Loci that show unusually low or high levels of genetic differentiation are often assumed to be subject to natural selection. We propose a method for the identification of loci showing such disparities. The differentiation can be quantified using the statistic F ST . For a range of population structures and demographic histories, the distribution of F ST is strongly related to the heterozygosity at a locus. Outlying values of F ST can be identified in a plot of F ST vs . heterozygosity using a null distribution generated by a simple genetic model. We use published data-sets to illustrate the importance of the relationship with heterozygosity. We investigate a number of models of population structure, and demonstrate that the null distribution is robust to a wide range of conditions. In particular, the distribution is robust to differing mutation rates, and therefore different molecular markers, such as allozymes, restriction fragment length polymorphisms (RFLPS) and single strand conformation polymorphisms (SSCPS) can be compared together. We suggest that genetic variation at a discrepant locus, Identified under these conditions, is likely to have been influenced by natural selection, either acting on the locus itself or at a closely linked locus.

Evaluating Loci for Use in the Genetic Analysis of Population Structure

The accuracy of gene flow estimates is unknown in most natural populations because direct estimates of dispersal are often not possible. These estimates can be highly imprecise or even biased because population genetic structure reflects more than a simple balance between genetic drift and gene flow. Most of the models used to estimate gene flow also assume very simple patterns of movement. As a result, multiple interpretations of population structure involving contemporary gene flow, departures from equilibrium, and other factors are almost always possible. One way to isolate the relative contribution of gene flow to population genetic differentiation is to utilize comparative methods. Population genetic statistics such as FST, heterozygosity and Nei's D can be compared between species with differing dispersal abilities if these species are otherwise phylogenetically, geographically and demographically comparable. Accordingly, the available literature was searched for all groups that meet these criteria ...

Dispersal, gene flow, and population structure

▪ Abstract Movement between discrete habitat patches can present significant challenges to organisms. Freshwater invertebrates achieve dispersal using a variety of mechanisms that can be broadly categorized as active or passive, and which have important consequences for processes of colonization, gene flow, and evolutionary divergence. Apart from flight in adult freshwater insects, active dispersal appears relatively uncommon. Passive dispersal may occur through transport by animal vectors or wind, often involving a specific desiccation-resistant stage in the life cycle. Dispersal in freshwater taxa is difficult to study directly, and rare but biologically significant dispersal events may remain undetected. Increased use of molecular markers has provided considerable insight into the frequency of dispersal in freshwater invertebrates, particularly for groups such as crustaceans and bryozoans that disperse passively through the transport of desiccation-resistant propagules. The establishment of propagule b...

https://www.jstor.org/stable/pdfplus/2678638.pdf

Dispersal in freshwater invertebrates

Patterns of variation in mitochondrial DNA (mtDNA) increasingly are being investigated in threatened or managed species, but not always with clearly defined goals for conservation. In this review I identify uses of mtDNA analysis which fall into two different areas: (i) ‘gene conservation’ - the identification and management of genetic diversity, and (ii) ‘molecular ecology’ - the use of mtDNA variation to guide and assist demographic studies of populations. These two classes of application have different conceptual bases, conservation goals and time-frames. Gene conservation makes extensive use of phylogenetic information and is, in general, most relevant to long-term planning. Appropriate uses here include identification of Evolutionarily Significant Units and assessment of conservation priority of taxa or areas from an evolutionary perspective. Less appropriate are inferences about fitness from within-population diversity and about species boundaries. Molecular ecology makes more use of allele frequencies and provides information useful for short-term management of populations. Powerful applications are to identify Management Units and to define and use naturally occurring genetic tags. Estimating demographic parameters, e.g migration rate and population size, from patterns of mtDNA diversity is fraught with difficulty, particularly where populations are fluctuating, and is unlikely to produce quantitative estimates sufficiently accurate to be useful for practical management of contemporary populations. However, through comparative studies, mtDNA analysis can provide qualitative signals of population changes, allowing efficient targeting of resource-intensive ecological studies. Thus, there are some relatively straightforward uses of mtDNA, preferably in conjunction with assays of nuclear variation, that can make a significant contribution to the long-term planning and short-term execution of species recovery plans.

Applications of mitochondrial DNA analysis in conservation: a critical review

We examined the impact of three forms of dispersal, stepping-stone, normal and leptokurtic, on spatial genetic structure of expanding populations using computer simulations. When dispersal beyond neighbouring demes is allowed, rare long-distance migration leads to the establishment of pocket populations in advance of the main invasion front and results in spatial clustering of genotypes which persists for hundreds of generations. Patchiness is more pronounced when dispersal is leptokurtic as is the case in many animal and plant species. These results are of particular interest because population genetic parameters such as gene flow and effective population size are commonly estimated using gene frequency divergence information assuming equilibrium conditions and island models. We show how the three forms of dispersal during colonization bring about contrasting population genetic structures and how this affects estimates of gene flow. The implications for experimental studies of the spatial dimension of population genetic structure are discussed.

/pdf/spatial-patterns-of-genetic-variation-generated-by-different-4mw7gjgwps.pdf

Spatial patterns of genetic variation generated by different forms of dispersal during range expansion

The estimation of gene flow using gene frequency divergence information has become increasingly popular because of the difficulty involved in the direct determination of gene flow among populations. The present study examined allozyme gene frequencies in populations of eighteen aquatic invertebrate taxa at two sites in northern Canada. Gene frequencies at polymorpic loci were significantly different among 8–31 localized populations of all species at Igloolik and among 10–36 populations at Churchill confirming the generality of gene pool fragmentation in pond-dwelling organisms. Measures of gene flow estimated from gene frequency divergence, which assume that gene frequency distributions are at equilibrium, were inconsistent with the probable dispersal capacities of taxa. This provoked an examination of historical events as alternative explanations. Both theory and computer simulations demonstrated that when populations grow rapidly in size after founding from few individuals, the gene frequency divergence established during colonization is resistant to decay by gene exchange. Our work suggests that gene frequency distributions are often not in equilibrium and that caution should be employed in attempts to infer gene flow from them in natural populations.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1420-9101.1992.5010025.x

Non-equilibrium gene frequency divergence : persistent founder effects in natural populations

Pond-dwelling copepods have colonized habitats throughout North America after glaciers have receded. Most species are passively transported via resting eggs into new habitats. Colonists originating in a glacial refugium could lose some of the ancestral genetic diversity when they establish new populations and the attenuation may be substantial in populations far removed from the refugium due to multiple founder events. Genetic variation was measured in Heterocope septentrionalis from 27 populations at arctic sites near potential refugia and those more recently deglaciated to determine the effects of postglacial dispersal on patterns of genetic relatedness and diversity. Some populations were more distant, genetically, from others within the same site than those from other more distant sites. Eleven polymorphic enzyme loci were significantly more variable (F [1,294 df] = 5.94, P < 0.025) among individuals from populations near the Alaskan refuge than those at the eastern limit of their distribution. Because populations are typically extremely large and stable this loss of genetic diversity is attributed primarily to repeated founder events during colonization. This result suggests profound genetic changes may occur on a continental scale in passively dispersed copepods due to founder events alone. Their potential for divergence and speciation is greater than currently recognized.

Genetic consequences of passive dispersal in pond‐dwelling copepods

The Onychophora are a relic taxon which diverged early in the arthropod radiation and have shown little morphological differentiation for several hundred million years. Jamaica has the richest onychophoran fauna in the Caribbean and although only five species are known, they represent 7 per cent of the global fauna. The present study involved an analysis of allozyme and mtDNA diversity in Plicatoperipatus jamaicensis, an endemic species which is the commonest onychophoran on the island. The work showed that P. jamaicensis includes at least two different species. These taxa are largely allopatric, but co-occur at some sites without interbreeding. Allozyme and mtDNA analyses suggest their divergence in the early Pleistocene, following the Pliocene origin of the Plicatoperipatus lineage from a Macroperipatus ancestor. Allozyme variation in both species was unusually low, but sufficient to confirm that each taxon reproduces sexually. Mitochondrial DNA diversity was abundant in both species and indicated that local aggregations did not consist of kin groups. The mitochondrial genomes of both species were small enough to suggest that their organizations are unusual.

/pdf/genetic-variation-in-the-onychophoran-plicatoperipatus-1zty274oen.pdf

Genetic variation in the onychophoran Plicatoperipatus jamaicensis

Freshwater pond copepod species at a low-arctic site show distributional and life history differences which may reflect different dispersal efficiencies. In order to ascertain levels of gene flow among populations, cellulose acetate gel electrophoresis was used to examine allozyme variation in Heterocope septentrionalis, Hesperodiaptomus victoriaensis and Leptodiaptomus tyrrelli near Churchill, Canada. Differentiation of gene frequencies among populations of these species, plus Hesperodiaptomus eiseni and Hesperodiaptomus arcticus at other sites along Hudson Bay was moderate. The variation in gene frequencies was less than that of other passively dispersing organisms from the same habitats, and only slightly greater than that reported for an intertidal copepod with pelagic larvae. The mean number of dispersers exchanged among populations per generation, estimated from Wright’s island model, averages 4.1 for the three species. Dispersal efficiency, calculated using population size estimates, revealed differences among the three species. However, these differences were not consistent with that expected from their distributional patterns. This suggests that factors other than dispersal alone determine the distributions of copepod species.

https://link.springer.com/content/pdf/10.1007%2F978-94-009-3103-9_43.pdf

Marc G. Boileau

Papers

Non-equilibrium gene frequency divergence : persistent founder effects in natural populations

Genetic consequences of passive dispersal in pond‐dwelling copepods

Genetic variation in the onychophoran Plicatoperipatus jamaicensis

Genetic differentiation of freshwater pond copepods at arctic sites

A genetic determination of cryptic species (Copepoda: Calanoida) and their postglacial biogeography in North America