Abstract. A goal of phylogeography is to relate patterns of genetic differentiation to potential historical geographic isolating events. Quaternary glaciations, particularly the one culminating in the Last Glacial Maximum ;21 ka (thousands of years ago), greatly affected the distributions and population sizes of temperate marine species as their ranges retreated southward to escape ice sheets. Traditional genetic models of glacial refugia and routes of recolonization include these predictions: low genetic diversity in formerly glaciated areas, with a small number of alleles/haplotypes dominating disproportionately large areas, and high diversity including ''private'' alleles in glacial refugia. In the Northern Hemisphere, low diversity in the north and high diversity in the south are expected. This simple model does not account for the possibility of populations surviving in relatively small northern periglacial refugia. If these periglacial populations experienced extreme bottlenecks, they could have the low genetic diversity expected in recolonized areas with no refugia, but should have more endemic diversity (private alleles) than recently recolonized areas. This review examines evidence of putative glacial refugia for eight benthic marine taxa in the temperate North Atlantic. All data sets were reanalyzed to allow direct comparisons between geographic patterns of genetic diversity and distribution of particular clades and haplotypes including private alleles. We contend that for marine organisms the genetic signatures of northern periglacial and southern refugia can be distinguished from one another. There is evidence for several periglacial refugia in northern latitudes, giving credence to recent climatic reconstructions with less extensive glaciation.

Recent glaciation covered the full extent of rocky intertidal habitat along the coasts of New England and the Canadian Maritimes. To test whether this glaciation in fact caused wholesale extinction of obligate rocky intertidal invertebrates, and thus required a recolonization from Europe, we compared American and European populations using allelic diversity and techniques adapted from coalescent theory. Mitochondrial DNA sequences were collected from amphi-Atlantic populations of three cold-temperate obligate rocky intertidal species (a barnacle, Semibalanus balanoides, and two gastropods, Nucella lapillus and Littorina obtusata) and three cold-temperate habitat generalist species (a seastar, Asterias rubens; a mussel, Mytilus edulis, and an isopod, Idotea balthica). For many of these species we were able to estimate the lineage-specific mutation rate based on trans-Arctic divergences between Pacific and Atlantic taxa. These data indicate that some obligate rocky intertidal taxa have colonized New England from European populations. However, the patterns of persistence in North America indicate that other life-history traits, including mech anisms of dispersal, may be more important for surviving dramatic environmental and climatic change.

A long-standing issue in marine biology is identifying spatial scales at which populations of sessile adults are connected by planktonic offspring. We examined the genetic continuity of the acorn barnacle Balanus glandula, an abundant member of rocky intertidal communities of the northeastern Pacific Ocean, and compared these genetic patterns to the nearshore oceanography described by trajectories of surface drifters. Consistent with its broad dispersal potential, barnacle populations are genetically similar at both mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1-alpha) loci across broad swaths of the species' range. In central California, however, there is a striking genetic cline across 475 km of coastline between northern and southern populations. These patterns indicate that gene flow within central California is far more restricted spatially than among other populations. Possible reasons for the steep cline include the slow secondary introgression of historically separated populations, a balance between diversifying selection and dispersal, or some mix of both. Geographic trajectories of oceanic drifters closely parallel geographical patterns of gene flow. Drifters placed to the north (Oregon; approximately 44 degrees N) and south (Santa Barbara, California; approximately 34 degrees N) of the cline disperse hundreds of kilometers within 40 days, yet over the long-term their trajectories never overlapped. The lack of communication between waters originating in Oregon and southern California probably helps to maintain strong genetic differentiation between these regions. More broadly, the geographical variation in gene flow implies that focusing on species-level averages of gene flow can mask biologically important variance within species which reflects local environmental conditions and historical events.

Our ability to make inferences about the processes acting upon a biological system can be dramatically improved through integration of information from other fields. In particular, this is true of the field of phylogeography, a discipline that attempts to describe geographical variation in species using neutral genetic diversity as a correlate of time. Through comparisons of information from multiple species, as well as background information about the abiotic environment and how it has changed over time, we should be able to reassemble many of the forces that were important in assembling the communities and community interactions found in a given region. Here I review the information available for coastal species in the northwestern Atlantic, and argue that an integration of ecological, genetic, geological and oceanographic information can illustrate emergent patterns of community genetics.

Microbes and other organisms smaller than one to a few millimeters in size are hypothesized to have global populations, in contrast to the geographically restricted ranges of larger organisms. However, fungi, which routinely have reproductive propagules no larger than 10 micrometers, challenge the generality of this hypothesis because recent studies have shown that globally distributed morphological species embrace two or more geographically restricted phylogenetic species. We used the concordance of gene genealogies to recognize phylogenetic species in the globally distributed opportunistic human pathogenic fungus, Aspergillus fumigatus. Based on DNA sequence data of five loci for each of 63 individuals collected from five continents, we have delineated two phylogenetic species in this single morphological species. Unlike all other fungi examined to date, both genetically isolated groups showed a global distribution with no evidence of a correlation between genotype and geographic location. Sexual reproduction has never been observed in A. fumigatus, but when the same data were used to explore the association of alleles at the five loci for one of the phylogenetic species, evidence was found to support recombination. The discovery of a cryptic species is medically relevant because different species are likely to differ in virulence or drug resistance. The discovery of a globally distributed A. fumigatus species clade highlights the need for ecological studies of the fungus to either document global dispersal or propose alternative mechanisms by which it persists as single, global phylogenetic population.

In many nonclonal, benthic marine species, geographic distribution is mediated by the dispersal of their larvae. The dispersal and recruitment of marine larvae may be limited by temperature gradients that can affect mortality or by ocean currents that can directly affect the movements of pelagic larvae. We focus on Point Conception, a wellknown biogeographic boundary between the Californian and Oregonian biogeographic provinces, to investigate whether ocean currents affect patterns of gene flow in intertidal marine invertebrates. The predominance of pelagically dispersing species with northern range limits at Point Conception suggests that ocean currents can affect species distributions by erecting barriers to the dispersal of planktonic larvae. In this paper, we investigate whether the predominantly southward currents have left a recognizable genetic signature in species with pelagically dispersing larvae whose ranges span Point Conception. We use patterns of genetic diversity and a new method for inferring cladistic migration events to test the hypothesis that southward currents increase southward gene flow for species with pelagically dispersing larvae. We collected mitochondrial DNA (mtDNA) sequence data for the barnacles Balanus glandula and Chthamalus fissus and also reanalyzed a previously published mtDNA dataset (Strongylocentrotus purpuratus, Edmands et al. 1996). For all three species, our cladistic approach identified an excess of southward migration events across Point Conception. In data from a fourth species with nondispersing larvae (Nucella emarginata, Marko 1998), our method suggests that ocean currents have not played a role in generating genetic structure.

Human activities have strongly impacted natural communities through the introduction of non‐native species in historical times. A frequently cited marine example is Littorina littorea, a common intertidal gastropod that was first reported in North America in 1840. The seemingly sudden appearance and rapid geographical spread of this species southward from Nova Scotia has led many researchers to consider L. littorea a human‐mediated species introduction. This is despite allozyme and subfossil evidence that the `European periwinkle' was in North America long before 1840. Our mitochondrial and nuclear DNA sequence data confirm that L. littorea has been in continuous residence in North America for at least 8000 years. It appears most likely that ecological interactions, rather than oceanographic or climatic forces, maintained the limited geographical distribution of L. littorea prior to the 19th century.

Although pairs of species often interact over broad geographic ranges, few studies have explored how interactions vary across these large spatial scales. Surveys along 1500 kilometers of the Pacific coast of North America documented marked variation in the frequency of predation by the snail Nucella canaliculata on the intertidal mussel Mytilus californianus. Laboratory rearing experiments suggest that regional differences in drilling behavior have a genetic basis, and mitochondrial sequence variation confirms that gene flow is low among these snail populations. Marine communities separated by hundreds of kilometers may have intrinsically different dynamics, with interactions shaped by restricted gene flow and spatially varying selection.