Figure 2.
Gut microbes as potentiators of host dietary shifts. Graphs depict fitness surfaces, or the fitness outcomes of hosts (vertical axis) as a function of variation in a host phenotype value (horizontal axis), in this case, the degree of milk consumption as an adult. Presume that acquisition and consumption of milk has increasing fitness costs to the host, and fitness rewards that plateau at a certain point. Presume also that ability to digest lactose, either through secretion of endogenous host enzymes or via Bifidobacterium, increases the point at which that fitness reward plateaus. In a background population without adult production of lactase (LCTwt allele; (a)), a change in phenotype that increased milk acquisition and consumption (Plow → Phigh) might lead to a fitness decrease in the absence of Bifidobacterium in the gut, but a fitness increase in its presence. After fixation of the lactase-production allele (LCTmut) in the population (b), the host may no longer be sensitive to the presence of the lactose-degrading bacterium in the gut. In this way, the evolution of the host diet could be potentiated by gut bacteria, even without ultimately depending on the bacteria.