in the main experiment, nematode worms were divided into two groups. one group was entirely outcrossing, the other was entirely selfing. the groups were subjected to a rugged terrain and repeatedly subjected to a mutagen. after 50 generations, the selfing population showed a substantial decline in fitness (= survival), whereas the outcrossing population showed no decline. this is one of a number of studies that show sexuality to have real advantages over non-sexual types of reproduction.