Predation and the Evolution of a Stickleback (Gasterosteus)

In western North America there are populations of Gasterosteus in which the male nuptial colour is a deep black rather than the normal bright red. Such black populations are usually allopatric to red populations, but in some areas the two forms occur in adjacent sympatry.The difference in male nuptial colour and a number of other morphological and physiological traits reflect genetic differences between black and red populations. Male nuptial colour functions as a partial isolating mechanism. Mate selection experiments indicate that females from red populations rarely accept a black male when they have a choice between a red or black male. Females from allopatric black populations also show a preference for red males, but females from black populations immediately upstream from a hybrid zone appear to mate randomly. The significant difference in mate preference between allopatric and sympatric populations of black females is evidence for the reinforcement of male colour as an isolating mechanism. The selective mechanism reinforcing colour as an isolating mechanism is hybrid inferiority due to genetic incompatibility and aberrant behaviour in hybrid males.A hypothesis is presented that persistent differential predation by an endemic predator (Novumbra) on the young concentrated near the male nest site was the selective force responsible for the evolution of the black genotype. Experiments demonstrate that Novumbra is differentially attracted to red males when given a choice between red and black males, and that the young of black sticklebacks possess an innate behavioural response to Novumbra that makes them significantly less vulnerable to predation by Novumbra.