路径依赖的选择——统一自然选择与中性选择

Path-dependent selection—Integrating natural selection and neutral selection

进化思想和方法论并行的、相互独立的自然选择理论和中性选择理论是否能够有效统一起来,一直是理论生物学重点关注的问题,而路径依赖的选择提供了一种可能。与中性选择理论一致,路径依赖的选择也采用了完全对称性的前提假设——自然界的突变完全是基于统计随机性,不具备方向性,因而突变没有适合度的差异。但是在一些特定的时空环境下,环境的正反馈效应导致中性选择对称性的破缺,促使该突变产生递归效应——拉马克的主动选择或者获得性遗传效应,这种适宜环境正反馈作用下的生物主动选择与环境自然选择压力下的被动选择产生相互拮抗作用,并因此在多维的空间条件下形成路径。路径依赖的选择认为生物演化的过程将是一个路径频率的选择过程而不是适合度提高的过程,并强烈依赖于其历史上所经历的事件或者环境变化等外界条件;由于不同演化路径之间存在转移概率(比如生物演化中的质粒转移、转座子、生态过程相互作用等),路径的演化会存在加速或者减速效应,解释了古尔德等主张的间断平衡论。当环境选择压力很小或者为零的时候,多数或者所有路径都等价(类似中性选择结果);而随着环境压力的增大,不同路径的频率将差异化,且路径频率比较高的更容易被选择。当环境压力很大,历史事件等外界因素或者演化过程对路径本身的演化没有影响的时候(静态的、均衡状态),频率最高的那条路径就是演化所用时间最短或者最优的路径——达尔文自然选择理论的结果。在借鉴了现代物理学,尤其是量子力学所用的路径积分方法,路径依赖的选择也许为我们提供了一个不同的视角和方法来解释生物的演化过程。

Background&Aims:Natural selection and neutral selection are two parallel and independent theories on the biology evolution,both in terms of evolutionary ideas and methodology.However,whether these two theories are intrinsically linked or can be integrated into a same theory has remained as one of the big scientific questions in evolutionary biology science community.The path-dependent selection based on the idea of path theories of modern physics and economics might be potential to integrate natural selection and neutral selection.Summary:Path-dependent selection assumes that mutations in nature are completely random and non-directional,but that,in some specific spatio-temporal environments,positive feedback effects with the environment lead to recursive effects of that mutation and hence to the formation of paths.The process of biological evolution will be a process of selection of path frequency rather than fitness enhancement and will be strongly dependent on the evolutionary paths experienced in its history;there will be acceleration or deceleration effects in the formation of paths due to the existence of transition probabilities between different evolutionary paths,explaining the discontinuity-equilibrium principle advocated by Gould et al.The theoretical model also implies that there might exist a link between natural selection and neutral selection based on an evolutionary game model,using the path integral form of the transition probability density function.When the environmental selection pressure is small or zero,most or all paths are equivalent;whereas,as the environmental pressure increases,the frequency of different paths will differentiate and paths with higher frequency are more likely to be selected.When environmental pressures are high and history or evolutionary processes have no effect on the evolution of the path itself(static,equilibrium state),the path with the highest frequency is the one that has taken the shortest time to evolve or is optimal—a result of Darwin’s theory of natural selection.Prospects:Path-dependent selection based on path-integral methods used in modern physics,especially quantum mechanics,may provide us with a different perspective and approach to explaining the evolution of life.In existing theoretical models of pathway evolution,transition probabilities are continuously distributed,and the trajectories on which these transition probabilities themselves occur are paths.However,in the evolution of life,transition probabilities like the plasmid transfer or transposons,are intermittently discontinuous across paths.The mathematical structure of these problems in the life sciences will place new demands on physics and mathematical methods,and will likely provide new ideas for new developments in physics.