Evolutionary quantitative genetics: Are we in danger of throwing out the baby with the bathwater?

Quantitative genetics has a long and venergenetic theory is able to account for the inher able history: the question I wish to consider in itance of quantitative traits such as body size, this essay is the future of quantitative genetics. fecundity, etc., within the Mendelian framework. Evolutionary biology, like most fields of seiIndeed in many ways Mendelian genetics is ence, tends to suffer from a band-wagon effect, really a subset of quantitative genetics rather which eventually leads to disillusionment and than vice versa. The units of quantitative genetics abandonment. The reason for this is that too are discrete elements, the genes, but the behavior much is expected of some new or revived area of these elements is considered in the aggregate of research. Quantitative genetics as applied to in the same manner as the properties of a gas are evolutionary biology received a major boost studied as the epiphenomenon of the collection from the extensive work of Lande, who showed of individual gas molecules. This is not to say how the equations of quantitative genetics could that the behavior of individual particles is not of be expanded and applied to problems beyond interest or might not be the focus of investiga livestock and crop improvement. This research tion under some circumstances. Rather, if the initiated a flurry of activity in quantitative genetquestion revolves around the epiphenomenon, ics, both in further theoretical development and such as the phenotype then the appropriate scale empirical determination of genetical parameters is beyond the individual elements. One of the such as heritability, evolvability and genetic corareas of growing interest to which I shall return relations. This early enthusiasm has largely been is indeed the dissection of the behavior of these expended and there is the risk of quantitative elements at varying levels, genetics falling on hard times, being considered Another significant contribution of quantita old-fashioned and out-moded in comparison tive genetics is to an understanding of inbreeding to the new area of molecular genetics. My purdepression. The deleterious effects of inbreeding pose in this essay is to try to place quantitative have been recognized for a considerable amount genetics in proper perspective, point out that it of time, possibly being the basis of many mar has had some fabulous successes, and that there riage taboos. It is not a priori obvious that for are central questions of evolutionary biology that inbreeding depression to occur there must be require a quantitative genetic perspective in condirectional dominance: a trait that shows only junction with other modalities of investigation. additive genetic variation will not suffer inbreed The first achievement of quantitative geneting depression. That this directional dominance ics was the resolution of the debate between can arise from heterozygosity per se or del the Mendelians and Biometricians. Quantitative eterious recessives is a corollary of the theory