Isolation by environment is more important than isolation by distance along a tropical gradient in direct developing frogs

Abiotic factors are important for defining population structure and limiting gene flow, especially in ectotherm species, the challenge is to identify which ones are the most important and can produce a pattern of isolation by environment. Our study aim is to quantify the extent to which divergence is driven by abiotic factors such as temperature, precipitation and elevation. To do so, we used a direct-developing frog species, Craugastor loki that occurs along a steep elevation gradient. Using restriction-site associated DNA sequencing (RADseq) from individuals collected from 100 m to 2250 m of elevation at 13 localities at Sierra Madre de Chiapas in southern Mexico we described population structure using a variety of model-based clustering and landscape genomics approaches. We found that populations sampled at higher elevation correspond probably to an undescribed new species of Craugastor , and that populations from Craugastor loki between 120 m and 1500 m are clustered in two different genetic groups: a Pacific slope group and a Central Depression slope group. We found signatures of isolation by environment more important that isolation by distance in contributing to genetic divergence in this group of frogs at a fine scale. The environmental variables such as: mean temperature of wettest and warmest quarter, annual mean temperature, and seasonality in temperature and precipitation play an important role on population differentiation. Our results underscore the importance of abiotic factors as drivers and highlight the use of different approaches to illuminate fine scale population divergence given the complexity of disentangling the contribution of both patterns of isolation.