Approach of Complex-Systems Biology to Reproduction and Evolution

Abstract

Two basic issues in biology – the origin of life and evolution of phenotypes – are discussed on the basis of statistical physics and dynamical systems. In section “A Bridge Between Catalytic Reaction Networks and Reproducing Cells”, we survey recent developments in the origin of reproducing cells from an ensemble of catalytic reactions. After surveying several models of catalytic reaction networks briefly, we provide possible answers to the following three questions: (1) How are nonequilibrium states sustained in catalytic reaction dynamics? (2) How is recursive production of a cell maintaining composition of a variety of chemicals possible? (3) How does a specific molecule species carry information for heredity? In section “Evolution”, general relationships between plasticity, robustness, and evolvability are presented in terms of phenotypic fluctuations. First, proportionality between evolution speed, phenotypic plasticity, and isogenic phenotypic fluctuation is proposed by extending the fluctuation–response relationship in physics. We then derive a general proportionality relationship between the phenotypic fluctuations of epigenetic and genetic origin: the former is the variance of phenotype due to noise in the developmental process, and the latter due to genetic mutation. The relationship also suggests a link between robustness to noise and to mutation. These relationships are confirmed in models of gene expression dynamics, as well as in laboratory experiments, and then are explained by a theory based on an evolutionary stability hypothesis For both sections “A Bridge Between Catalytic Reaction Networks and Reproducing Cells” and “Evolution”, consistency between two levels of hierarchy (i.e., molecular and cellular, or genetic and phenotypic levels) is stressed as a principle for complex-systems biology.