Geissmann, T. 1999: Taxonomy and evolution of gibbons. In: Primatology and Anthropology into the Third Millennium. Centenary Congress of the Anthropological Insitute in Zürich: 1899-1999, Programme and Abstracts, p. 20, Anthropological Insitute, Zürich.

Taxonomy and evolution of gibbons

T. Geissmann

Institute of Zoology, Tieraerztliche Hochschule Hannover

There is little fossil material relating to gibbon evolution. The few specimens (mainly teeth) reliably identified as members of the radiation are not older than Mid-Pleistocene and are only marginally different from their modern counterparts. As a result, gibbon evolution can only be reconstructed from a comparative analysis of evolutionarily informative characteristics of modern gibbons and, to some degree, of related primate taxa that can be used as outgroups.

Traditionally, two sets of data have been available for this task: Set 1 consists of mainly morphological and anatomical data, and set 2 uses characteristics of fur colouration. Results based on set 1 suggested the recognition of 4 distinct subgroups of gibbons. Set 2 produced quite different results, with certain members of one subgenus Hylobates (H. klossii) being identical in fur colouration to the members of another (Symphalangus), and with some populations of different species resembling each other more than populations of the same species (i.e. the "H. muelleri abbotti" and "H. agilis albibarbis" problems). In addition, set 2 has proved particularly difficult to analyse because some gibbon populations are either extremely variable in fur colouration, clearly polychromatic or sexually dichromatic. In addition, some gibbons go through as many as three distinct, age-related colouration phases linked by relatively extended phases during which individuals show intermediate colour patterns.

Results derived from set 1 have, therefore, become widely accepted, with each of the 4 subgroups tentatively having been assigned to a distinct subgenus (Symphalangus, Nomascus, Bunopithecus, and Hylobates) within the genus Hylobates, although their phylogenetic relationships remain unclear.

In recent years, two additional types of data have become available: vocal characteristics (set 3) and DNA sequences (set 4). Both sets have been able to confirm the results gained from set 1 and support a split of the genus into the 4 subgroups mentioned above. Although these newer types of data are widely believed to bear a considerable potential for eventually resolving some of the questions regarding gibbon systematics and evolution, neither has been able, so far, to reliably resolve the order of the 4-way split in the genus or the relationship of the taxa within the larger subgroups (subgenera Hylobates and Nomascus).

This study presents a re-evaluation of sets 1-3, as well as phylogenetic trees resulting from a cladistic analysis of each. Gibbons appear to have undergone such a radical "rebuilding" since their split from the last common ancestor with the other modern hominoids that it is difficult to find gibbon chracteristics which can be compared in any way with those of potential outgroups. This problem is shared by all available data sets. The tempo of evolutionary change, however, appears to differ among the data sets, similar to DNA sequences derived from different parts of the genome. Most data from set 1 appear to have changed considerably less than most of those from set 3, whereas set 2 takes an intermediate position. As a result, each set appears to be suited to the analysis of different levels of resolution within the hylobatid radiation.