The effect of bottleneck size on evolution in nested Darwinian populations

Previous work has shown how a minimal ecological structure consisting of patchily distributed resources and recurrent dispersal between patches can scaffold Darwinian properties onto collections of cells. When the timescale of dispersal is long compared with the time to consume resources, patch fitness increases but comes at a cost to cell growth rates. This creates conditions that initiate evolutionary transitions in individuality. A key feature of the scaffold is a bottleneck created during dispersal, causing patches to be founded by single cells. The bottleneck decreases competition within patches and, hence, creates a strong hereditary link at the level of patches. Here, we construct a fully stochastic model to investigate the effect of bottleneck size on the evolutionary dynamics of both cells and collectives. We show that larger bottlenecks simply slow the dynamics, but, at some point, which depends on the parameters of the within-patch model, the direction of evolution towards the equilibrium reverses. Introduction of random fluctuations in bottleneck sizes with some positive probability of smaller sizes counteracts this, even when the probability of smaller bottlenecks is minimal.

Automated summary

Previous work has found that a minimal ecological structure involving patchily distributed resources and recurrent dispersal between patches can introduce Darwinian properties to collections of cells. The introduction of a bottleneck during dispersal, causing patches to be founded by single cells, decreases competition within patches and leads to a strong hereditary link at the patch level. A fully stochastic model was constructed to show that larger bottlenecks slow down the evolutionary dynamics, but at a certain point, the direction of evolution reverses, which can be counteracted by introducing random fluctuations in bottleneck sizes.