Evolutionary Dynamics on Graphs
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letters to nature Received 24 September; accepted 16 November 2004; doi:10.1038/nature03211. often individuals place offspring into adjacent vertices. The 3 1. MacArthur, R. H. & Wilson, E. O. The Theory of Island Biogeography (Princeton Univ. Press, homogeneous population, described by the Moran process ,is Princeton, 1969). the special case of a fully connected graph with evenly weighted 2. Fisher, R. A., Corbet, A. S. & Williams, C. B. The relation between the number of species and the number of individuals in a random sample of an animal population. J. Anim. Ecol. 12, 42–58 (1943). edges. Spatial structures are described by graphs where vertices 3. Preston, F. W. The commonness, and rarity, of species. Ecology 41, 611–627 (1948). are connected with their nearest neighbours. We also explore 4. Brown, J. H. Macroecology (Univ. Chicago Press, Chicago, 1995). evolution on random and scale-free networks5–7. We determine 5. Hubbell, S. P. 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Novak, J. Duke, J. Orcutt, K. Barnes, C. Neufeld and L. Weintraub for field assistance; and NSF, EPA (CISES) and the Andrew W. Mellon foundation for partial financial support. Competing interests statement The author declares that he has no competing financial interests. Correspondence and requests for materials should be addressed to J.T.W. ([email protected]). .............................................................. Evolutionary dynamics on graphs Erez Lieberman1,2, Christoph Hauert1,3 & Martin A. Nowak1 1 Program for Evolutionary Dynamics, Departments of Organismic and Figure 1 Models of evolution. a, The Moran process describes stochastic evolution of a Evolutionary Biology, Mathematics, and Applied Mathematics, Harvard finite population of constant size. In each time step, an individual is chosen for University, Cambridge, Massachusetts 02138, USA reproduction with a probability proportional to its fitness; a second individual is chosen for 2Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA death. The offspring of the first individual replaces the second. b, In the setting of 3Department of Zoology, University of British Columbia, Vancouver, British evolutionary graph theory, individuals occupy the vertices of a graph. In each time step, an Columbia V6T 1Z4, Canada individual is selected with a probability proportional to its fitness; the weights of the ............................................................................................................................................................................. outgoing edges determine the probabilities that the corresponding neighbour will be Evolutionary dynamics have been traditionally studied in the replaced by the offspring. The process is described by a stochastic matrix W, where w ij context of homogeneous or spatially extended populations1–4. denotes the probability that an offspring of individual i will replace individual j. In a more Here we generalize population structure by arranging individ- general setting, at each time step, an edge ij is selected with a probability proportional to uals on a graph. Each vertex represents an individual. The its weight and the fitness of the individual at its tail. The Moran process is the special case weighted edges denote reproductive rates which govern how of a complete graph with identical weights. 312 NATURE | VOL 433 | 20 JANUARY 2005 | www.nature.com/nature © 2005 Nature Publishing Group letters to nature are sensitive to subtle differences in selective values. The tension tee—of fixation, whereas disadvantageous mutants are likely—but between selection and drift lies at the heart of the famous dispute again, no guarantee—to become extinct. between Fisher and Wright8–10. There is evidence that population We introduce population structure as follows. Individuals are structure affects the interplay of these forces11–15. But the celebrated labelled i ¼ 1, 2, …N. The probability that individual i places its 16 17 results of Maruyama and Slatkin indicate that spatial structures offspring into position j is given by w ij. are irrelevant for evolution under constant selection. Thus the individuals can be thought of as occupying the vertices Here we introduce evolutionary graph theory, which suggests a of a graph. The matrix W ¼ [w ij] determines the structure of the promising new lead in the effort to provide a general account of how graph (Fig. 1b). If w ij ¼ 0 and wji ¼ 0 then the vertices i and j are population structure affects evolutionary dynamics. We study the not connected. In each iteration, an individual i is chosen for simplest possible question: what is the probability that a newly reproduction with a probability proportional to its fitness. The introduced mutant generates a lineage that takes over the whole resulting offspring will occupy vertex j with probability w ij. Note population? This fixation probability determines the rate of evolu- that W is a stochastic matrix, which means that all its rows sum to tion, which is the product of population size, mutation rate and one. We want to calculate the fixation probability r of a randomly fixation probability. The higher the correlation between the placed mutant. mutant’s fitness and its probability of fixation, r, the stronger the Imagine that the individuals are arranged on a spatial lattice that effect of natural selection; if fixation is largely independent of can be triangular, square, hexagonal or any similar tiling. For all fitness, drift dominates. We will show that some graphs are such lattices r remains unchanged: it is equal to the r 1 obtained for governed entirely by random drift, whereas others are immune to the homogeneous population. In fact, it can