XVI.3 The Lamarckian model of evolution assumed that changes in the behavior of animals precedes changes in their phenotype
The original Lamarckian theory assumed that adaptive traits of organisms arise in evolution so that an organism first starts to behave adaptively, e.g. a giraffe’s ancestor starts to feed on tree leaves and in consequence the exercise of particular body parts will cause morphological changes: lengthening of the neck and legs in giraffes. In the chapter on the evolution of ontogenesis (XII.7), we showed that a similar mechanism can partly be active in some tissues and organs, for example in bone trabecule morphology. However, if the useful adaptation of an individual’s morphology could be important in the evolutionary process, it is necessary to fix the changes genetically. This claim is not as restrictive as it seems. In theory, the same repertoire of ecophenotype changes (i.e. non-genetic changes in phenotype caused and maintained by direct influences of the outer environment) should be found in the offspring of the first long-necked giraffe, if they learned to feed on tree leaves from their parents. However, general experience indicates that a vast majority of morphological traits are passed on genetically and develop in the same form even in individuals who did not have the opportunity to learn typical behavioral patterns from their parents.
Since the times of Darwin and Weisman, most biologists consider ideas about the possibility of genetic fixation of acquired characters to be obsolete; similarly, the possibility of influencing the evolution of an organism’s phenotype through adaptive changes in its behavior is also now thought to be minimal. Two mechanisms are accepted today: the Baldwin effect and genetic assimilation, which can have similar consequences (see XVI.3.2).