ABSTRACT
A common attitude is to admit the possibility of direct adaptive changes in principle but to ignore, dismiss or minimize the significance of the evidence that they occur. A large variability in tolerance to a constraint is generally observed in natural populations; hence, one may expect a progressive selection toward preadapted individuals. The differentiation of ecotypes adapted to toxic concentrations of heavy metals is well documented. The formation of salt-adapted ecotypes has been less investigated than emergence of heavy-metal-tolerant genotypes, probably because the progressive differentiation of salt-adapted ecotypes is more difficult to follow. Physiological adaptation to salinity has long been described. An increase in salinity tolerance in cotton has been reported following gradual pretreatments with moderate concentrations of NaCl. Environmental constraints are able to induce adaptive, transmissible changes in physiology and development. This evidence asks for a reexamination of the role of natural selection and of physiological adaptation in the evolutionary process.
