Continental copepods have been derived from several independent invasive events from the sea, but the subsequent evolutionary processes that account for the current diversity in lacustrine environments are virtually unknown. Salinity is highly variable among lakes and constitutes a source of divergent selection driving potential reproductive isolation. We studied four populations of the calanoid copepod Leptodiaptomus cf. sicilis inhabiting four neighboring lakes with a common history (since the Late Pleistocene) located in the Oriental Basin, Mexico; one lake is shallow and varies in salinity periodically (1.4–10 g L-1), while three are deep and permanent, with constant salinity (0.5, 1.1 and 6.5 g L-1, respectively). We hypothesized that (1) these populations belong to a different species than L. sicilis sensu stricto and (2) are experiencing ecologically based divergence due to salinity differences. We assessed morphological and molecular (mtDNA) COI variation, as well as fitness differences and tests of reproductive isolation. Although relationships of the Mexican populations with L. sicilis s.s. could not be elucidated, we identified a clear pattern of divergent selection driven by salinity conditions. The four populations can still be considered a single biological species (sexual recognition and hybridization are still possible in laboratory conditions), but they have diverged into at least three different phenotypes: two locally adapted, specialized in the lakes of constant salinity (saline vs. freshwater), and an intermediate generalist phenotype inhabiting the temporary lake with fluctuating salinity. The specialized phenotypes are poorly suited as migrants, so prezygotic isolation due to immigrant inviability is highly probable. This implication was supported by molecular evidence that showed restricted gene flow, persistence of founder events, and a pattern of allopatric fragmentation. This study showed how ecologically based divergent selection may explain diversification patterns in lacustrine copepods.

During the analysis of littoral samples collected from a high-altitude saline crater lake in Central Mexico

RESUMENEn este trabajo se propone una adecuación al método panbiogeográfi co utilizando modelos digitales de elevación (MDE). Se analizaron las distribuciones de 12 especies de roedores de la provincia biogeográfi ca Faja Volcánica Transmexicana (FVT) y los resultados se compararon utilizando un modelo tridimensional. Se generaron los trazos individuales de las 12 especies empleando una herramienta automatizada con lo que se identifi caron tres trazos generalizados. Por otro lado, sobre la base de un mapa de curvas de nivel, se generó un MDE raster, se calculó un coefi ciente de aumento de la distancia en la topología, y los trazos individuales se midieron sobre el MDE. Además, se calculó la longitud de cada trazo individual sobre un MDE vector, donde fi nalmente se observaron diferencias en la topología de un trazo con respecto al obtenido en el espacio bidimensional. Los restantes once trazos individuales no presentaron cambio alguno, lo cual podría indicar que los trabajos panbiogeográfi cos realizados a la fecha en dos dimensiones, han representado bien los patrones de los grupos taxonómicos estudiados. Sin embargo, es necesario continuar realizando modifi caciones al método para acercarse lo más posible a la realidad, en particular en áreas con alta heterogeneidad altitudinal.Palabras clave: Biogeografía, trazos, 3D, mamíferos, México. ABSTRACTThis paper proposes an adequacy to the panbiogeographic method using digital elevation models (DEM). The distributions of 12 species of rodents from the Transmexican Volcanic Belt (TVB) were analyzed, and the results were compared using a three-dimensional model. Individual tracks for the 12 species were draw employing an automated tool which identifi ed three generalized tracks. On the other hand, upon the base of a contour lines map, a raster DEM was built, a rate of increase of distance in the topology was calculated, and the individual tracks were measured on the DEM. Additionally, the length of each individual track was calculated on a vector DEM, where differences in the topology of one track compared to the two-dimensional track were observed. The other eleven tracks did not show any changes, which could means that the actual panbiogeographic analyses in two-dimensions had represented adequately the patterns of the taxonomic groups studied. However, it is necessary to keep making modifi cations to the method in order to approach as near as possible to the reality, mainly in areas with high altitudinal heterogeneity.