Content area
Full Text
REVIEWS
THE GENETIC THEORY OF ADAPTATION: A BRIEF HISTORY
H. Allen Orr
Abstract | Theoretical studies of adaptation have exploded over the past decade. This work has been inspired by recent, surprising findings in the experimental study of adaptation. For example, morphological evolution sometimes involves a modest number of genetic changes, with some individual changes having a large effect on the phenotype or fitness. Here I survey the history of adaptation theory, focusing on the rise and fall of various views over the past century and the reasons for the slow development of a mature theory of adaptation. I also discuss the challenges that face contemporary theories of adaptation.
Adaptation is not natural selection. As Ronald A. Fisher1 emphasized in 1930, adaptation is characterized by the movement of a population towards a phenotype that best fits the present environment. The result is an often astonishingly precise match between an organism and the world in which it lives. But as Fisher also emphasized, the steady increase in the frequency of an allele under selection need not invariably result in the adaptive modification of specific forms. Competition among selfish genes, for instance, can change a populations sex ratio but we have no reason to expect any improved fit between an organism and its environment.
To an evolutionary geneticist, there is another, simpler way of distinguishing between selection and adaptation: we know a lot about the former but little about the latter. Many important questions about the genetic basis of adaptation remain unanswered. Do most adaptations involve new mutations or STANDING GENETIC
VARIATION? Do most adaptations involve single genes of large phenotypic effect (major genes)? If so, can we say anything about the expected effect of this major gene? Can we describe the distribution of phenotypic effects among the mutations that are substituted during a typical bout of adaptation? How does FITNESS change as a
population approaches an optimum? For example, do populations evolve quickly at first and then more slowly? Because random mutations are more likely to be deleterious in complex organisms than in simple organisms, do complex organisms adapt more slowly than simple ones?
STANDING GENETIC VARIATION
Allelic variation that is currently segregating within a population; as opposed to alleles that appear by new mutation events.