A natural laboratory for evolution
The streams that flow down from Trinidad's Northern Range mountains contain a series of natural evolutionary experiments. Waterfalls punctuate these rivers, creating impassable barriers for many fish species. This divides the streams into distinct ecological zones. Downstream, in the lower pools, the Trinidadian guppy (Poecilia reticulata) lives alongside a host of predators. These "high-predation" sites contain visually hunting fish like the pike cichlid, Crenicichla alta, and the wolf fish, Hoplias malabaricus.
Above the waterfalls, these larger predators cannot follow. In these upstream "low-predation" sites, guppies find a relative sanctuary. The only other fish commonly found here is the killifish, Anablepsoides hartii, which preys on small, immature guppies but leaves adults alone. This sharp divide in predation pressure, repeated in multiple parallel rivers, creates the ideal conditions to observe evolution happening at a rapid pace. The guppies in the upper pools are genetically distinct from their relatives living just a few hundred meters downstream.
Evolution in a decade
In 1976, biologist John Endler conducted a now-classic experiment in a tributary of the Aripo River. He captured 200 guppies from a high-predation community downstream and transferred them to an upstream pool that contained killifish but had no native guppy population. When he returned just two years later, the male guppies in the new population had already changed. They showed significantly more colorful spots, an adaptation to attract mates that is a liability in predator-filled waters.
Biologist David Reznick later expanded on this work, showing that life-history traits also evolve with incredible speed. Guppies from high-predation zones mature quickly at a small size, and produce many small offspring to ensure some reproductive success before being eaten. When Reznick moved these guppies to predator-free sites, their evolutionary trajectory reversed. Within four to eleven years, about 30 to 60 guppy generations, they began to mature later at a larger size and produced fewer, but bigger, offspring. The measured rate of this change was thousands of times faster than evolutionary rates calculated from the fossil record. The research demonstrated that significant evolutionary shifts can occur not over millennia, but in a matter of years.