Alpha and sigma taxonomy of Pan (chimpanzees) and Plio-Pleistocene hominin species

• Published in: South African Journal of Science 2018-11, Vol.114 (11-12), p.1-2
• Main Author: Thackeray, J. Francis
• Format: Article
• Language: English
• Subjects: Biological species constant; Boundaries; Chimpanzees; Conspecificity; Cranium; Hypotheses; Morphometrics; Paleontology; Pleistocene; Regression analysis; Species; Species variation; Statistical analysis; Taxonomy
• ISSN: 0038-2353; 1996-7489
• DOI: 10.17159/sajs.2018/a0291

A fundamental question in biology, and more specifically in palaeontology, is ‘how much variation is there within a biological species?’ To answer that question, it is necessary to define a species, notably in a way that can be applied in palaeontological contexts. Recognising that boundaries between taxa may not always be clear, an appeal has been made for a probabilistic definition of a species1-3, based on pairwise comparisons of specimens and morphometric analyses using least squares linear regression analysis associated with a general equation of the form y=mc+c, where x and y are linear dimensions of a skeletal element such as a cranium4. The degree of scatter around the regression equation (associated with morphology) is quantifiable using the log of the standard error of the m co-efficient (log sem). Here it is shown how this morphometric approach can be applied to cranial specimens attributed to two extant species of Pan, and to extinct Plio-Pleistocene hominins in a temporal sequence, indicating the lack of clear boundaries between species, thereby challenging the prevailing concept of alpha taxonomy5 which assumes discrete entities. An appeal is made for an alternative concept, namely sigma taxonomy.3