Using Gene Trees to Infer Species from Testable Null Hypothesis: Cohesion Species in the Spalax ehrenbergi Complex

Abstract

The cohesion species concept defines a species as an evolutionary lineage whose boundaries arose from the forces that create reproductive communities. Such forces are collectively called cohesion mechanisms and consist of two main subtypes: genetic exchangeablity and ecological interchangeablity. To make this definition operational, populations that behaved as separate evolutionary lineages are first identified. A method is reviewed for inferring lineages using explicit statistical criteria from geographical overlays upon gene trees and is illustrated with data from the Spalax ehrenbergi superspecies complex. This step infers three statistically significant lineages of mole rats and several range expansion events within this group. Once lineages have been identified, the next step is to use the cohesion mechanisms to identify candidate traits that should contribute to genetic exchangeability and/or ecological interchangeability. The cohesion species are then identified by performing overlays upon gene trees to identify significant transitions in the candidate traits. Cohesion species are recognized only when statistically significant genetic/ecological transitions occur that are concordant with the lineages defined earlier. All three mole rat lineages reject the null hypotheses of both genetic exchangeability and ecological interchangeability among the lineages, and hence there is statistically significant evidence for three cohesion species in this complex. More species could exist, and the testing procedure provides detailed guidance for future research on species status in these mole rats. This data-rich method of recognizing species automatically generates much information about the biogeography, population structure, historical events, and ecology and/or reproductive biology of the group under study. This information in turn provides much insight into the process of speciation.