Charles F. Baer

How and When Selection Experiments Might Actually be Useful

Abstract Laboratory natural selection and artificial selection are vital tools for addressing specific questions about evolutionary patterns of variation. Laboratory natural selection can illuminate whether a putative selective agent is capable of generating long-term, sustained changes in individual traits and suites of traits. Artificial selection is the essential tool for understanding the general evolvability of traits and the extent to which genetic correlations constrain evolution. We review the contexts in which each type of experiment seems capable of offering key insights into important evolutionary issues. We also discuss theoretical and methodological considerations that play critical roles in designing selection experiments that are relevant to evolutionary patterns of trait variation. In particular, we focus on the critical role of selection intensity and the consequences of experiments with different intensities. While selection experiments are not practical in many cases, sophisticated selection experiments—designed with careful consideration of the theory of selection—should be taken beyond model organisms and used in well-chosen natural systems to understand natural patterns of variation.

Phylogeography of a parasitoid wasp (Diaeretiella rapae): no evidence of host‐associated lineages

The exceptional diversity of insects is often attributed to the effects of specialized relationships between insects and their hosts. Parasite–host interactions are influenced by current natural selection and dispersal, in addition to historical effects that may include past selection, vicariance, and random genetic drift. Both current and historical events can lead to reduced fitness on some hosts. If trade‐offs in fitness on alternate hosts are common, adaptation to one host can prevent adaptation to another, giving rise to genetic differentiation among host‐associated lineages. Previous studies of Diaeretiella rapae (Hymenoptera: Aphidiidae), a parasitoid of aphids, have revealed additive genetic differences in performance between populations that parasitize different aphid host species. To determine whether D. rapae populations collected from different aphid hosts have diverged into genetically independent lineages, we constructed a haplotype network based on sequence variation in mitochondrial DNA (mtDNA). We used single strand conformation polymorphism (SSCP) analysis to examine 2041 base pairs of mtDNA and to identify nucleotide sequences of 42 unique SSCP haplotypes. We found no association between mtDNA haplotypes and host species in either the ancestral range (Europe, Mediterranean region, Middle East, Asia) or part of the introduced range (western North America). Haplotypes likely to be ancestral were geographically widespread and found on both hosts, suggesting that the ability to use both hosts evolved prior to the diversification of the mtDNA. Ongoing gene flow appears to prevent the formation of host races.

Cumulative Effects of Spontaneous Mutations for Fitness in Caenorhabditis: Role of Genotype, Environment and Stress

It is often assumed that the mutation rate is an evolutionarily optimized property of a taxon. The relevant mutation rate is for mutations that affect fitness, U, but the strength of selection on the mutation rate depends on the average effect of a mutation. Determination of U is complicated by the possibility that mutational effects depend on the particular environmental context in which the organism exists. It has been suggested that the effects of deleterious mutations are typically magnified in stressful environments, but most studies confound genotype with environment, so it is unclear to what extent environmental specificity of mutations is specific to a particular starting genotype. We report a study designed to separate effects of species, genotype, and environment on the degradation of fitness resulting from new mutations. Mutations accumulated for >200 generations at 20° in two strains of two species of nematodes that differ in thermal sensitivity. Caenorhabditis briggsae and C. elegans have similar demography at 20°, but C. elegans suffers markedly reduced fitness at 25°. We find little evidence that mutational properties differ depending on environmental conditions and mutational correlations between environments are close to those expected if effects were identical in both environments.

DIRECT AND CORRELATED RESPONSES TO ARTIFICIAL SELECTION ON ACUTE THERMAL STRESS TOLERANCE IN A LIVEBEARING FISH

Abstract.— Tradeoffs in performance or fitness across environments have important implications regarding the nature of evolutionary constraints. It remains controversial whether tradeoffs such as these reflect genetic correlations that are genuine evolutionary constraints. However, if such long‐term genetic constraints do exist, they must be due to underlying pleiotropy such that alleles that confer high performance in one environment invariably confer low performance in another. The distribution of genetic correlations within and among populations can provide insight about the existence of such pleiotropic tradeoffs. The long‐term association of certain teleost fish taxa with particular abiotic environments suggests that tradeoffs in performance across environments have constrained the geographic distribution of those taxa. Here we report the results of an experiment in which we artificially selected on acute heat‐ and cold‐stress tolerance in two stocks of the poeciliid fish Heterandria formosa from source populations with different thermal histories. Unexpectedly, we observed no direct responses to selection. Under certain conditions, fish from the different source populations differed significantly in cold tolerance, but not in heat tolerance. The results suggest there are no strong pleiotropic tradeoffs between heat‐ and cold‐stress tolerance in these populations.

Mutational Bias for Body Size in Rhabditid Nematodes

Mutational bias is a potentially important agent of evolution, but it is difficult to disentangle the effects of mutation from those of natural selection. Mutation-accumulation experiments, in which mutations are allowed to accumulate at very small population size, thus minimizing the efficiency of natural selection, are the best way to separate the effects of mutation from those of selection. Body size varies greatly among species of nematode in the family rhabditidae; mutational biases are both a potential cause and a consequence of that variation. We report data on the cumulative effects of mutations that affect body size in three species of rhabditid nematode that vary fivefold in adult size. Results are very consistent with previous studies of mutations underlying fitness in the same strains: two strains of Caenorhabditis briggsae decline in body size about twice as fast as two strains of C. elegans, with a concomitant higher point estimate of the genomic mutation rate; the confamilial Oscheius myriophila is intermediate. There is an overall mutational bias, such that mutations reduce size on average, but the bias appears consistent between species. The genetic correlation between mutations that affect size and those underlying fitness is large and positive, on average.

SPECIES-WIDE POPULATION STRUCTURE IN A SOUTHEASTERN U.S. FRESHWATER FISH, HETERANDRIA FORMOSA: GENE FLOW AND BIOGEOGRAPHY

The phylogeography of the freshwater fish fauna of the southeastern United States has almost achieved paradigm status in evolutionary biology (Avise 1992), and the major geographic features responsible for shaping species distributions are well‐characterized. Nevertheless, variation among species in distributions of allele or haplotype frequencies suggests that species‐specific processes (e.g., migration) may also play a role in establishing those distributions. There has also been relatively little investigation into how population structure may differ among subregions in the Southeast, for example, on the Florida peninsula versus the U.S. mainland to the northwest and/or northeast. The geology of the peninsula is such that both physical and biotic fluctuations may have been (and still be) particularly important in establishing the population structure of freshwater taxa. This possibility leads to two interesting questions in population genetics. (1) Does gene flow in freshwater species of the region better approximate a one‐ or two‐dimensional pattern? (2) Are populations on the peninsula farther from migration‐genetic drift equilibrium than their counterparts on the mainland? These questions were addressed by examining the population strucuture of a livebearing fish, Heterandria formosa; several features of the biology of the species make it particularly likely that recent gene flow has been important in its evolution. I surveyed electrophoretic variation in 34 populations distributed throughout the species range. The phylogeographic patterns observed are in general concordance with those found in other species, although with some differences. A two‐dimensional hypothesis of gene flow on the Florida peninsula better explains the data than does a one‐dimensional one. There is no evidence that populations on the peninsula are farther from migration‐drift equilibrium than those to the northwest. Populations in the northeast have lower genetic diversity than those to the south and west and show no isolation by distance; those results are consistent with a recent range expansion into the northeast, although smaller historical effective population sizes could also explain the pattern.

Population structure in a south-eastern US freshwater fish, Heterandria formosa . II. Gene flow and biogeography within the St. Johns River drainage

A previous study of gene flow in Heterandria formosa, the least killifish, suggested that populations on the Florida peninsula may be close to equilibrium for migration and genetic drift, but the high average value of Nem calls that conclusion into question. In this study I examine patterns of gene flow at a fine scale within the St. Johns River drainage to test the predictions that (i) Nem within a drainage should be equal or greater than that among drainages, and (ii) gene flow within a drainage should approximate a one-dimensional stepping-stone system at equilibrium. I used four isozyme systems (six loci) that were polymorphic in the previous study to characterize population structure and gene flow. Nem within each river was equal to or greater than the average value reported in the previous study, which strengthens the equilibrium interpretation of the species-wide results. However, the results showed no isolation by distance within two forks of the St. Johns drainage (the upper St. Johns and the Ocklawaha rivers), but significant differentiation between the two forks. This result has two implications: (i) that populations within each river are not at equilibrium; and (ii) that there was a historical barrier to migration between the two forks. There were substantial differences among loci, but an (appropriate) application of Lewontin and Krakauers test revealed no evidence that locus-specific forces led to a violation of the assumption of neutrality.

Experimental evolution in Heterandria formosa ,a livebearing fish: group selection on population size

Group selection has historically been an important and controversial subject in evolutionary biology. There is now a compelling body of evidence, both theoretical and experimental, that group selection not only can be effective, but can be effective in situations when individual selection is not. However, experiments in which true population-level traits have been shown to evolve in response to group selection are currently limited to two species of flour beetle in the genus Tribolium and RNA viruses. Here we report the results of an experiment wherein we imposed group selection via differential extinction for increased and decreased population size at 6-week intervals, a true population-level trait, in the poeciliid fish Heterandria formosa. In contrast to most other group selection experiments, we observed no evolutionary response after six rounds of group selection in either the up- or down-selected lines. Populational heritability for population size was low, if not actually negative. Our results suggest that group selection via differential extinction may be effective only if population sizes are very small and/or migration rates are low.

The mutational structure of metabolism in Caenorhabditis elegans.

A properly functioning organism must maintain metabolic homeostasis. Deleterious mutations degrade organismal function, presumably at least in part via effects on metabolic function. Here we present an initial investigation into the mutational structure of the Caenorhabditis elegans metabolome by means of a mutation accumulation experiment. We find that pool sizes of 29 metabolites vary greatly in their vulnerability to mutation, both in terms of the rate of accumulation of genetic variance (the mutational variance, VM) and the rate of change of the trait mean (the mutational bias, ΔM). Strikingly, some metabolites are much more vulnerable to mutation than any other trait previously studied in the same way. Although we cannot statistically assess the strength of mutational correlations between individual metabolites, principal component analysis provides strong evidence that some metabolite pools are genetically correlated, but also that there is substantial scope for independent evolution of different groups of metabolites. Averaged over mutation accumulation lines, PC3 is positively correlated with relative fitness, but a model in which metabolites are uncorrelated with fitness is nearly as good by Akaikes Information Criterion.