Stress and Limits to Selection in Single Traits

Animal breeding and laboratory-based selection experiments on model species have shown that many species have the ability to evolve large phenotypic changes over short periods of time. Despite this there are many examples of evolutionary stasis within populations. Clearly, a simple explanation for this is an absence of genetic variance in traits under selection; however, a lack of genetic variance is rarely implicated when selection fails to produce a response. Nonetheless, examples of stasis in the presence of genetic variation are scattered throughout the literature so other factors must also be involved. There are several reasons why an evolutionary response may be low in populations including an absence of genetic variation, selection, negative consequences of low population sizes, asymmetrical gene flow and finally, complex genetic, ecological, or environmental interactions (Figure 1). These limits to stress adaptation are discussed below.

The most obvious factor that may limit stress adaptation is low levels of genetic variance. Variation is thought to be maintained in most traits through a balance between mutation and selection, and there is almost no empirical

Stressful conditions

Migration

Extinction

Migration

Trait

interactions

Asymmetrical gene flow

Adaptation

Asymmetrical gene flow

Figure 1 When faced with stressful conditions, organisms have three options: adapt, migrate, or face extinction. Migration will be influenced by ability to disperse and the availability of suitable habitat elsewhere. Adaptation is primarily influenced by the presence of genetic variance, which is influenced by population processes, including selection, population size, and genetic factors such as genetic interactions, mutation, recombination, and canalization. Trait interactions and asymmetrical gene flow may also limit adaptation to stress even when additive genetic variance is present.

Figure 1 When faced with stressful conditions, organisms have three options: adapt, migrate, or face extinction. Migration will be influenced by ability to disperse and the availability of suitable habitat elsewhere. Adaptation is primarily influenced by the presence of genetic variance, which is influenced by population processes, including selection, population size, and genetic factors such as genetic interactions, mutation, recombination, and canalization. Trait interactions and asymmetrical gene flow may also limit adaptation to stress even when additive genetic variance is present.

evidence in the literature for a lack of genetic variation in quantitative traits. However, the presence of additive genetic variance is required in specific traits for adaptation to occur, and the heritability of relevant traits has rarely been measured. Although directional selection may increase genetic variation while rare beneficial alleles increase in frequency, if continued selection causes these alleles to increase in frequency and approach and reach fixation, genetic variance is predicted to be reduced. Mutation may be adequate to maintain variation in traits even under intense levels of selection if the mutational target is large (enough loci are involved). However, a small mutation target size (small number of loci) or a low mutation rate may limit the amount of new variation being generated, resulting in lower levels of additive genetic variance for key traits involved in adaptation.

Adaptive shifts may also be influenced by low population sizes. Low size can adversely affect populations due to the negative effects of inbreeding and the increased action of genetic drift decreasing additive genetic variance. Breeding between relatives increases homozygosity, unmasking unfavorable allele combinations which tend to lead to a reduction in fitness and limit evolutionary change. Furthermore, the effects of genetic drift are greater in small populations where favorable alleles may be lost by chance. Human-induced habitat fragmentation is increasingly decreasing the size of many populations. Habitat fragmentation isolates populations, preventing gene flow and thus reducing effective population size. Restoring gene flow between habitats represents one effective way of reversing the effects of inbreeding and genetic drift.

Finally, interactions among alleles within (dominance) or between (epistasis) loci may enhance or constrain adaptation, depending on the intensity and directionality of the interactions. For example, positive epistatic interactions, where alleles systematically reinforce the effect of one another can increase a selection response, while negative epistatic interactions may constrain evolution by diminishing the effect of the interacting allele, 'hiding' this variation from selection. Chromosome inversion polymorphisms, where the arrangement of genes on particular chromosomes is reversed, can lock up particular combinations of alleles that are involved in an adaptive shift - because gametes become inviable when recombination occurs between inverted and noninverted chromosomes. Inversions and epistatic interactions are thought to evolve because certain combinations of alleles are favored in an adapted population. However, if environmental conditions alter, these interactions may no longer be favored and having alleles locked up could prevent an adaptive response.

While a lack of additive genetic variance can reduce the evolutionary potential ofa population, many traits reach an adaptive limit with ample genetic variation for selection to act upon. There are several reasons why genetic variation does not always ensure a selection response.

One reason is that a trait may still fail to respond because of the way that selection influences a trait. Selection may act on a nonheritable component of the phenotype rather than the heritable component. For instance while there is heritable variation for antler size in red deer (Cervus elaphus) and directional selection for increased size, this trait has not changed for many years. It appears that only the environmental component of variation in antler size is under selection - this component reflects the nutritional state of the organisms and is unrelated to their antler genotype.

Another reason is that while low or infrequent migration can adversely influence the amount of genetic variation available for selection, high and/or asymmetrical gene flow can negatively influence a population's ability to respond to selection. Unidirectional gene flow from central/source populations may prevent local adaptation within marginal border/sink populations. As a consequence of asymmetrical gene flow, border populations will not be locally adapted to their surrounding environment, preventing further range expansion. This hypothesis may explain the presence of species borders in the absence of any geographical barriers. There is evidence for low differentiation in marginal populations in the butterfly Atalopedes campestris despite strong selection for cold resistance, in support of this hypothesis. However, there are also examples of genetic differentiation in populations despite the presence of high gene flow. While asymmetrical gene flow has the potential to explain evolutionary stasis in the presence of stressful conditions, especially in border populations, there is little direct empirical evidence for gene flow limiting adaptive responses.

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