Fig. 2.

In pulled waves, expansion velocity depends on the growth rate only at low densities irrespective of the carrying capacity. (A) A population of S. cerevisae grows exponentially at low densities in 0.125% glucose and 0.5% galactose. Growth rate at low densities is higher on 0.125% glucose compared with 0.5% galactose and decreases monotonically in both environments. (B) The galactose environment has a higher carrying capacity compared with glucose. The two environments thus show a trade-off between the low-density growth rate and the carrying capacity. (C) Although the galactose environment is more favorable in terms of the total nutrient availability (carrying capacity), expansions are faster in glucose because the populations grow faster in glucose at low density.