Extended Data Fig. 1: Increased stochasticity in small host populations.

In Fig. 2, we show how the microbiome transmission fidelity shapes host phenotype distributions. In doing so, we simulated large populations, consisting of 500 individuals, in order to obtain robust results. This results in a limited role of stochasticity, explaining the relatively low variation across replicated simulations (see shaded regions in Fig. 2a,b). In smaller populations, however, populations are, unsurprisingly, more sensitive to stochastic processes. Here, we set transmission fidelity τ at 1, implying strict vertical transmission, and assessed the average deviation from P = 0 for varying population sizes. In small populations, there is an increase in the number of maladapted populations (that is a larger deviation from the optimal phenotype). Grey dots indicate individual simulations (30 per population size), red lines indicate median values for each population size. τ = 1; ω² = 1; \(\sigma _\varphi ^2\)=2; V_α=0.01.