question: Can a given genotype exhibit opposite fitness effects (beneficial and detrimental) within the same environment?
answer: A given genotype can be either beneficial or detrimental, even deleterious, depending on the environment in which an organism lives. This is known as antagonistic pleiotropy. Antagonistic pleiotropy can operate even within the same environment. For example, in Escherichia coli, certain mutations can exhibit beneficial, deleterious or neutral fitness effects at different growth rates. Also, antagonistic pleiotropy is involved in the evolution of ageing, since a certain genotype may affect late- and early-life fitness in opposite directions.