The ‘birth’ of guppies Poecilia reticulata as a paradigm of evolutionary biology was in the Arima Valley of Trinidad, where Caryl Haskins first described the simple gradient of fish communities found in the ascent up the drainages of the Northern Range Mountains and its consequence for guppies. At lower elevations, guppies lead a fugitive existence to avoid the constant threat of predation. Above waterfalls that exclude predators but not guppies, the guppies have a greatly reduced risk of being eaten and thus occupy the whole river.