Selection pressures have caused genome-wide population differentiation of Anthoxanthum odoratum despite the potential for high gene flow.

The extent to which divergent selection can drive genome-wide population differentiation remains unclear. Theory predicts that in the face of ongoing gene flow, population differentiation should be apparent only at those markers that are directly or indirectly (i.e. through linkage) under selection. However, if reproductive barriers limit gene flow, genome-wide population differentiation may occur even in geographically proximate populations. Some insight into the link between selection and genetic differentiation in the presence of ongoing gene flow can come from long-term experiments such as The Park Grass Experiment, which has been running for over 150 years, and provides a unique example of a heterogeneous environment with a long and detailed history. Fertilizer treatments applied in the Park Grass Experiment have led to rapid evolutionary change in sweet vernal grass Anthoxanthum odoratum, but until now, nothing was known of how these changes would be reflected in neutral molecular markers. We have genotyped ten A. odoratum populations from the Park Grass Experiment using Amplified Fragment Length Polymorphisms (AFLPs). Our data show that nutrient additions have resulted in genome-wide divergence among plots despite the high potential for ongoing gene flow. This provides a well-documented example of concordance between genomes and environmental conditions that has arisen in continuous populations across a time span of fewer than 75 generations.

Gene flow and genetic diversity: a comparison of freshwater bryozoan populations in Europe and North America.

We have used microsatellite and mitochondrial sequence data to gain insight into patterns of gene flow and genetic diversity among North American and European populations of the freshwater bryozoan Cristatella mucedo. Mitochondrial sequence data reveal numerous, widely distributed, divergent genetic lineages in North America that can be broadly categorized into two groups, one of which is genetically homogeneous and relatively similar to the European haplotypes, the other of which is more diverse. The maximum North American sequence differentiation translates into a divergence time of approximately 1.5 Myr BP. In contrast, European populations contained only three haplotypes that are all closely related. Microsatellite data reveal higher overall levels of genetic diversity in North America than Europe, although levels of within-population genetic variation are similar on the two continents. In North America, two of the three microsatellite loci show bimodal distributions of allele sizes which are significantly associated between the two loci. As a result, two microsatellite lineages are evident, and these are assortatively distributed between the mitochondrial haplotype groupings. The combined mitochondrial and microsatellite data suggest two distinct genetic lineages in North America that may represent cryptic species. Hybridization between the two presumptive species or subspecies may have contributed to the high levels of genetic diversity in North America. The overall lower levels of genetic diversity in Europe can be attributed to postglacial derivation of extant populations from a single mitochondrial lineage, and conformation to a metapopulation structure.

Genetic diversity of North American populations of Cristatella mucedo, inferred from microsatellite and mitochondrial DNA.

Research over the past 20 years has shown, with the help of molecular markers, that the population genetics and distribution patterns of freshwater invertebrates in North America are often more complex than was previously believed. Here we extend this research to an, as yet, unstudied but widespread and common group, the freshwater bryozoans. Colonies of the bryozoan Cristatella mucedo were collected from a number of lakes across central North America, and were characterized genetically by analysis of microsatellite loci and mitochondrial DNA (mtDNA) cytochrome b sequences. The microsatellites illustrate a pattern of generally diverse and highly differentiated populations that contain little evidence of recent gene flow. The mtDNA sequences yielded highly variable levels of divergence, ranging from 0.0 to 8.8% within populations, and 0.0 to 9.8% among populations. The multiple divergent mtDNA lineages within populations provide evidence for repeated colonization events. The lack of clustering of haplotypes by site suggests that there has been widespread dispersal of multiple genetic lineages since the last ice age. While some of the haplotype lineages may have evolved in disjunct glacial refugia, the maximum levels of divergence predate the time since the last glacial-interglacial cycles. It is likely that multiple factors including vicariance events, patterns of dispersal, localized extinction, and an unusual life history, explain the unique phylogeographic patterns evident today in populations of C. mucedo.

A molecular phylogeny of warbling-finches (Poospiza): paraphyly in a neotropical Emberizid genus.

We investigated the phylogenetic relationships of 12 species within a single genus of neotropical passerine (Poospiza) using 849 bp (283 codons) of the cytochrome b mitochondrial gene. We further explored evolutionary affinities of these taxa using sequence from an additional 47 thraupine (tanagers) and 7 emberizine (sparrows and buntings) genera, members of the predominantly New World family Emberizidae. Poospiza have traditionally been considered part of the emberizine radiation. However, our analyses suggest that members of this genus are more closely related to some thraupine lineages than they are to the other neotropical emberizine genera included in our study (Atlapetes, Embernagra, Melopyrrha, Phrygilus, Saltatricula, Tiaris). Although member taxa are closely related, the genus Poospiza appears to be paraphyletic with representatives of 6 thraupine genera (Cnemoscopus, Cypsnagra, Hemispingus, Nephelornis, Pyrrhocoma, Thylpopsis) interspersed among four well-supported Poospiza clades. The majority of species within this Poospiza-thraupine clade have geographic ranges that are exclusive to, or partially overlap with, the Andes Mountains. It is probable that these mountains have played an important role in driving cladogenesis within this group. Sequence divergence (transversions only; mean 4.7+/-1.3%) within the clade suggests that much of this diversification occurred within the late Miocene and Pliocene, a period coincident with major orogenic activity in central-western South America.