Guppies (Poecilia reticulata) are a classic model for studying how predator pressure shapes the evolution of life history traits such as size at maturity and timing of reproduction. In their native Trinidadian streams, different predators exert different selective pressures on guppy populations, resulting in remarkable local adaptation.

Predation and Life History Evolution in Guppies

• Habitat A: Pike cichlid fishes are the main predators and preferentially feed on large, adult guppies. In response, guppies in these habitats tend to mature early at a smaller size to reproduce before they are likely to be eaten.

• Habitat B: Killer fish target small, juvenile guppies. Here, the highest mortality risk is at the juvenile stage, so guppies that survive this period can afford to grow larger and mature later, as adult mortality is lower.

What Happens When Guppies Are Transferred from Habitat A to Habitat B?

When guppies from Habitat A (where adult predation is high) are experimentally moved to Habitat B (where juvenile predation is high), the selective pressure shifts:

• In Habitat B, the main threat is to juveniles, not adults. Guppies that can survive the risky juvenile stage benefit from delaying reproduction and growing larger, as their risk of dying as adults is lower.

• Over generations, natural selection will favor guppies that mature later and at a larger size. This is because early-maturing, small adults (an adaptation to adult predation) are no longer at a survival advantage; instead, larger size and later maturity become advantageous.

Scientific Evidence

These patterns have been repeatedly documented in field and laboratory studies. When guppies are transplanted from high adult-predation sites to high juvenile-predation sites, their descendants evolve to mature later and at a larger size—the opposite of what is seen in their original habitat.

Conclusion

If guppies from habitat ‘A’ (pike cichlid, adult predation) are transferred to habitat ‘B’ (killer fish, juvenile predation), they will evolve to mature late but at a large size as adults.

Correct answer: (4) mature late but large size adults