Data from: Repeatability of adaptation in experimental populations of different sizes

The degree to which evolutionary trajectories and outcomes are repeatable
across independent populations depends on the relative contribution of
selection, chance and history. Population size has been shown
theoretically and empirically to affect the amount of variation that
arises among independent populations adapting to the same environment.
Here, we measure the contribution of selection, chance and history in
different-sized experimental populations of the unicellular alga
Chlamydomonas reinhardtii adapting to a high salt environment to determine
which component of evolution is affected by population size. We find that
adaptation to salt is repeatable at the fitness level in medium (Ne = 5 ×
104) and large (Ne = 4 × 105) populations because of the large
contribution of selection. Adaptation is not repeatable in small (Ne = 5 ×
103) populations because of large constraints from history. The threshold
between stochastic and deterministic evolution in this case is therefore
between effective population sizes of 103 and 104. Our results indicate
that diversity across populations is more likely to be maintained if they
are small. Experimental outcomes in large populations are likely to be
robust and can inform our predictions about outcomes in similar
situations.