Richard Lenski

Experimental Evolution

Cell Size Data Through Generation 10,000

The table below provides the cell-size data used by Lenski and Travisano (1994). Measurements were obtained for the ancestor and at generations 500, 1000, 2000, 5000, 8000, 9000, 9500, and 10000 for each of the 12 evolving populations. In that paper, Fig. 1 shows the cell-size trajectory for population Ara-1 using the actual data; Fig. 2 shows trajectories for all 12 populations using curves fit to a hyperbolic model; and Fig. 3 shows the trajectory for the diversification of the 12 populations from one another based on the among-population standard-deviation in cell volume.

Average cell volumes were measured in fL (= 10^-15 L) using a Coulter counter, which measures the volume displaced by each of several thousand cells as they pass through a small aperture. Cell cultures were grown to stationary-phase density for 24 h in the same DM25 liquid medium used in the long-term evolution experiment. Averages were calculated after removing particles < 0.25 fL, except for the ancestor (gen. 0) where particles < 0.15 fL were removed. These criteria gave clear separation between bacterial cells and background debris.

Each datum is the mean of two assays, except for the ancestor (gen. 0) whose value is the mean of four assays (two each for the Ara^- and Ara⁺ states). All data were obtained as part of a single experiment, so uncontrolled temporal heterogeneity in assay conditions is not a factor. The mixed samples were used in this experiment.

The final column, labeled SD-AP, is the estimate of the among-population standard-deviation in cell volume based on analysis of variance components.

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