What might natural selection ever cause population to lose

An endemic fish living in freshwater blue hole in the Bahamas, Poecilia bahamania, has two color morphs, a red and blue morph. You have observed that an invasive species of cichlid that was introduced to the blue hole five years before seems to prefer to eat the red morph of the native fish. You even go so far as to conduct an experiment that the cichlid will eat the red Poecilia twice as often as the blue. However, in those five years, the relative frequency of Poecilia has not noticeably changed. What might account for this observation and might natural selection ever cause the population to lose the red morph?

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