Evolutionary Accessibility of Modular Fitness Landscapes
Abstract
A fitness landscape is a mapping from the space of genetic sequences, which is modeled here as a binary hypercube of dimension L, to the real numbers. We consider random models of fitness landscapes, where fitness values are assigned according to some probabilistic rule, and study the statistical properties of pathways to the global fitness maximum along which fitness increases monotonically. Such paths are important for evolution because they are the only ones that are accessible to an adapting population when mutations occur at a low rate. The focus of this work is on the block model introduced by A.S. Perelson and C.A. Macken (Proc. Natl. Acad. Sci. USA 92:9657, 1995) where the genome is decomposed into disjoint sets of loci (`modules') that contribute independently to fitness, and fitness values within blocks are assigned at random. We show that the number of accessible paths can be written as a product of the path numbers within the blocks, which provides a detailed analytic description of the path statistics. The block model can be viewed as a special case of Kauffman's NKmodel, and we compare the analytic results to simulations of the NKmodel with different genetic architectures. We find that the mean number of accessible paths in the different versions of the model are quite similar, but the distribution of the path number is qualitatively different in the block model due to its multiplicative structure. A similar statement applies to the number of local fitness maxima in the NKmodels, which has been studied extensively in previous works. The overall evolutionary accessibility of the landscape, as quantified by the probability to find at least one accessible path to the global maximum, is dramatically lowered by the modular structure.
 Publication:

Journal of Statistical Physics
 Pub Date:
 January 2014
 DOI:
 10.1007/s1095501308688
 arXiv:
 arXiv:1306.1938
 Bibcode:
 2014JSP...154..334S
 Keywords:

 Evolution;
 Fitness landscapes;
 Adaptive walks;
 Spin glasses;
 Quantitative Biology  Populations and Evolution;
 Condensed Matter  Disordered Systems and Neural Networks
 EPrint:
 26 pages, 12 figures