   Biologists have discovered many ways in which the genetic details of trait determination and/or ecological details of the population can prevent the evolution of optima. For example, interaction between loci (linkage or pleiotropy) can prevent the optimal phenotype from being fixed in a population, and thereby prevent the maximization of mean fitness (Moran 1964). Such a deviation from the state of highest fitness is called genetic load. The standard adaptationist response is that genetic and ecological constraints are eliminated over evolutionary time by the introduction of new genetic variation via mutation (Parker and Maynard Smith 1990).