An example of natural selection is the red color of a male cardinal. the females of the species choose mates based on the vibrant colors of the males’ feathers. if females begin using different criteria than feather color when they choose mates, what would most likely happen to the tails of peacocks over time?

If female’s species would chose their mates using different criteria other than feather, it may lead to decrease in variation in tail feather color because the only reason for the variation was selection pressure.  

Explanation:

The opposite gender for mating are chosen based on specific criteria like color of feathers, eye color etc..The selection of opposite gender is the natural selection considering the red color of the male cardinal. This red color male cardinal which depends only on the sexual selection pressure will now reduce. As this was now neutralized it may lead to decreased variation.

"do a testcross with a homozygous recessive plant."

that scientist would be robert hooke