  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.
