Genetic assimilation is responsible for genetic fixation of a phenotype trait originally produced non-genetically in the individual. For example, in the nineteen fifties, Conrad Hal Waddington observed that during artificial selection, a certain change of wing morphology in drosophila (absence of certain veins – i.e. cross-veinless phenotype), originally a response to heightened temperature during larvae ontogenesis of a small portion of the flies population, occurs in the offspring of altered individuals in further generations more and more often. After 23 generations of the selection, 96% of the flies respond to increased temperature by change of the wing morphology. Most importantly, the morphological change started to occur also in flies that were not exposed to increased temperature. (Waddington 1961; Grodnitsky 2001) (Fig. XVI.4). This means, that during several generations of selection for the ability to response to increased temperature through wing modification, a phenotype change that was originally conditioned by the environment (phenocopy) has become genetically dependent.
At the present time, it is assumed that genetic assimilation occurs in that changed environmental conditions or a behavior pattern achieved by learning cause manifestation of already existing minor inter-individual genetically dependent differences in a particular trait in the individual members of the population (i.e. manifestation of latent genetic variability). Manifestation of these differences subsequently enables selection and thus genetic fixation of the new forms of the traits (Hall 2001; Flegr 2002). In the above mentioned hypothetical snail-shelling case, both genetic assimilation and the Baldwin effect can play a role. When the organisms start to exhibit a certain behavioral pattern, the so-far hidden differences in the predispositions of the individual members of the population for performing a certain activity, in our case the predispositions for snail-shelling became “visible” for natural or artificial selection. This enables spreading and finally fixation of already existing alleles that cause or at least facilitate development of a particular trait, e.g. launching a particular behavioral pattern (snail-shelling in a bird) or modification of wing morphology (in drosophila), even without the necessity to learn it individually or without any external stimulus.
Along with this, the Baldwin effect is responsible for the fact that selection makes this behavioral pattern more effective in time by suitable modification of the organism’s phenotype – e.g. by selecting birds with larger or stronger beaks.