For
the majority of time, these populations therefore experienced the
same environment as the unstressed populations. At the start of
this experiment, all ancestral populations exhibited pronounced
oscillations of several orders of magnitude in cell density (between
109/ml and 105/ml), as previously described with this species [21].
However, within 500 generations these fluctuations were uniformly
lost [22], and all populations retained a stable density of 109
cells/ml. As a consequence of this change, which made it
impossible to directly compete evolved and ancestral cells, fitness
differences between treatments after experimental evolution were
estimated using pair-wise competition assays between differentially
marked terminal populations [23].