Instructions

QUESTION 2 Guppies (Poecilia reticulata) are a very popular species of tropical ornamental fish that can be found in small ponds in South America. Throughout its distribution, guppies are known to inhabit mountain streams containing waterfalls. Because guppies are unable to swim up waterfalls, populations up and down the waterfall are commonly isolated from each other. In one such system, in the Island of Trinidad, populations that were separated by a waterfall started to diverge morphologically. While all guppies present some heritable color variation, individuals from upstream started to present more brightly intense coloration than downstream ones. These differences seemed to be almost exclusively restricted to male guppies, and not females. Furthermore, some aspects of the male coloration pattern seem to be associated with environmental factors. For example, the number of bright yellow spots on the flank of males is associated with the amount of carotenoids an individual consumes. An investigation of both upstream and downstream environments shows no difference in the type of food available for guppies. The same investigation revealed that there is a clear difference in predation risk in both environments: while downstream ponds are home for pike cichlids (Crenicichla alta), a common visual predator of guppies, upstream ponds are devoid of predators. Upper pond Low predation Figure 2- Schematics of guppies morphological differences between upper and lower ponds. Explain the selective forces that are at play in shaping the color divergence between upstream and downstream populations of guppies. A complete answer should take into account which and how different selective pressures affect the components of fitness.