.