Biogeographical change in the tiger, Panthera tigris

Abstract

The number of tiger subspecies is a major conservation issue that is difficult to resolve owing to small fragmented extant populations and limited historical samples in museums, which compromise the rigour of both molecular and morphological taxonomic studies. Rather than considering a static taxonomic approach to geographical variation in the tiger, we consider the changing biogeographical range of the tiger through the last glacial-interglacial cycle, based on habitat associations of modern tiger specimen records, and environmental reconstructions from the Last Glacial Maximum (LGM; approximately 20000 years before present (B.P.)). We regard this cycle as representative of the numerous glacial cycles that span the evolutionary history of the tiger since its appearance in the fossil record about two million years ago, thereby giving a deep-time perspective. The key issue is to determine the extent to which ancestral populations of the tiger were geographically isolated. If no geographical isolation is likely, and gene flow between tiger populations could be maintained until modern times, then diagnosably distinct populations could not have evolved. Our reconstructions show that only two tiger populations were likely to have experienced significant geographical isolation from the main species distribution; these were to the west of Tibet (during the LGM) and on Japan (throughout the glacial cycle). In addition, the LGM is likely to have seen the partial separation of peninsular Malayan and Sunda Islands tigers from mainland populations. From a biogeographical perspective it seems probable that only three contemporary populations were sufficiently separated for the evolution of distinct populations, which can be regarded as subspecies or evolutionary significant units. Therefore, most variation in modern tiger populations is probably clinal, which has important implications for future conservation strategies both in the wild and in captivity.