Wednesday, 5 February 2014

Preprint: Evolutionary dynamics of shared niche construction

I have just uploaded a new preprint on arXiv (and bioaRxiv, a new preprint repository for biology) that explores the evolutionary dynamics of shared niche construction.

In the model we assume that the carrying capacity of each species in the population consists of the sum of two parts: an intrinsic part, and a contribution from all species present in the system. If the constructed niche is highly specific, only the first part is included, while a non-specific niche construction corresponds to the second contribution dominating.

Now it turns out that the evolutionary dynamics of the system strongly depends on the specificity: when the carrying capacity is intrinsic, selection is almost exclusively for mutants with higher carrying capacity, while a shared carrying capacity yields selection purely on growth rate.

The below figure illustrates this fact. In the upper panel, where specificity is low, the invasion of a mutant can lead to a decrease in total population size, while in the lower panel, where carrying capacity is intrinsic, each successful invasion increases the total population size. 

Coming from a background in cancer I prefer interpret this result in the context of tumour growth. If you think of different types (or subclones) of cancer cells as being able to withstand and survive different cell densities (i.e. the niche is specific to each subclone) then growth rate of a rare mutant is irrelevant for determining if it spreads in a tumour populated at the maximal cell density of the resident subclone. Only if it can divide and survive at higher densities will it spread and take over the tumour.

The other extreme can be viewed in terms of diffusible factors, such as angiogenetic factors that attract new blood vessels to the growing tumour. The release of a factor benefits all cells (within a reasonable distance) and hence increases the carrying capacity of all subclones. Now a mutant that produces less factors compared to the resident will still receive the benefit, and if it divides faster, will spread in the population. This situation is analogous to the appearance of cheaters in the classical public goods game.


Abstract:

Many species engage in niche construction that ultimately leads to an increase in the carrying capacity of the population. We have investigated how the specificity of this behaviour affects evolutionary dynamics using a set of coupled logistic equations, where the carrying capacity of each genotype consists of two components: an intrinsic part and a contribution from all genotypes present in the population. The relative contribution of the two components is controlled by a specificity parameter $\gamma$, and we show that the ability of a mutant to invade a resident population depends strongly on this parameter. When the carrying capacity is intrinsic, selection is almost exclusively for mutants with higher carrying capacity, while a shared carrying capacity yields selection purely on growth rate. This result has important implications for our understanding of niche construction, in particular the evolutionary dynamics of tumor growth.
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