Final answer:
Endler's research on the male guppy suggests that male colorfulness is influenced by both predation and sexual selection by females, with brighter colors in low predation areas due to female preference and drab colors in high predation environments for camouflage.
Step-by-step explanation:
Endler's research on the male guppy (Poecilia reticulata) indicates that male colorfulness is determined by both predation and sexual selection by females. His studies show that under conditions where there is little to no predation, male guppies develop more conspicuous colors, possibly because these bright colors are favored by females during mate selection. However, in environments with a high predation risk, male guppies exhibit less conspicuous colors, which likely serves as a camouflage against predators. Endler demonstrated these findings through a series of experiments testing guppies in environments with varying levels of predation, observing changes in male coloration over time.