Guppies

The Trinidadian guppy (Poecilia reticulata) is an excellent species for studying the relationship between adaptive evolution across different environmental circumstances because:
  • It matures rapidly (one generation = 3-4 months)
  • It inhabits different ecological environments that can be easily manipulated
On Trinidad, guppies live in streams, or portions of streams, that can differ in the species of predators that the guppies have to contend with. Some streams are high-predation environments, others  low-predation. Different predation environments are often right next to one another, separated by a waterfall (which neither guppies nor predators can cross). 
Guppies from high-predation environments experience much higher mortality rates than do guppies in low-predation environments. High mortality is associated with the following characteristics, all of which have a genetic basis:
  • Earlier maturity
  • Greater investment of resources in reproduction
  • More and smaller offspring.
We have found that mortality rates can be manipulated by:
  • Transplanting guppies from high-predation localities into sites from which they and their predators had previously been excluded by natural waterfalls, thus lowering mortality rates;
  • Introducing predators into low-predation sites, thus increasing mortality rates. 
Such experiments have shown that species evolve as predicted by theory. We have also found that evolution by natural selection can be remarkably fast, on the order of four to seven orders of magnitude faster than had been inferred from the fossil record.

Killifish

The killifish (Rivulus hartii) follows a similar pattern of evolutionary changes across different ecological settings in the streams of Trinidad. Several traits, however, distinguish killifish from guppies, especially their unique dispersal capabilities. Killifish can tolerate being out of the stream for extended periods and can navigate around barrier waterfalls to colonize upstream environments. 
Because of their dispersal ability, killifish occur across a gradient of predation environments separated by barrier waterfalls:
  • Downstream, high-predation habitats contain large predatory fish.
  • Upstream, low-predation habitats contain guppies (and sometimes non-predatory fish species)
  • Further upstream, killifish-only habitats have no other fish species
Killifish have evolved in response to this gradient of ecological predation pressure and mortality. As predation pressure (and mortality) increases downstream, killifish have evolved the following characteristics:
  • Earlier maturity
  • Greater investment of resources in offspring
  • More and smaller offspring
Our experiments with killifish have demonstrated that these changes have a genetic basis and can evolve rapidly (within 25 years).