F. Vasi et al., LONG-TERM EXPERIMENTAL EVOLUTION IN ESCHERICHIA-COLI .2. CHANGES IN LIFE-HISTORY TRAITS DURING ADAPTATION TO A SEASONAL ENVIRONMENT, The American naturalist, 144(3), 1994, pp. 432-456
Twelve populations of the bacterium Escherichia coli were propagated f
or 2,000 generations in a seasonal environment, which consisted of alt
ernating periods of feast and famine. The mean fitness of the derived
genotypes increased by approximately 35% relative to their common ance
stor, based on competition experiments in the same environment. The ba
cteria could have adapted, in principle, by decreasing their lag prior
to growth upon transfer to fresh medium (L), increasing their maximum
growth rate (V(m)), reducing the the concentration of resource requir
ed to support growth at half the maximum rate (K(s)), and reducing the
ir death rate after the limiting resource was exhausted (D). We estima
ted these parameters for the ancestor and then calculated the opportun
ity for selection on each parameter. The inferred selection gradients
for V(m) and L were much steeper than for K(s) and D. The derived geno
types showed significant improvement in V(m) and L but not in K(s) or
D. Also, the numerical yield in pure culture of the derived genotypes
was significantly lower than the yield of the common ancestor, but the
average cell size was much larger. The independently derived genotype
s are somewhat more variable in these life-history traits than in thei
r relative fitnesses, which indicates that they acquired different gen
etic adaptations to the seasonal environment. Nonetheless, the evoluti
onary changes in life-history traits exhibit substantial parallelism a
mong the replicate populations.