Growth rates of populations evolved and assayed at two temperatures for 6500 generations

Evolutionary biologists have long sought to understand what factors affect
the repeatability of adaptive outcomes. To better understand the role of
temperature in determining the repeatability of adaptive trajectories, we
evolved populations of different genotypes of the ciliate Tetrahymena
thermophila at low and high temperatures and followed changes in growth
rate over 6,500 generations. As expected, growth rate increased with a
decelerating rate for all populations; however, there were differences in
the patterns of evolution at the two temperatures. The growth rates of the
different genotypes tended to converge as evolution proceeded at both
temperatures, but this convergence was quicker and more pronounced at the
higher temperature. Additionally, over the first 4,000 generations we
found greater repeatability of evolution, in terms of change in growth
rate, among replicates of the same genotype at the higher temperature.
Finally, we found limited evidence of trade-offs in fitness between
temperatures, and an asymmetry in the correlated responses, whereby
evolution in a high temperature increases growth rate at the lower
temperature significantly more than the reverse. These results demonstrate
the importance of temperature in determining the repeatability of
evolutionary trajectories for the eukaryotic microbe Tetrahymena
thermophila and may provide clues to how temperature affects evolution
more generally.