Although we have so far focused on the effects of stressful conditions on single traits, there is a growing realization that the direction of evolution can be restricted by interactions among traits. Different traits can be genetically correlated, mostly due to the fact that the same genes can influence the expression of multiple traits (i.e., they exhibit pleiotropy). This means that variation in one trait will often be correlated with variation in different sets of traits. When selection is pushing a trait in one direction, the response to selection can be constrained because selection is at the same time pushing different traits into other directions.
The simplest form of pleiotropy that constrains selection on traits involves antagonistic pleiotropy. This process is commonly envisaged as occurring among two traits; for instance, increased cold resistance may be associated with a decrease in the early reproductive output of females. Selection under stressful conditions may act to increase cold resistance, but there will be an associated reduction in reproductive output, ultimately providing a limit on the extent to which cold resistance might increase. In reality, pleiotropic interactions involve multiple sets of traits, and several of these might be under selection. Identifying constraints to evolution depends on understanding whether the direction of selection can proceed when levels of genetic variation in a set of traits and genetic interactions among these traits are understood. The response to selection for a complex of traits is given by Az = Gfi, where z is a vector of the response of individual traits, G is the genetic variance-covariance matrix, and @ is the vector of linear selection gradients. The response of traits within the vector z depends critically on G, which is effectively a matrix that includes VA in its leading diagonal (as a measure of genetic variance) and genetic interactions among traits in its off-diagonal components. Particular forms of G may mean that traits might not respond at all to selection or they may respond in a direction opposite to the one in which selection acts.
A difficulty in making predictions based on G is that genetic interactions among traits, just like the genetic variance of traits, can change dramatically depending on environmental conditions particularly when these are stressful. For instance, there is a well-known interaction between development time and egg production (faster developing genotypes often have lower levels of egg production), but this interaction can change sign when organisms are exposed to stressful conditions such that rapidly developing individuals have a higher reproductive output.
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