Integration, phylogeny, and the hominid cranial base

  • Strait D
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Basicranial features were examined in catarrhine primates and early hominids in order to dem-onstrate how information about morphological integration can be incorporated into phylogenetic analysis. Hypothe-ses purporting to explain the functional and structural relationships of basicranial characters were tested using factor analysis. Characters found to be functionally or structurally related to each other were then further exam-ined in order to determine whether there was evidence that they were phylogenetically independent. If phyloge-netic independence could not be demonstrated, then the characters were presumed to be integrated and were grouped into a complex. That complex was then treated as if it were a single character for the purposes of cladistic analysis. Factor analysis revealed that five basicranial features may be structurally related to relative brain size in homi-noids. Depending on how one defines phylogenetic inde-pendence, as few as two, or as many as all of those char-acters might be morphologically integrated. A cladistic analysis of early hominids based on basicranial features revealed that the use of integrated complexes had a sub-stantial effect on the phylogenetic position of Australo-pithecus africanus, a species whose relationships are poorly resolved. Moreover, the use of complexes also had an effect on reanalyses of certain published cladistic data sets, implying that those studies might have been biased by patterns of basicranial integration. These results dem-onstrate that patterns of morphological integration need to be considered carefully in all morphology-based cladis-tic analyses, regardless of taxon or anatomical focus. How-ever, an important caveat is that the functional and struc-tural hypotheses tested here predicted much higher degrees of integration than were observed. This result warns strongly that hypotheses of integration must be tested before they can be adequately employed in phylo-genetic analysis. The uncritical acceptance of an untested hypothesis of integration is likely to be as disruptive to a cladistic analysis as when integration is ignored. Functional and developmental integration leads to the coinher-itance of character complexes. These coinherited character com-plexes are then constrained to evolve in a coordinated fashion. —Cheverud (1995, p. 84)

Author-supplied keywords

  • Australopithecus
  • Basicranium
  • Brain
  • Cladistics
  • Morphological integration

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  • David S. Strait

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