Evolution under pH stress and high population densities leads to increased density-dependent fitness in the protist Tetrahymena thermophila

Abiotic stress is a major force of selection that organisms are constantly
facing. While the evolutionary effects of various stressors have been
broadly studied, it is only more recently that the relevance of
interactions between evolution and underlying ecological conditions, that
is, eco-evolutionary feedbacks, have been highlighted. Here, we
experimentally investigated how populations adapt to pH-stress under high
population densities. Using the protist species Tetrahymena thermophila,
we studied how four different genotypes evolved in response to stressfully
low pH conditions and high population densities. We found that genotypes
underwent evolutionary changes, some shifting up and others shifting down
their intrinsic rates of increase (r0). Overall, evolution at low pH led
to the convergence of r0 and intraspecific competitive ability (α) across
the four genotypes. Given the strong correlation between r0 and α, we
argue that this convergence was a consequence of selection for increased
density-dependent fitness at low pH under the experienced high density
conditions. Increased density-dependent fitness was either attained
through increase in r0 , or decrease of α, depending on the genetic
background. In conclusion, we show that demography can influence the
direction of evolution under abiotic stress.