Abstract
Alpha and sigma taxonomy of Pan (chimpanzees) and Plio-Pleistocene hominin species
Highlights
A fundamental question in biology, and 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 species[1,2,3], 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 cranium[4]
The degree of scatter around the regression equation is quantifiable using the log of the standard error of the m co-efficient. 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 taxonomy[5] which assumes discrete entities
Summary
The approach has been applied to measurements obtained from more than 70 taxa[1], and more recently to measurements of crania of Pan troglodytes, the common chimpanzee, and to those of P. paniscus, the bonobo[2,6]. The results were remarkable in the sense that, in the case of both species analysed separately (using alpha taxonomy), a mean log sem value of -1.6 was obtained for conspecific pairs. The data confirmed a hypothesis proposed by Thackeray[1] that -1.61 for mean log sem values constitutes an approximation of a biological species constant (T), relating to a central tendency for the degree of variation within a species. An associated standard deviation for this proposed biological constant was given as 0.1 when using more than 2000 regression analyses for pairwise comparisons of specimens of the same species.[2].
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