FORTY YEARS OF GENECOLOGY
215
factor.” In short, the physiological economy of producing directly an
adapted form is of selective value.
Turesson’s argument may be linked with that advanced by Waddington (1953a), concerning the “genetic assimilation” of acquired characteristics, the so-called Baldwin effect. According to this, if there is
variability in a population in the capacity to react to a particular environment by the production of a modified phenotype which is at a
selective advantage in that environment, then the reacting genotypes
will necessarily be favoured. Successive generations of selection for the
capacity to react to the effective stimuli may be expected to lower the
intensity of stimulation required to produce a given phenotype, and
ultimately to bring about an approach to a,state where the favoured
phenotype is produced spontaneously. The feasibility of this kind of
genetic assimilation has been demonstrated in a well-known experiment
of Waddington (1953b) with Drosophila, the characteristic being a wing
venation defect induced in the normal genotype by a high-temperature
shock during pupation. In the line selected for ready expression of the
defect, flies appeared in the 14th generation which revealed it without
temperature treatment.
Given that there is advantage of the kind envisaged by Turesson in
the direct rather than the enforced production of adapted phenotypes,
and given further that genetic variability exists in a population in the
capacity to respond to the effective environmental factors, then it seems
inevitable that some form of genetic assimilation will occur. The significance for genecological differentiation is evident. The ability to exploit
an unusual habitat in consequence of inherent developmental flexibility
will, as it were, provide a species with a beach-head. To the extent that
more facile phenotypic adaptation to the new environment is favoured,
genes contributing to this will accumulate in the immigrant sub-population, and so the threshold at which the specialized form appears will be
lowered. Since the intensity of selection will decrease as this process
progresses, the adjustment will be asymptotic, and finality w i l l never be
reached. This means that some adaptation w i l l always depend upon
developmental flexibility, and it is significant in this connection that, on
transplantation to neutral environments, individuals of ecotypic populations adapted to extreme environments invariably regress in some degree
towards the norm of the species.
Waddington’s principle of genetic assimilation offers a plausible interpretation of one form of quasi-Lamarckian response to habitat. The
possibility of other, more direct, mechanisms is, of course, still to be ke@,
open. Dmant’s experiments on environmental conditioning in flax
(1958,1962) seem to show unequivocally that one major group of habitat
factors, nameIy soil nutrients, can induce heritable changes in pheno-
215
factor.” In short, the physiological economy of producing directly an
adapted form is of selective value.
Turesson’s argument may be linked with that advanced by Waddington (1953a), concerning the “genetic assimilation” of acquired characteristics, the so-called Baldwin effect. According to this, if there is
variability in a population in the capacity to react to a particular environment by the production of a modified phenotype which is at a
selective advantage in that environment, then the reacting genotypes
will necessarily be favoured. Successive generations of selection for the
capacity to react to the effective stimuli may be expected to lower the
intensity of stimulation required to produce a given phenotype, and
ultimately to bring about an approach to a,state where the favoured
phenotype is produced spontaneously. The feasibility of this kind of
genetic assimilation has been demonstrated in a well-known experiment
of Waddington (1953b) with Drosophila, the characteristic being a wing
venation defect induced in the normal genotype by a high-temperature
shock during pupation. In the line selected for ready expression of the
defect, flies appeared in the 14th generation which revealed it without
temperature treatment.
Given that there is advantage of the kind envisaged by Turesson in
the direct rather than the enforced production of adapted phenotypes,
and given further that genetic variability exists in a population in the
capacity to respond to the effective environmental factors, then it seems
inevitable that some form of genetic assimilation will occur. The significance for genecological differentiation is evident. The ability to exploit
an unusual habitat in consequence of inherent developmental flexibility
will, as it were, provide a species with a beach-head. To the extent that
more facile phenotypic adaptation to the new environment is favoured,
genes contributing to this will accumulate in the immigrant sub-population, and so the threshold at which the specialized form appears will be
lowered. Since the intensity of selection will decrease as this process
progresses, the adjustment will be asymptotic, and finality w i l l never be
reached. This means that some adaptation w i l l always depend upon
developmental flexibility, and it is significant in this connection that, on
transplantation to neutral environments, individuals of ecotypic populations adapted to extreme environments invariably regress in some degree
towards the norm of the species.
Waddington’s principle of genetic assimilation offers a plausible interpretation of one form of quasi-Lamarckian response to habitat. The
possibility of other, more direct, mechanisms is, of course, still to be ke@,
open. Dmant’s experiments on environmental conditioning in flax
(1958,1962) seem to show unequivocally that one major group of habitat
factors, nameIy soil nutrients, can induce heritable changes in pheno-