Abstract. Well-studied model systems present ideal opportunities to understand the relative roles of contemporary selection versus historical processes in determining population differentiation and speciation. Although guppy populations in Trinidad have been a model for studies of evolutionary ecology and sexual selection for more than 50 years, this work has been conducted with little understanding of the phylogenetic history of this species. We used variation in nuclear (X-src) and mitochondrial DNA (mtDNA) sequences to examine the phylogeographic history of Poecilia reticulata Peters (the guppy) across its entire natural range, and to test whether patterns of morphological divergence are a consequence of parallel evolution. Phylogenetic, nested clade, population genetic, and demographic analyses were conducted to investigate patterns of genetic structure at several temporal scales and are discussed in relation to vicariant events, such as tectonic activity and glacial cycles, shaping northeast South American river drainages. The mtDNA phylogeny defined five major lineages, each associated with one or more river drainages, and analysis of molecular variance also detected geographic structuring among these river drainages in an evolutionarily conserved nuclear (X-src) locus. Nested clade and other demographic analyses suggest that the eastern Venezuela/ western Trinidad region is likely the center of origin of P. reticulata. Mantel tests show that the divergence of morphological characters, known to differentiate on a local scale in response to natural and sexual selection pressures, is not associated with mtDNA genetic distance; however, TreeScan analysis identified several significant associations of these characters with the haplotype tree. Parallel upstream/downstream patterns of morphological adaptation in response to selection pressures reported in P. reticulata within Trinidad rivers appears to persist across the natural range. Our results together with previous studies suggest that, although morphological variation in P. reticulata is primarily attributed to selection, phylogeographic history may also play a role. The relative importance of contemporary selection versus historical processes in determining population differentiation and the potential for speciation is a critical question in evolutionary biology. Several studies and reviews have recently shown the importance of local divergent selection in driving reproductive isolation (Schluter 2000;Nosil 2002;Rundle and Nosil 2005;Thorpe et al. 2005). Many other studies have taken a phylogeographic approach to determine the chronology of evolutionary diversification and test the congruence between distribution of lineages and geological and climatic events (Avise 1998;Moritz and Bermingham 1998;Bernatchez 2001). Fewer studies have examined these processes together at multiple scales of time and space (Thorpe et al. 1995;Hoekstra et al. 2005). The complex geological history and topography of northeastern South America suggests that both historical event...

Well-studied model systems present ideal opportunities to understand the relative roles of contemporary selection versus historical processes in determining population differentiation and speciation. Although guppy populations in Trinidad have been a model for studies of evolutionary ecology and sexual selection for more than 50 years, this work has been conducted with little understanding of the phylogenetic history of this species. We used variation in nuclear (X-src) and mitochondrial DNA (mtDNA) sequences to examine the phylogeographic history of Poecilia reticulata Peters (the guppy) across its entire natural range, and to test whether patterns of morphological divergence are a consequence of parallel evolution. Phylogenetic, nested clade, population genetic, and demographic analyses were conducted to investigate patterns of genetic structure at several temporal scales and are discussed in relation to vicariant events, such as tectonic activity and glacial cycles, shaping northeast South American river drainages. The mtDNA phylogeny defined five major lineages, each associated with one or more river drainages, and analysis of molecular variance also detected geographic structuring among these river drainages in an evolutionarily conserved nuclear (X-src) locus. Nested clade and other demographic analyses suggest that the eastern Venezuela/ western Trinidad region is likely the center of origin of P. reticulata. Mantel tests show that the divergence of morphological characters, known to differentiate on a local scale in response to natural and sexual selection pressures, is not associated with mtDNA genetic distance; however, TreeScan analysis identified several significant associations of these characters with the haplotype tree. Parallel upstream/downstream patterns of morphological adaptation in response to selection pressures reported in P. reticulata within Trinidad rivers appears to persist across the natural range. Our results together with previous studies suggest that, although morphological variation in P. reticulata is primarily attributed to selection, phylogeographic history may also play a role.