Local adaptation is an important principle in a world of environmental change
and might be critical for species persistence. We tested the hypothesis that
replicated populations can attain rapid local adaptation under two varying laboratory
environments. Clonal subpopulations of the cyclically parthenogenetic
rotifer Brachionus calyciflorus were allowed to adapt to two varying harsh and a
benign environment: a high-salt, a food-limited environment and untreated
culture medium (no salt addition, high food). In contrast to most previous
studies, we re-adjusted rotifer density to a fixed value (two individuals per ml)
every 3–4 days of unrestricted population growth, instead of exchanging a fixed
proportion of the culture medium. Thus our dilution regime specifically
selected for high population growth during the entire experiment and it allowed
us to continuously track changes in fitness (i.e., maximum population growth
under the prevailing conditions) in each population. After 56 days (43 asexual
and eight sexual generations) of selection, the populations in the harsh environments
showed a significant increase in fitness over time relative to the beginning
compared to the population in untreated culture medium. Furthermore,
the high-salt population exhibited a significantly elevated ratio of sexual offspring
from the start of the experiment, which suggested that this environment
either triggered higher rates of sex or that the untreated medium and the foodlimited
environment suppressed sex. In a following assay of local adaptation we
measured population fitness under “local” versus “foreign” conditions (populations
adapted to this environment compared to those of the other environment)
for both harsh habitats. We found significantly higher fitness values for
the local populations (on average, a 38% higher fitness) compared to the
foreign populations. Overall, local adaptation was formed rapidly and it seemed
to be more pronounced in the high-salt treatment.