Allometry unleashed: an adaptationist approach of brain scaling in mammalian evolution

Abstract

The idea that allometry in the context of brain evolution mainly result from constraints channelling the scaling of brain components is deeply embedded in the field of comparative neurobiology. Constraints, however, only prevent or limit changes, and cannot explain why these changes happen in the first place. In fact, considering allometry as a lack of change may be one of the reasons why, after more than a century of research, there is still no satisfactory explanatory framework for the understanding of species differences in brain size and composition in mammals. The present paper attempts to tackle this issue by adopting an adaptationist approach to examine the factors behind the evolution of brain components. In particular, the model presented here aims to explain the presence of patterns of covariation among brain components found within major taxa, and the differences between taxa. The key determinant of these patterns of covariation within a taxon-cerebrotype (groups of species whose brains present a number of similarities at the physiological and anatomical levels) seems to be the presence of taxon-specific patterns of selection pressures targeting the functional and structural properties of neural components or systems. Species within a taxon share most of the selection pressures, but their levels scale with a number of factors that are often related to body size. The size and composition of neural systems respond to these selection pressures via a number of evolutionary scenarios, which are discussed here. Adaptation, rather than, as generally assumed, developmental or functional constraints, thus appears to be the main factor behind the allometric scaling of brain components. The fact that the selection pressures acting on the size of brain components form a pattern that is specific to each taxon accounts for the peculiar relationship between body size, brain size and composition, and behavioural capabilities characterizing each taxon. While it is important to avoid repeating the errors of the “Panglossian paradigm”, the elements presented here suggests that an adaptationist approach may shed a new light on the factors underlying, and the functional consequences of, species differences in brain size and composition.

Imaging evolution of the primate brain: the next frontier?

“…Allometric changes may provide a window for understanding the adaptations of specific neural systems in response to evolutionary pressure ( Willemet, 2019 ; Finlay et al, 2001 ). In this regard, evolutionary psychology and neuroscience suggest that allometry arises from evolutionary developmental constraints, as a brain adjustment to optimise its functional organisation ( Montgomery 2013 ; Willemet, 2015a, Montgomery et al, 2016 ).…”

Commentary: Mosaic and Concerted Brain Evolution: The Contribution of Microscopic Comparative Neuroanatomy in Lower Vertebrates

“…Finally, the hypothesis implies that brain evolution is mostly mediated by functional selection (under the constraints of energetic/physiological costs). Yet, part of the evolution of brain components may be due to selection on their structural properties (Willemet, 2013(Willemet, , 2019. D'Aniello et al seem to adhere to the traditional belief that the debate is polarized between these two hypotheses, and that, so to speak, keeping count on how many studies support one hypothesis or the other would allow to determine which constraint (functional or developmental) is more important in a given context.…”

“…The developmental and functional constraints discussed above are an integral part of this framework, although their effect are limited compared to the other factors. Such an adaptationist approach gives new insights and allows to make unique predictions regarding the changes in brain size and composition between and within taxa, as well as their functional consequences (Willemet, 2019).…”