Crossover behaviour of 3-species systems with mutations or migrations

Abstract

We study the ABC model in the cyclic competition (A+B → 2B, B+C → 2C, C+A → 2A) and the neutral drift (A+B → 2B or 2A, B+C → 2C or 2B, C+A → 2A or 2C) versions, with mutations and migrations introduced into the model. When stochastic phenomena are taken into account, there are three distinct regimes in the model. (i) In the “fixation” regime, the first extinction time scales with the system size N and has an exponential distribution, with an exponent that depends on the mutation/migration probability per particle μ. (ii) In the “diversity” regime, the order parameter remains nonzero for very long times, and becomes zero only rarely, almost never for large system sizes. (iii) In the critical regime, the first passage time for crossing the boundary (one of the populations becoming zero) has a power law distribution with exponent −1. The critical mutation/migration probability scales with system size as N ⁻¹. The transition corresponds to a crossover from diffusive behaviour to Gaussian fluctuations about a stable solution. The analytical results are checked against computer simulations of the model.