In 2007, we set out across Texas and northern Mexico to collect specimens of the Amazon molly (Poecilia formosa)—a sperm-dependent asexual fish—and its sexual host species. Our goal was to test whether the morphology of the asexuals co-varied with that of their hosts. We wondered: could these clonal fish evolve to resemble the females of their host species, thereby deceiving males into mating and providing the sperm needed for reproduction?
What we found surprised us. Instead of mimicry, the Amazon mollies differed from their host species—and we weren’t sure whether this pattern was real or just a quirk of our sampling design.
Seventeen years later, Waldir Berbel-Filho revisited this question using a new dataset and found the same pattern: Amazon mollies differ morphologically from their sexual hosts, and in a host-specific way. They resemble P. mexicana when coexisting with P. latipinna hosts, and vice versa. This suggests character displacement, potentially driven by ecological interactions or male mate choice.
You can read the full paper in Proceedings of the Royal Society B.