Abstract
Darwin insisted that evolutionary change occurs very slowly over long periods of time, and this gradualist view was accepted by his supporters and incorporated into the infinitesimal model of quantitative genetics developed by R. A. Fisher and others. It dominated the first century of evolutionary biology, but has been challenged in more recent years both by field surveys demonstrating strong selection in natural populations and by quantitative trait loci and genomic studies, indicating that adaptation is often attributable to mutations in a few genes. The prevalence of strong selection seems inconsistent, however, with the high heritability often observed in natural populations, and with the claim that the amount of morphological change in contemporary and fossil lineages is independent of elapsed time. I argue that these discrepancies are resolved by realistic accounts of environmental and evolutionary changes. First, the physical and biotic environment varies on all time-scales, leading to an indefinite increase in environmental variance over time. Secondly, the intensity and direction of natural selection are also likely to fluctuate over time, leading to an indefinite increase in phenotypic variance in any given evolving lineage. Finally, detailed long-term studies of selection in natural populations demonstrate that selection often changes in direction. I conclude that the traditional gradualist scheme of weak selection acting on polygenic variation should be supplemented by the view that adaptation is often based on oligogenic variation exposed to commonplace, strong, fluctuating natural selection.
Highlights
Darwin insisted that evolutionary change occurs very slowly over long periods of time, and this gradualist view was accepted by his supporters and incorporated into the infinitesimal model of quantitative genetics developed by R
THE OLIGOGENIC MODEL The renaissance of evolutionary biology as an empirical and experimental science began in the 1950s, from a variety of sources
(i) The physical conditions of life change continually on all time-scales, (ii) this contributes to the continual change in community composition, (iii) changes in the physical and biotic conditions of life impose strong directional selection on populations, (iv) as the agents of selection vary continually over time, the direction of selection often changes, and adaptive walks are often interrupted, (v) beneficial alleles of large effect are likely to be the first to be fixed in an adaptive walk, and (vi) adaptation will often involve alleles of large effect
Summary
The central problem of evolutionary biology is to explain how adaptation to particular environmental conditions evolves through natural selection. The acolyte of Darwin: Mr Darwin was rather inclined to exaggerate the necessary slowness of the action of natural selection; but with the knowledge we possess of the great amount and range of individual variation, there seems no difficulty in an amount of change, quite equivalent to that which usually distinguishes allied species, sometimes taking place in less than a century, should any rapid change in conditions necessitate an rapid adaptation The gradual modification of characters in fossil time-series is attributed to stabilizing selection (acting against extreme phenotypes) around an optimal value that changes slowly over time so as to generate very weak but consistent directional selection In this way, the mathematical theory of population genetics that was developed during the twentieth century formalized and reinforced the gradualist foundation laid down in the nineteenth century
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