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.

Evolutionary vestigialization of sex in a clonal plant: selection versus neutral mutation in geographically peripheral populations

“…A simulation model by Peck et al (1998) predicts strong disequilibrium between sexual sterility and increased fitness in populations at geographical range margins, a situation that we have observed in D. verticillatus. The model is based on the premise that only rare gene combinations provide tolerance to the extreme environments in peripheral populations (Antonovics 1976;Bradshaw 1991). Sterility mutations protect these high fitness genotypes from being broken up by sexual recombination (e.g.…”

Change in genetic architecture resulting from the evolution of insecticide resistance: a theoretical and empirical analysis

“…Accordingly, both major and minor mutations can now contribute to the evolution of the resistant phenotypes in this novel habitat (Lande, 1983). The spread of the resistance allele(s), however, apparently resulted in local increases in pleiotropic costs affecting the fitness components, which is one element that may impose limits on evolution (Antonovics, 1976;Bradshaw, 1991), especially in the absence of strong directional selection (Wright, 1982;Lande, 1983;Lenski, 1988). This study also provides a basis for understanding how the genetic properties of populations vary during the transient period over which the resistance allele(s) increases toward fixation.…”

Gene flow among established Puerto Rican populations of the exotic tree species, Albizia lebbeck

“…For many species, there is a strong genetic component to invasiveness, indicated by the presence of both invasive and noninvasive conspecific populations (eg, Sakai et al, 2001). Extensive gene flow may produce novel combinations of alleles, allowing invasive genotypes to form (Ellstrand and Schierenbeck, 2000), and providing the genetic diversity needed to adapt to a wide range of habitats during range expansion (Antonovics, 1976;Crawley, 1986;Hengeveld, 1990). In introduced populations of reed canary grass (Phalaris arundinacea), for instance, one genotype grew faster under strong competition, while a different genotype had the fastest growth rate with minimal competition (Morrison and Molofsky, 1999).…”