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Fixation probabilities in weakly compressible fluid flows

A. Plummer,R. Benzi,D. Nelson,F. Toschi

Published 2018 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance Marine fronts, where two different water masses meet, are characterized by high biological productivity. Determining which organisms are able to thrive in these environments is crucial to understanding ocean ecosystems. However, many questions about how natural selection functions in an advective environment remain unanswered. We study flows with sources and sinks to model the upwellings and convergences experienced by plankton at marine fronts. Remarkably, even when the flows are so weak that they cannot significantly alter the spatial distribution of organisms, they substantially affect genetic outcomes. The flow reduces the effective population size and makes the fate of organisms strongly dependent on local flow conditions. In particular, organisms born in upwelling regions experience a considerably enhanced probability of survival. Competition between biological species in marine environments is affected by the motion of the surrounding fluid. An effective 2D compressibility can arise, for example, from the convergence and divergence of water masses at the depth at which passively traveling photosynthetic organisms are restricted to live. In this report, we seek to quantitatively study genetics under flow. To this end, we couple an off-lattice agent-based simulation of two populations in 1D to a weakly compressible velocity field—first a sine wave and then a shell model of turbulence. We find for both cases that even in a regime where the overall population structure is approximately unaltered, the flow can significantly diminish the effect of a selective advantage on fixation probabilities. We understand this effect in terms of the enhanced survival of organisms born at sources in the flow and the influence of Fisher genetic waves.

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