Aim Geographical gradients of vertebrate species richness are determined jointly by evolutionary and environmental factors. Support for these factors comes mostly from tetrapods and, recently, marine fishes, but their validity and relative importance in freshwater fishes is not well understood. Here, we describe the species richness gradient for the major component of the viviparous freshwater fishes of the Americas, the poeciliids, and test the effects of evolutionary and environmental factors on this pattern. Location The Americas. Time period Approximatley 56–0 Ma. Major taxa studied Poeciliidae (Teleostei: Cyprinodontiformes). Methods We constructed geographical ranges for 93% (256) of poeciliids to describe and evaluate their species richness gradient. Evolutionary factors (evolutionary time and speciation rate) were derived from a recent phylogeny. Environmental factors were represented as basin area, topographical heterogeneity, energy, climate seasonality and past climatic stability. We tested the influence of these factors with a piecewise structural equation model (pSEM). Results The distribution of Poeciliidae is biased to the Atlantic coast, with species richness showing a bimodal latitudinal gradient, peaking in middle latitudes near the Tropics of Cancer and Capricorn, and exhibiting the highest richness in Middle America. The pSEM showed that this species richness pattern was influenced positively by evolutionary time and past climatic stability and negatively by climate seasonality. Main conclusion The species richness gradient of Poeciliidae has been shaped by the interplay of evolutionary time in addition to current and historical climate. Indeed, regions with high poeciliid richness were those containing by ancient lineages, supporting the time‐for‐speciation effect, and that have experienced low historical stability in temperature and currently show low temperature seasonality. Conversely, species‐poor regions contained younger lineages and experienced greater temperature seasonality. Our study highlights the need to assess jointly the evolutionary, historical and climatic drivers of species richness in order to unravel the causes of diversity gradients.

Background Ictalurus is one of the most representative groups of North American freshwater fishes. Although this group has a well-studied fossil record and has been the subject of several morphological and molecular phylogenetic studies, incomplete taxonomic sampling and insufficient taxonomic studies have produced a rather complex classification, along with intricate patterns of evolutionary history in the genus that are considered unresolved and remain under debate. Results Based on four loci and the most comprehensive taxonomic sampling analyzed to date, including currently recognized species, previously synonymized species, undescribed taxa, and poorly studied populations, this study produced a resolved phylogenetic framework that provided plausible species delimitation and an evolutionary time framework for the genus Ictalurus. Conclusions Our phylogenetic hypothesis revealed that Ictalurus comprises at least 13 evolutionary units, partially corroborating the current classification and identifying populations that emerge as putative undescribed taxa. The divergence times of the species indicate that the diversification of Ictalurus dates to the early Oligocene, confirming its status as one of the oldest genera within the family Ictaluridae.

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Identification of species boundaries within the genus Aspidoscelis is one of the most complex tasks in herpetological systematics due to the extensive morphological variation and complex evolutionary history involved. The whiptail lizard Aspidoscelis lineattissimus is a polymorphic species that inhabits tropical ecosystems over a wide region of western Mexico and is currently classified into four subspecies, A. l. exoristus, A. l. lineattissimus, A. l. lividus and A. l. duodecemlineatus. In this study, we used phylogenetic and coalescent‐based approaches to disentangle the phylogenetic relationships among the subspecies of A. lineattissimus and to assess the level of differentiation between these subspecific taxa. We also inferred the divergence times and historical biogeography to reconstruct the evolutionary history of the A. lineattissimus complex. Three mitochondrial genes (ND4, 12S and 16S) and one nuclear exon (BACH1) were used, along with comprehensive sampling that included individuals representing the four subspecies. Coalescence analyses supported three well‐differentiated lineages as independent evolutionary units, corresponding to A. l. exoristus, A. l. lineattissimus + A. l. lividus and A. l. duodecemlineatus, partially recovering the previous classification. Furthermore, phylogenetic analyses support three additional discrete lineages within A. l. duodecemlineatus. Finally, divergence time estimates and reconstructions of ancestral areas indicated that the origin and diversification of the five differentiated lineages within the A. lineattissimus complex was strongly associated with oscillations in sea level due to glacial/interglacial climatic fluctuations beginning in the Mid‐Pliocene. Our research highlights the importance of phylogenetic studies within the genus Aspidoscelis to disentangle their evolutionary history and reveal hitherto underestimated diversity.