Question

A male stickleback fish will attack other male sticklebacks that invade its nesting territory. it will only attack male fish, which display the red belly characteristic of the species. why has natural selection favored this behavior?

Answer 1

The conduct enables the male to build up a characterized space for reproducing with female sticklebacks. 
Natural selection is the differential survival and generation of people because of contrasts in phenotype. It is a key instrument of development, the change in heritable qualities of a populace after some time.