The Nature of Limits to Natural Selection

Abstract

Insufficient genetic variability and the swamping effects of gene flow are inadequate explanations of limits to natural selection.Comparison of evolutionary responses in different populations subjected to similar selective forces, comparison of rare and widespread species, and comparison of marginal and central populations are all neglected research areas that bear on the nature of limits to natural selection.Plant populations provide us with well-defined, operationally viable systems for addressing these comparisons.Several possible constraints on range extension of ecologically marginal populations are considered in detail.Selection on fitness components that are themselves negatively correlated will be ineffective: such negative correlations are to be expected in natural populations.Small size of marginal populations will reduce severely the probability of obtaining appropriate character combinations; it will increase the swamping effects of gene flow; and it may lead to inbreeding depression effects.Gene flow will have different effects depending on whether the genes concerned are effectively neutral, advantageous, or deleterious in the population into which they migrate.Gene flow will spread beneficial genes rapidly, but may retard divergence if density of marginal populations is low and swamping effects are high.Finally a population entering a new habitat is likely to meet new competitors and predators: the revolutionary responses of the latter may counteract adaptive responses by the species undergoing range extension.All these factors are likely to interact in important ways in marginal populations.The study of limits to natural selection is likely to be a fruitful future research area, and one in which the detailed documentation of the systematist will provide invaluable baseline information.