Causes and Consequences of Robustness and Plasticity in Biological Systems: Two Sides of the Same Coin

Frederik Nijhout (June 26, 2019)

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Two universal Rules of Life are that all organisms are subject to variable environments, and all are also subject to continuous mutations in genes that are important for normal function and survival. Organisms have evolved a variety of mechanisms that buffer form and function against deleterious environmental and genetic variables. These are collectively called homeostatic and robustness mechanisms, which stabilize the phenotype, so that the same phenotype is produced in spite of genetic and environmental variation. Insofar as natural selection acts only on phenotypes, but heritable change comes from genotypes, it has been thought that robustness mechanisms produce a constraint on evolution by decoupling phenotype form genotype. An apparently contradictory fact is that many organisms have a variable phenotype that depends on environmental conditions. This is called plasticity, and produces different phenotypes from the same genotype. Plasticity, therefore, also seems to uncouple phenotype and genotype. Plasticity, like robustness, can be an adaptation to a variable environment. Using conceptual and mathematical models, I will discuss a diversity of mechanisms that produce robustness and plasticity and show they are closely related. I will also discuss why such mechanisms, rather than constraining evolution, actually enable rapid evolution.