The Darwinian Dynamic

Abstract

We argue that the evolution of order in living systems and certain nonliving physical systems obeys a common fundamental principle which we call the Darwinian dynamic. Such ordered systems deviate greatly from the thermodynamic equiprobability rule according to which, for example, all nucleotide sequences of comparable length should be found in roughly equal abundance. We formulate the Darwinian dynamic by first considering how macroscopic order is generated in a simple nonbiological system far from the thermodynamic equilibrium. We then extend our consideration to short, replicating RNA molecules, which we assume to be like the earliest forms of life, and show that the underlying order-generating process is basically similar. The equation we use as an example of the Darwinian dynamic for these simple replicators contains variables that express the basic conditions necessary for the process of natural selection as conceived by Darwin: variation of type, heritability, and competition for limited resources. By starting with such uncomplicated systems, we believe we have clarified the essential elements of natural selection. Specifically, we show that the fitness of an RNA replicator (its per capita rate of increase) is a function of adaptive capacities which are intrinsic (in the sense that they are determined by the nucleotide sequence) and of the availibility of resources. The three primary adaptive capacities are, we think, the capacity to replicate, the capacity to avoid decay, and the capacity to acquire and process resources. Competitive sucess among different replicators depends on the relative value of these adaptive capacities. We show for phage T4, the most complex organism for which nearly all gene functions are characterized, that its approximately 140 known genes fall naturally into the above three categories. Although the Darwinian dynamic can be expressed in simple mathematical form as it applies to such primitive life as self-replicating RNA or bacteria, its application to higher organisms requires a more complex treatment. We have indicated the main kinds of complexity that emerge, and how these can be taken into account, in consonance wich current treatments of natural selection in population genetics and evolutionary ecology. By defining the essential features of natural selection, we think we have additionally clarified two specific areas of prior confusion. These are the relationship of thermodynamics to evolution, and the empirical content of Darwin's theory.