Using niche centrality within the scope of the nearly neutral theory of evolution to predict genetic diversity in a tropical conifer species-pair

Aim: Estimating genetic diversity is key for understanging biogeographic and evolutionary processes. However, gathering genetic information is not feasible for all taxa or populations, particularly in the tropical regions. Identifying proxies for inferring such values has thus become essential. Here, we built on the niche centrality hypothesis (NCH; or central-abundance hypothesis) and the nearly neutral theory of evolution (NNT) to identify some of such proxies using a montane tropical conifer species-pair as model. The NCH predicts more genetic diversity under optimal ecological conditions, which should also allow for more efficient purifying selection, according to the NNT. Location: The Transmexican Volcanic Belt, central Mexico. Taxa: A fir species-pair endemic to central Mexico, Abies flinckii and A. religiosa. Methods: We estimated patterns of genetic diversity from nuclear SSRs (A, H_E), and gene-coding sequences (π_S, π_N), together with the efficacy of purifying selection, measured as π_N/π_S. After testing for niche overlap, we used several geographic and ecological proxies (i.e. longitude, latitude, elevation, estimated area and distance to the niche centroid in the present and in the LGM) to predict genetic diversity and π_N/π_S using general linear models. Results: Populations at the west of the Trans Mexican Volcanic Belt (TVB) had lower genetic diversity than populations in the east of this mountain chain. Both species had significant niche overlap. The principal predictors for neutral genetic diversity (H_E, A and π_S) were longitude and latitude, followed by the current distance to the niche centroid; the efficiency of purifying selection was mostly accounted for by the current distance to the niche centroid (which was also correlated with elevation). No correlation was observed between genetic diversity or π_N/π_S and current population area. Main conclusions: Historical and ecological factors have to be taken into account for explaining the amounts of genetic diversity in mountain tropical species. Following the NTT, populations closer to the niche centroid are more efficient at eliminating slightly deleterious mutations than marginal stands, independently of their size or geographic location (longitude). Expanding the central-abundance theory within the scope of the NTT might help reconciling conflicting views concerning the extent of its empirical support.