Qualitative Analysis

There have been a number of outstanding evolutionary analyses of classic issues in neurobiology, and I describe three of these very briefly to suggest the topics and flavor: those by Ebbesson, by Karten, and by Killackey (complete citations for this discussion are in the chapter referred to in footnote on first page). Ebbesson provided a superb case history on the difficulties in interpreting and reasoning from available anatomical data on the brain. Ebbesson argued that connections are created by a process of "parcella-tion (segregation-isolation)'' that occurs ontogeneti-cally as well as phylogenetically, with originally diffuse and extravagantly proliferating neurons and connections eventually becoming reduced and segregated from one another during the course of development. Northcutt's commentary was noteworthy, pointing out not only the problems with the data used to support the position but also the semantic and philosophical difficulties: the need for rigorous specification of homologies and homoplasies in using comparative data for cladistic analysis.

Karten analyzed the origin of neocortex as a "uniquely" mammalian brain system, pointing out that neocortex is not functionally unique since its connections are comparable to those of the "neostriatum" in birds (and even in reptiles). The enlarged neostriatum in birds is homologous with mammalian neocortex. His microscopic analysis of these forebrain systems points to their comparability with respect to information processing, despite the very different ways that the brain is organized in these classes of vertebrates. This is in agreement with data that show birds and mammals to be comparable in "grade" of encephalization.

Killackey argued from ontogenetic data on the sequence of appearance of various neocortical regions, making the important point for evolutionists that the detailed organization of the neocortex is established to a significant extent by experience, and the evolution of its organization is therefore likely to be difficult to specify with standard genetic models.

Although informed by modern evolutionary theory, with the exception of Northcutt's commentary the discussion that I just reviewed was traditional in its evolutionary approach. The concern was to develop insight into the origin of neural systems and to the degree of specialization in different species. The analysis proceeded from data on morphology and development, and from educated intuition rather than from the rigorous application of cladistic methodology. All would agree that the nervous system parallels other systems in the body in reflecting adaptations to various environmental niches. Killackey and Ebbes-son emphasized the lability of the fate of neural structures, and there is a consensus about the significant extent to which use determines fate for the circuitry of the brain—that brains can be normal only if they develop in a normal environment.

Comparative brain data have also been used for more formal analyses of relationships, either with the methods of modern cladistics or with other multi-variate methods. The results of these analyses can be summarized in a few sentences. Performing a factor analysis on brain traits in fish helps to clarify issues on the classification of particular groups offish. The most helpful traits were the size of the olfactory apparatus. The contribution, however, is primarily to taxonomic issues rather than to neurobiology. The cladistic analyses, using only data on brain traits in constructing a species-by-traits matrix, produce essentially the same phylogenetic tree as when a full suite of traits is used. The diversity of species as determined rigorously by a full suite of traits predicts the measured diversity as determined from brain traits. The similarity between Hoplophoneus and F. catus in Fig. 1 is the expected finding in any comparative analysis of mammalian brains and confirms the taxonomic conclusion that these are relatively closely related species despite their separation by 30 million years of evolution. The brain can serve as well as other organs of the body for evidence on phylogenetic relationships.

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