Biogeography of Cyprinodon across the Great Plains-Chihuahuan Desert region and adjacent areas

Cyprinodon is renowned for localized endemism across the North American desert. Competing molecular studies have made elucidating timing of diversification across the desert controversial. Debate has focused on Mojave Desert species, with limited evaluation of other evidence. However, the Great Plains and Chihuahuan Desert harbor more taxonomic diversity and are geographically positioned between the Gulf of México (place of origin for the genus) and Mojave Desert, making them central to understanding the evolution of all desert Cyprinodon. This study is a detailed assessment of evidence from literature spanning geomorphology, climate, and biogeography vis à vis the mtDNA phylogeny for Cyprinodon. Conclusions of Late Miocene-Early Pleistocene diversification are supported across all major clades. Future studies that could improve understanding and address ongoing dilemmas are identified. Importantly, the geography of each clade corresponds to drainage configurations and their evolution through the proposed period of diversification. Eight hypotheses are presented to address major evolutionary events, with emphasis on exploring interpretive challenges within the phylogeny. Broadly, aridity within the Late Miocene apparently facilitated inland invasion of coastal Cyprinodon along the ancestral Brazos River and Río Grande. The following Pliocene warm, wet period enabled survival and range expansion through aridland drainages and into adjacent ones. Mio-Pliocene development of the Río Grande Rift and Gila River drainages, causing inter-drainage transfers, was crucial to range expansion. Development of other Gulf of California drainages (Colorado River, Río Yaqui) played peripheral roles. Climatic cooling in the Quaternary Period evidently caused range contractions for populations living at higher latitudes and elevations. Living Cyprinodon of the desert represent an incredible legacy of Pliocene range expansion memorialized by subsequent persistence of tenacious endemic populations. Human impacts now threaten this legacy.