Roles of adaptation, chance and history in the evolution of the dinoflagellate Prorocentrum triestinum

Evolutionary changes depend on adaptive evolution and two other components that traditionally have not been taken into account: chance and history. However, concepts in evolutionary biology are very often supported by descriptive studies but not by empirical approach. In order to disentangle the three components of evolutionary change, we have investigated the effects of long-term selection pressure on fitness and cell size in the bloom-forming dinoflagellate, Prorocentrum triestinum Schiller, under increased nutrient concentrations and temperature. Two strains of P. triestinum, previously adapted to grow under standard conditions (f/2 medium, 20 degrees C), were grown under increased nutrient concentrations and temperature (f medium, 25 degrees C) until fitness and cell size were invariant as a function of time (720 days, corresponding to c. 400 generations). The mean value for fitness of both strains of P. triestinum showed a significant increase whereas a decrease in mean cell size was found. The changes in fitness have adaptive value, but the changes in cell size were due to both adaptation and the contribution of chance-plus-history.