Centre for the Study of Existential Risk, University of Cambridge, Cambridge, UK.
Department of Sociology, Philosophy and Anthropology, University of Exeter, Exeter, UK.
BMC Biol. 2021 May 10;19(1):97. doi: 10.1186/s12915-021-01024-1.
Vertebrate brain structure is characterised not only by relative consistency in scaling between components, but also by many examples of divergence from these general trends.. Alternative hypotheses explain these patterns by emphasising either 'external' processes, such as coordinated or divergent selection, or 'internal' processes, like developmental coupling among brain regions. Although these hypotheses are not mutually exclusive, there is little agreement over their relative importance across time or how that importance may vary across evolutionary contexts.
We introduce an agent-based model to simulate brain evolution in a 'bare-bones' system and examine dependencies between variables shaping brain evolution. We show that 'concerted' patterns of brain evolution do not, in themselves, provide evidence for developmental coupling, despite these terms often being treated as synonymous in the literature. Instead, concerted evolution can reflect either functional or developmental integration. Our model further allows us to clarify conditions under which such developmental coupling, or uncoupling, is potentially adaptive, revealing support for the maintenance of both mechanisms in neural evolution. Critically, we illustrate how the probability of deviation from concerted evolution depends on the cost/benefit ratio of neural tissue, which increases when overall brain size is itself under constraint.
We conclude that both developmentally coupled and uncoupled brain architectures can provide adaptive mechanisms, depending on the distribution of selection across brain structures, life history and costs of neural tissue. However, when constraints also act on overall brain size, heterogeneity in selection across brain structures will favour region specific, or mosaic, evolution. Regardless, the respective advantages of developmentally coupled and uncoupled brain architectures mean that both may persist in fluctuating environments. This implies that developmental coupling is unlikely to be a persistent constraint, but could evolve as an adaptive outcome to selection to maintain functional integration.
脊椎动物的大脑结构不仅具有成分之间相对一致的缩放特征，而且还有许多背离这些总体趋势的例子。替代假说通过强调“外部”过程（如协调或发散选择）或“内部”过程（如大脑区域之间的发育耦合）来解释这些模式。尽管这些假设不是相互排斥的，但对于它们在时间上的相对重要性或在进化背景下的重要性如何变化，几乎没有达成一致意见。
我们引入了一个基于代理的模型来模拟“基本”系统中的大脑进化，并研究了影响大脑进化的变量之间的依赖关系。我们表明，尽管这些术语在文献中经常被视为同义，但“协调”的大脑进化模式本身并不能提供发育耦合的证据。相反，协调进化可以反映功能或发育整合。我们的模型还使我们能够澄清在哪些条件下，这种发育耦合或解偶联可能是适应性的，这为神经进化中维持这两种机制提供了支持。至关重要的是，我们说明了偏离协调进化的概率如何取决于神经组织的成本/收益比，当大脑整体大小本身受到限制时，这种比率会增加。
我们的结论是，发育上耦合和不耦合的大脑结构都可以提供适应性机制，具体取决于大脑结构、生活史和神经组织成本之间的选择分布。然而，当约束也作用于大脑整体大小时，大脑结构之间的选择异质性将有利于特定区域或镶嵌进化。无论如何，发育上耦合和不耦合的大脑结构的各自优势意味着它们都可能在波动的环境中持续存在。这意味着发育耦合不太可能成为持续的限制，但可能作为一种适应性结果进化出来，以维持功能整合。
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