Interaction of Selection, Mutation, and Drift

This chapter examines the joint impact of selection, mutation, and drift on the allele frequencies at a locus. One key finding is that if the strength of selection is sufficiently weak relative to drift, alleles behave as if they are effectively neutral. Hence, as a population attempts to evolve toward some ideal (optimal) value, the beneficial increment from new mutations eventually becomes sufficiently weak (relative to drift) they are efficiently neutral, implying that perfect adaptation is never possible. This is the notion of the drift barrier. Another key ideal is Haldane's principle: the decline in mean population fitness generated by deleterious mutations is largely independent of their selective effects, but instead is simply a function of their mutation rate.

Captive Ancestry Upwardly Biases Estimates of Relative Reproductive Success

Supplementation programs, which release captive-born individuals into the wild, are commonly used to demographically bolster declining populations. In order to evaluate the effectiveness of these programs, the reproductive success of captive-born individuals released into the wild is often compared to the reproductive success of wild-born individuals in the recipient population (relative reproductive success, RRS). However, if there are heritable reductions in fitness associated with captive breeding, gene flow from captive-born individuals into the wild population can reduce the fitness of the wild population. Here, we show that when captive ancestry in the wild population reduces mean population fitness, estimates of RRS are upwardly biased, meaning that the relative fitness of captive-born individuals is over-estimated. Furthermore, the magnitude of this bias increases with the length of time that a supplementation program has been releasing captive-born individuals. This phenomenon has long-term conservation impacts since management decisions regarding the design of a supplementation program and the number of individuals to release can be based, at least in part, on RRS estimates. Therefore, we urge caution in the interpretation of relative fitness measures when the captive ancestry of the wild population cannot be precisely measured.

Evolution of clonal populations approaching a fitness peak

Populations facing novel environments are expected to evolve through the accumulation of adaptive substitutions. The dynamics of adaptation depend on the fitness landscape and possibly on the genetic background on which new mutations arise. Here, we model the dynamics of adaptive evolution at the phenotypic and genotypic levels, focusing on a Fisherian landscape characterized by a single peak. We find that Fisher's geometrical model of adaptation, extended to allow for small random environmental variations, is able to explain several features made recently in experimentally evolved populations. Consistent with data on populations evolving under controlled conditions, the model predicts that mean population fitness increases rapidly when populations face novel environments and then achieves a dynamic plateau, the rate of molecular evolution is remarkably constant over long periods of evolution, mutators are expected to invade and patterns of epistasis vary along the adaptive walk. Negative epistasis is expected in the initial steps of adaptation but not at later steps, a prediction that remains to be tested. Furthermore, populations are expected to exhibit high levels of phenotypic diversity at all times during their evolution. This implies that populations are possibly able to adapt rapidly to novel abiotic environments.

THE EVOLUTION OF EPISTASIS AND THE ADVANTAGE OF RECOMBINATION IN POPULATIONS OF BACTERIOPHAGE T4

ABSTRACT Experiments reported here test two hypotheses about the evolution of recombination: first, the Fisher-Muller concept that sexual organisms respond to selection more rapidly than do asexual ones, and second, that epistasis is more likely to evolve in the absence of recombination. Populations of bacteriophage T4 were selected by the drug proflavine in discrete generations and the change in mean population fitness was monitored. Three separate selection series yielded results supporting the Fisher-Muller hypothesis. The amount of epistasis evolved was measured by partitioning the T4 map into regions and comparing the sum of the proflavine resistances of each region with the resistance of the whole. Significantly more interactions were found in phage isolated from the populations with lower total recombination than in those from populations with higher recombination. The degree to which these experiments fit preconceived notions about natural selection suggests that microorganisms may be advantageously used in other population genetics experiments.

An experiment on recombination load in Drosophila melanogaster

SUMMARYThis paper describes the results of an experiment to measure the effect on mean population fitness of recombination in the second chromosome of Drosophila melanogaster. There was a small and non-significant effect of recombination in lowering egg-to-adult viability of heterozygotes for wild-type chromosomes. A large (7%) and significant effect of recombinant chromosomes on the fecundity of Cy female carriers was detected.