What would happen if selective pressure changed and the recessive allele was selected FOR?

Answer 1

Well, it really depends on a lot of different variables. I guess you are saying that since it is being selected for, then the trait the allele codes for is important for survival. I would then have to assume that you mean that the recessive allele being selected for is homozygous. Then I would assume that the allele you are talking about was not being selected for originally.

If the dominant form of the trait was previously being selected before (before this 'switch' happened), then after the change, there would be a reduction in the number of organisms in the population. This would be because the homozygous dominant and heterozygous individuals would no longer have the best fitness.

Also from your question, it seems that you think recessive alleles are never selected for.