Figure 2.

Host-associated microbes cannot establish in a new population in which they are incapable of independent proliferation outside hosts, unless VT is high, and they increase host fitness significantly. In population 1, microbes can proliferate independently of hosts and there is environmental acquisition by the host. In population 2, microbes cannot be acquired by hosts from the environment, only via maternal VT from dispersers from population 1. Each line represents different steady-state microbe carrier proportions (0.1–0.9, increasing in 0.2 intervals). Multiples of each line represent additional degrees of host mate choice, e.g. the number of opportunities a male has to find a preferred mating partner from n = 1 (no choice) to 9 (choosiest). Black arrows in (b,d) are used to highlight the main differences in parameter combinations capable of producing a high carrier frequency (>0.9). Each figure shows a possible combination of host mate choice and host–microbe carrier fitness relative to non-carrier fitness, with details as specified in the figure labels. Here, the microbe cannot spread with its host carriers into a second population at anything less than equal fitness to non-microbe-carrying hosts. Positive assortative mating by hosts for the same carrier type (carrier–carrier versus non-carrier–non-carrier) in (a,b) strongly reinforces population separation. In (c, d) preference by males is for female microbe carriers and this has the opposite effect, relaxing the degree of VT required for carriers to persist in population 2. For full model outputs, see electronic supplementary material, figures S5b and S6b. (Online version in colour.)