Species don't exhibit responses, individuals do...just as populations don't have physical traits, individuals do.
What you're essentialy promoting here is psychology, and psychology is not a science.
If natural selection can not explain physical changes within an organism, that are't already explained by mutation, drift, recombination, and heredity, then it has no business being in a scientific theory.
What is so hard to understand about that.
I'll tell, you guys have worn me out. Why is it so hard to answer a simple question about a mechanism, that supposedly is an observabble fact of nature.
Ever been to Africa? Try selling sunscreen next time you're there