Can clustering in genotype space reveal “Niches”?

Community ecology lacks the success enjoyed by pop-ulation genetics to quantify the relative roles played by deterministic and stochastic processes. It has been proposed that clustered patterns of abundance in genotype space provide evidence of selection in microbial communities, since no such clustering would arise in the absence of se-lection. We critique this test for its unrealistic null hypothesis. We show mathematically and with simulations that point mutations alone lead to clustering in genotype space by causing correlations between abun-dances of similar genotypes.We also show potential deviations from the mutation-only pattern caused by immigration from a source pool. Clus-tered patterns in genotype space may still be revealing of selection if analyzed quantitatively but only if neutral and selective regimes can be distinguished once mutation and immigration are included in the null model.