Fig. 4.

Varying the effect of beneficial substitutions on fitness of free-living cells. Parameters: $s_b=0.1$, γ = 5, ${\mu }_b={10}^{-8}$ and a linear fitness function. When N_FL = 1000, the population size of free-living genomes is equal to the number of eukaryotic hosts; when $N_{FL}=50,000$, the population size of free-living genomes is equal to the number of cytoplasmic genomes (assuming N = 1000 and n = 50, as in fig. 2). The y-axis shows the mean number of generations to accumulate a single beneficial substitution (see fig. 2F legend for details). On the x-axis, we vary the effect mutations have on the fitness of free-living cells. A mutation on a free-living genome has an s_FL-fold effect on its cell’s fitness compared to the effect of a mutation on a cytoplasmic genome on its host’s fitness. The dashed line represents the mean number of generations required to accumulate a beneficial substitution assuming uniparental inheritance (relaxed bottleneck) under equivalent conditions ($\approx 272$; see fig. 2F). (A) Population size of free-living genomes equals 1000. (B) Population size of free-living genomes equals 50,000. Error bars are ± standard error of the mean.