Breaking the habit. Integrating plasticity into taxonomy

Focusing on records dealing with species within the colonial green algal genus Desmodesmus, we learn how the methods used, especially choice of culture medium, temperature, illumination as well as age of the population, influence the results we gather. Most, if not all, Desmodesmus species exhibit phenotypic plasticity, that is, as the population grows,they reproduce in an ordered sequence, making morphological adjustments to changing environmental signals, whether from nutrient availability, temperature or illumination. Progeny may soon have wall ornamentation unlike that of the parent. Although traditionally the display of wall features was used to separate Desmodesmus species, it is now clear that individuals are not mere copies of the parent, but rather adjust their growth/development, switching from one ecomorph to another. With this insight one can begin to interpret life histories of both organisms and populations, as they respond to these signals, be that in nature or the laboratory. In studies of such developmental complexity, there are advantages to some narrowly focused projects in which data on just one organism are assembled, so that an unambiguous sequence of ecomorph development over time is revealed. Eventually this should lead to use of names for taxa more wisely. Development of theory might sometimes be stalled, but the data concerning plastic responses will no doubt be valuable in resolving concerns over scenedesmoid classification, relationships and evolution. Finally we will have a framework in which to incorporate all new ultrastructural and molecular data.