It may sound too bizarre to be true, but the Amazon molly ( Poecilia formosa ), a fish that inhabits rivers, lakes, and swamps in Mexico and Texas, exists over much of its range in populations that are 100% female. In 1932, the Amazon molly became the first known vertebrate to reproduce by cloning itself , producing all-female populations. A new genetic study has given scientists insights into the longstanding mystery about how and why this happens. The proportion of females in the human