Recombining Pollution: Natural Selection's Influence

The recombination of pollution can influence natural selection by creating selective conditions that make recombinants directly and immediately advantageous. Natural selection itself can be a source of pervasive imbalance, with the rationale that the ubiquity of natural selection approximates the ubiquity of sex and recombination in nature. Natural selection is fed by heritable variation which may be produced by factors such as drift, founder effects, migration and mutation.

In the case of the Atlantic killifish (Fundulus heteroclitus), large populations of this species persist in some of the most contaminated estuaries of the United States. Studies have shown that selection by industrial pollutants and other stressors has acted on multiple populations of killifish and posit that extreme nucleotide diversity uniquely positions this species for successful evolutionary adaptation.

Natural selection can act at many loci across the genome

Recombination is an important component of adaptive evolution based on multi-locus selection. High-fitness genotypes are expected to be absent from the initial population when the frequencies of favourable alleles at the selected loci are low. As the allele frequencies increase in response to selection, the missing genotypes will be generated by recombination.

In the absence of recombination, selection will be limited to the highest-fitness genotypes that are already present in the initial population. Recombination acts to unblock the adaptive response to multi-locus selection in finite populations.

Recombination can also occur between two individuals within the same population. In this case, the measure of selective mismatch across loci affecting the evolution of recombination is linkage disequilibrium, which measures bias in allelic frequencies across loci but does not retain information about the selective value of those alleles.

Overall, the effect of natural selection is to always promote the evolution of recombination, on average, independently of the source of the variation that feeds it.

The number of possible genotypic combinations increases exponentially as the number of polymorphic loci increases linearly

Natural selection can act on many loci across the genome. However, as the number of polymorphic loci increases linearly, the number of possible genotypic combinations increases exponentially. This means that even a very large population will only contain a small sample of all possible multi-locus genotypes.

The classic Fisher-Muller models of recombination demonstrate this phenomenon, taking into account the abundant standing variation that is commonly seen in natural populations. Specifically, high-fitness genotypes are likely to be absent from the initial population when the frequencies of favourable alleles at the selected loci are low. However, as the allele frequencies increase in response to selection, the missing genotypes will be generated through recombination.

Given recombination, if the average frequency of the favoured alleles at the selected loci is equal to p, then the expected number of favourable alleles per chromosome will be equal to pL, where L is the number of loci. As the value of p approaches unity at the selected loci, the number of favourable alleles per chromosome will approach a value of L, i.e., at the end of the selection process, a favourable allele will be found at all loci.

In the absence of recombination, selection will be limited to the highest-fitness genotypes that are already present in the initial population. The fitness of such initial genotypes is far less than the theoretical maximum fitness because they contain a favourable allele at only a fraction of the loci. Consequently, recombination acts to unblock the adaptive response to multi-locus selection in finite populations.

Recombination is favoured under these conditions because it breaks up the mismatched combinations and assembles matched combinations

The following answer is based on the assumption that the user is referring to the following sentence from the provided text: "Recombination is favoured under these conditions because it breaks up the mismatched combinations and assembles matched combinations".

Recombination acts to unblock the adaptive response to multi-locus selection in finite populations

Natural selection can act at many loci across the genome. However, as the number of polymorphic loci increases linearly, the number of possible genotypic combinations increases exponentially. Consequently, a finite population – even a very large one – contains only a small sample of all possible multi-locus genotypes.

In this paper, we revisit the classic Fisher-Muller models of recombination, taking into account the abundant standing variation that is commonly seen in natural populations. We show that the generation of new genotypic combinations through recombination is an important component of adaptive evolution based on multi-locus selection.

High-fitness genotypes are expected to be absent from the initial population when the frequencies of favourable alleles at the selected loci are low. But as the allele frequencies rise in response to selection, the missing genotypes will be generated by recombination. Given recombination, if the average frequency of the favoured alleles at the various selected loci is equal to p, then the expected number of favourable alleles per chromosome will be equal to pL, where L is the number of loci. As the value of p approaches unity at the selected loci, the number of favourable alleles per chromosome will approach a value of L, i.e., at the end of the selection process, a favourable allele will be found at all loci.

In the absence of recombination, however, selection will be limited to the highest-fitness genotypes that are already present in the initial population. We point out that the fitness of such initial genotypes is far less than the theoretical maximum fitness because they contain a favourable allele at only a fraction of the loci. Consequently, recombination acts to unblock the adaptive response to multi-locus selection in finite populations.

Using simulations, we show that the sexual population can withstand invasion by newly arising asexual clones. These results help explain the maintenance of sexual reproduction in natural populations.

The evolutionary influence of anthropogenic stressors as selective agents occurs on a geographic scale much larger than expected

In the case of the killifish, large populations of Atlantic killifish (Fundulus heteroclitus) have been found to persist in some of the most contaminated estuaries of the United States. Killifish studies have provided some of the first insights into the types of genomic changes that enable rapid evolutionary rescue from complexly degraded environments. Mechanistic studies have identified some of the genetic underpinnings of adaptation to a well-studied class of toxic pollutants; however, multiple genetic regions under selection in wild populations seem to reflect more complex responses to diverse native stressors and/or compensatory responses to primary adaptation. The discovery of these pollution-adapted killifish populations suggests that the evolutionary influence of anthropogenic stressors as selective agents occurs widely.

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