Mosaic evolution of brain structure in mammals

Abstract

The mammalian brain comprises a number of functionally distinct systems. It might therefore be expected that natural selection on particular behavioural capacities would have caused size changes selectively, in the systems mediating those capacities. It has been claimed, however, that developmental constraints limited such mosaic evolution, causing co-ordinated size change among individual brain components. Here we analyse comparative data to demonstrate that mosaic change has been an important factor in brain structure evolution. First, the neocortex shows about a fivefold difference in volume between primates and insectivores even after accounting for its scaling relationship with the rest of the brain. Second, brain structures with major anatomical and functional links evolved together independently of evolutionary change in other structures. This is true at the level of both basic brain subdivisions and more fine-grained functional systems. Hence, brain evolution in these groups involved complex relationships among individual brain components.

The evolution of the cortico-cerebellar complex in primates: anatomical connections predict patterns of correlated evolution

“…These results corroborate the suggestion that, during the radiation of the primate order, the neocortex and cerebellum have undergone correlated evolution (Barton & Harvey 2000). They additionally show that other related structures, the pons, thalamus and vestibular complex, have also changed in concert.…”

“…The first part expands on studies showing that the cerebellum has undergone correlated evolutionary size changes with the neocortex 7,13,14 . The evolution of the cerebellum and the neocortex is investigated in more detail here to determine which particular areas of the cerebellum have shown correlated evolutionary changes with the neocortex.…”

“…At present, there is much controversy over whether the brain evolves as a coordinated whole 6 or whether individual structures or systems can evolve independently 4 of changes in other parts of the brain (mosaic evolution) 7 . This paper tests the mosaic change hypothesis by analysing patterns of correlated evolution among interconnected structures.…”

“…Cerebellar systems are of interest because recent evidence suggests that they have expanded in some groups of primates 11 . In addition, there is evidence for conjoint expansion of the cerebellum with an area that has been the subject of intense scrutiny, the neocortex 7 . Finally, volumetric comparative data are available on individual cerebellar and vestibular nuclei 12 .…”

Comparative methods for studying primate adaptation and allometry

“…For example, Figure 9 shows that anthropoid primates have larger neocortices relative to the rest of the brain than do prosimians, and that primates in general have larger relative neocortices than do other mammals, such as insectivores. 84 Contrary to the argument, however, the contrasts method tends to do better than analysis of species values in this situation. 26,30,100 This is because multiple data points in a species analysis are treated incorrectly as independent but, with the method of independent contrasts, only a single contrast is calculated for each grade shift, and this single data point will produce less bias (Fig.…”