David E. McCauley

Indirect measures of gene flow and migration: FST≠1/(4Nm+1)

The difficulty of directly measuring gene flow has lead to the common use of indirect measures extrapolated from genetic frequency data. These measures are variants of FST, a standardized measure of the genetic variance among populations, and are used to solve for Nm, the number of migrants successfully entering a population per generation. Unfortunately, the mathematical model underlying this translation makes many biologically unrealistic assumptions; real populations are very likely to violate these assumptions, such that there is often limited quantitative information to be gained about dispersal from using gene frequency data. While studies of genetic structure per se are often worthwhile, and FST is an excellent measure of the extent of this population structure, it is rare that FST can be translated into an accurate estimate of Nm.

Extinction and recolonization : Their effects on the genetic differentiation of local populations

In this paper, we use a model by Slatkin (1977) to investigate the genetic effects of extinction and recolonization for a species whose population structure consists of an array of local demes with some migration among them. In particular, we consider the conditions under which extinction and recolonization might enhance or diminish gene flow and increase or decrease the rate of genetic differentiation relative to the static case with no extinctions. We explicitly take into account the age‐structure that is established within the array of populations by the extinction and colonization process. We also consider two different models of the colonization process, the so‐called “migrant pool” and “propagule pool” models. Our theoretical studies indicate that the genetic effects of extinction and colonization depend upon the relative magnitudes of K, the number of individuals founding new colonies, and 2Nm, twice the number of migrants moving into extant populations. We find that these genetic effects are surprisingly insensitive to the extinction rate. We conclude that, in order to assess the genetic effects of the population dynamics, we must first answer an important empirical question that is essentially ecological: is colonization a behavior distinct from migration?

Some population genetic consequences of colony formation and extinction: genetic correlations within founding groups.

Extinction and recolonization in an island model affects genetic differentiation among subpopulations through a combination of sampling and mixing. We investigate the balance of these forces in a general model of population founding that predicts first the genetic variance among new groups and then the effect of these new groups on the total genetic variance among all populations. We allow for a broad range of types of mixing at the time of colonization and demonstrate the significant effects on differentiation from the probability of common origin of gametes (φ). We further demonstrate that kin‐structured founding and inbreeding within populations can have a significant effect on the genetic variance among groups and use these results to make predictions about lineal fission and fusion of populations. These results show that population structure is critically affected by non‐equilibrium dynamics and that the properties of new populations, especially founding number, probability of common origin, and kin structure, are vital in our understanding of genetic variation.

The use of chloroplast DNA polymorphism in studies of gene flow in plants

In many species of plants, the dispersal of genes is mediated by the movement of both seeds and pollen. The relative contributions of seed and pollen movement to total gene flow can be difficult to estimate. Chloroplast DNA (cpDNA) may prove useful for resolving this problem. Over the past several years, studies of numerous species of plants have shown that intraspecific variation in cpDNA is often sufficiently abundant to serve as a marker for studies of gene flow. Recent theoretical models have shown that estimates of population structure based on cpDNA polymorphism should be especially sensitive to the impact of seed movement on gene flow, because cpDNA is often maternally inherited.

Rapid Evolution of Enormous, Multichromosomal Genomes in Flowering Plant Mitochondria with Exceptionally High Mutation Rates

A pair of species within the genus Silene have evolved the largest known mitochondrial genomes, coinciding with extreme changes in mutation rate, recombination activity, and genome structure.

Genetic consequences of local population extinction and recolonization.

Theoretical results have shown that a pattern of local extinction and recolonization can have significant consequences for the genetic structure of subdivided populations; consequences that are relevant to issues in both evolutionary and conservation biology. The nature of those consequences depends largely on the mode of colony formation. Extinction and recolonization can either increase or decrease the genetic differentiation of local populations and can lead to a loss of the genetic diversity stored in an array of populations. Recent ecological studies of two insect species have revealed population structures resembling, in part, that considered in the models. They serve to illustrate the potential complexity of the processes of extinction and recolonizatiion in nature.

Silene as a model system in ecology and evolution.

The genus Silene, studied by Darwin, Mendel and other early scientists, is re-emerging as a system for studying interrelated questions in ecology, evolution and developmental biology. These questions include sex chromosome evolution, epigenetic control of sex expression, genomic conflict and speciation. Its well-studied interactions with the pathogen Microbotryum has made Silene a model for the evolution and dynamics of disease in natural systems, and its interactions with herbivores have increased our understanding of multi-trophic ecological processes and the evolution of invasiveness. Molecular tools are now providing new approaches to many of these classical yet unresolved problems, and new progress is being made through combining phylogenetic, genomic and molecular evolutionary studies with ecological and phenotypic data.

Local founding events as determinants of genetic structure in a plant metapopulation

The genetic structure of a metapopulation of the weedy plant, Silene alba, was quantified by calculating FST, Wrights measure of the among-deme component of genetic variance, for seven polymorphic allozyme loci and a chloroplast DNA (cpDNA) polymorphism. Specifically, FST was estimated separately for 12 recently founded demes and 11 demes that had been established in the same area for at least six years, in order to assess the influence of founding events on genetic structure. The FST value for the recently established populations was the larger of the two estimates for six of the seven allozyme polymorphisms and for the cpDNA polymorphism, showing that the metapopulation dynamics of extinction and colonization enhance genetic structure. Combined with an estimate of k, the size of colonizing groups, the FST values were also used to estimate φ, the probability that two alleles found in a colonizing group originated in the same source population. Information from allozymes and cpDNA resulted in estimates of φ of 0.73 and 0.89, respectively, suggesting that relatively little mixing of individuals from diverse source populations occurs during colonization.

THE INFLUENCE OF POPULATION SIZE AND ISOLATION ON GENE FLOW BY POLLEN IN SILENE ALBA

In a series of experiments conducted over two seasons, we used arrays of experimental populations to examine the effects of flower number and distance between patches on gene flow by pollen. For this study we used the dioecious, short‐lived perennial plant Silene alba (Caryophyllaceae). This species lives in disturbed roadside and agricultural habitats and displays a weedy population dynamic with high colonization and extinction rates. The motivation for the study was to understand what factors may be influencing genetic connectedness among newly colonized populations within a regional metapopulation. By using experimental populations composed of genotypes homozygous at a diagnostic locus, it was possible to identify explicitly pollen movement into a focal patch as a function of flower number and distance to the nearest neighboring patch. Overall, the mean immigration rate (measured as the fraction of seeds sired by males outside the focal patch) at 20 m was just over 47%, whereas at 80 m immigration rates were less than 6%. In addition, by knowing the context in which each of these gene‐flow events occurred, it was possible to understand some of the factors that influenced the exchange of genes. Both the number of flowers in the focal population (target) and in the neighboring populations (source) had a significant effect on the frequency of gene flow. Our experimental data also demonstrate that factors that influence gene flow at one spatial scale may not act in the same way at another. Specifically, the influence of target size and the relative size of the target and source patches on rates of gene flow depended on whether the patches were separated by 20 m or 80 m. These data suggest that the patterns of gene flow within a metapopulation system can be complex and may vary within a growing season.

FREQUENCY-DEPENDENT FITNESS IN SILENE VULGARIS, A GYNODIOECIOUS PLANT

In gynodioecious plants the selective processes that determine the relative number of female and hermaphroditic individuals are often frequency dependent. Frequency‐dependent fitness can occur in the two sexes through a variety of mechanisms, especially given pollen limitation and inbreeding depression when hermaphrodites are rare. Frequency dependence in several components of the fitness of female and hermaphroditic Silene vulgaris was tested in experiments in which the relative numbers of the two sexes was varied among 12 artificial populations. In females, the proportion of flowers that set fruit covaried positively among populations with the frequency of hermaphrodites in two separate experiments, whereas the number of flowers/plant covaried negatively in one case. In hermaphrodites, the number of seeds/fruit covaried positively with the frequency of hermaphrodites, whereas the fitness of hermaphrodites estimated through pollen transfer covaried negatively. The results are discussed as they relate to the selective maintenance of gynodioecy in S. vulgaris and in light of a recent model of the effect of population structure on selection in gynodioecious systems.