Does bigger mean better? Evolutionary determinants of brain size and structure.

Current perspectives on brain evolution relate brain size variability to two main parameters: a scaling factor that corresponds to overall body size and an ecological factor associated with behavioral capacity. I suggest in this paper that in evolution body weight and ecological conditions have different effects on brain structure, resulting in distinct differences in neural architecture, even if both factors may produce brain size increases. There are two postulated modalities of brain growth, one passive that lags behind increases in body size, and one active that relates to selection of specific behavioral abilities and hence increased processing capacity. These two modes of growth differ in three main aspects: (i) cellular and connectional rearrangements are modest in passive brain growth while they are conspicuous in active growth, corresponding to increases in processing capacity; (ii) passive brain growth follows a rather conservative allometric rule between brain components, while active growth usually affects only a few brain parts, thereby producing much steeper allometric relations between these parts and sometimes also in brain/body relations; and (iii) passive growth may either affect early periods of ontogenic brain development or produce a generalized increase in cell proliferation in later periods. On the other hand, active growth is restricted to relatively late developmental periods. Finally, an evolutionary scenario for the active mode is proposed where phylogenetic selection of an increased number of cells in particular brain regions occurs in order to facilitate neural reorganization and to increase the specificity of connections. This view emphasizes the role of connectional modifications in increasing brain capacity and contrasts with current ideas of a unitary process of phylogenetic brain growth, where a larger brain size per se produces better processing capacity, regardless of the causal factor behind it.