Hybridisation or Incomplete Lineage Sorting: The convoluted story about the origin of the capped-golden langur lineage (Cercopithecidae: Colobinae)
Hybridisation and incomplete lineage sorting (ILS) are two of the many naturally occurring evolutionary processes that contribute to variability in individual gene histories. However, distinguishing between hybridisation and ILS is challenging and these processes can lead to serious difficulties in inferring phylogenies. These processes have been invoked as a possible explanation for the evolution of the endangered golden langur and its sister species, the capped langur. Previous molecular studies have retrieved conflicting phylogenies, with mitochondrial tree grouping capped and golden langur (CG) lineage with a largely Indian genus Semnopithecus while nuclear markers supporting their affiliation with a Southeast Asian genus, Trachypithecus. However, as pointed by others, the use of nuclear copies of mitochondrial DNA in the above studies might have generated the discordance. Because of this discordance, the phylogenetic position of CG lineage has been much debated in recent times. In this study, we have used nine nuclear and eight mitochondrial markers in conjunction with coalescent based species tree approach to better understand the evolutionary origin of CG lineage. Concatenated nuclear as well as the mitochondrial dataset recovered congruent relationships where CG lineage was sister to Trachypithecus. However nuclear species tree estimated using different multispecies coalescent methods were incongruent with the above result, suggesting presence of ILS/hybridisation. Furthermore, CG lineage is morphologically intermediate between Semnopithecus and Trachypithecus with respect to skull and body measurements. Based on these evidences, we argue that CG lineage evolved through hybridisation between Semnopithecus and Trachypithecus. Taxonomic and conservation implications of these results are also discussed.