Phenotypic determinism and contingency in the evolution of hypothetical tree-like organisms

Whether evolutionary history is mostly contingent or deterministic has been given much focus in the field of evolutionary biology. Studies addressing this issue have been conducted theoretically, based on models, and experimentally, based on microcosms. It has been argued that the shape of the adaptive landscape and mutation rate are major determinants of replicated phenotypic evolution. In the present study, to incorporate the effects of phenotypic plasticity, we constructed a model using tree-like organisms. In this model, the basic rules used to develop trees are genetically determined, but tree shape (described by the number and aspect ratio of the branches) is determined by both genetic components and plasticity. The results of the simulation show that the tree shapes become more deterministic under higher mutation rates. However, the tree shape became most contingent and diverse at the lower mutation rate. In this situation, the variances of the genetically determinant characters were low, but the variance of the tree shape is rather high, suggesting that phenotypic plasticity results in this contingency and diversity of tree shape. The present findings suggest that plasticity cannot be ignored as a factor that increases contingency and diversity of evolutionary outcomes.

进化生物学领域长期以来高度关注演化历史究竟以偶然性为主还是决定性为主这一核心议题。针对该议题的研究已从理论层面（基于模型构建）与实验层面（基于微宇宙（microcosm）体系）展开。已有研究指出，自适应景观（adaptive landscape）的形态与突变率是重复表型演化的主要决定因素。本研究为纳入表型可塑性（phenotypic plasticity）的影响，以树形生物为对象构建了演化模型。该模型中，树木发育的基本规则由遗传决定，但树形（以分枝数量与分枝长宽比表征）同时受遗传组分与可塑性共同调控。模拟结果显示，在较高突变率条件下，树形演化更趋近于决定性模式；而在较低突变率条件下，树形反而呈现出最强的偶然性与最高的多样性。在此情形下，遗传决定性状的方差较低，但树形的方差却相对较高，这表明表型可塑性正是导致树形演化出现偶然性与多样性的关键因素。本研究结果表明，在提升演化结果的偶然性与多样性方面，表型可塑性是不容忽视的重要因素。