  The selection pressures that have shaped the evolution of complex traits in
humans remain largely unknown, and in some contexts highly contentious, perhaps
above all where they concern mean trait differences among groups. To date, the
discussion has focused on whether such group differences have any genetic
basis, and if so, whether they are without fitness consequences and arose via
random genetic drift, or whether they were driven by selection for different
trait optima in different environments. Here, we highlight a plausible
alternative, that many complex traits evolve under stabilizing selection in the
face of shifting environmental effects. Under this scenario, there will be
rapid evolution at the loci that contribute to trait variation, even when the
trait optimum remains the same. These considerations underscore the strong
assumptions about environmental effects that are required in ascribing trait
differences among groups to genetic differences.
