Fig. 3. Surfing depends sensitively on location and the strength of genetic drift.

Time-lapse microscopy (top row) and individual-based simulations (bottom row) reveal cell-scale dynamics at the front of expanding colonies. (a, c) Segmented micrographs of the initial front (bottom cells) and the front after three hours of growth (top cells) in S. cerevisiae (a) and E. coli (c) colonies, respectively. Colored lines track lineages backward in time (see also Figs. B8-B10). The histograms in (b, d, h) quantify how surfing success depends on position: The probability density p(Δ) that the lineages tracked for 3 hours back in time lead to an ancestor that had a distance Δ (in unit of cell diameters) to the front. Note the pronounced peak in both experiments (b, c) and simulations (h). (e) Illustration and measurement of the random meandering of tracked lineages. We measure the lateral displacement δy (in units of cell diameters) a lineage has undergone while moving a distance δx along the direction of the front propagation, and average 〈δy²〉 over all lineages. (f) Average (root mean square) lateral displacement of lineages in expanding colonies, showing that E. coli lineages are fluctuating substantially more strongly than S. cerevisiae lineages (absolute value at a given δx). The lateral displacement in both cases follows a characteristic scaling (slope), as expected for a spatially unbiased growth process with a rough front (Appendix A). These experimental observations can be reproduced in simulations (j) of expanding rough and smooth fronts, respectively. (g) In simulations with rough fronts, surfing beneficial mutations (light green) are frequently occluded by neighboring wild-type domains (dark green). (i) As a consequence, the number of sectors are much lower for rough than smooth fronts, for identical initial mutant frequency P_i and front length L.