FORTY YEARS O F QENECOLOQY
213
Mather’s phrase, fitness for existing environments is not acquired at the
cost of a narrow specialization which sacrifices the flexibility offered by
a large gene pool. The change of circumstance which renders the original
close adaptation to previously existing habitats valueless may simultaneously permit the survival of hybrid derivatives that otherwise
would have been eliminated in favour of the strictly adapted types.
What Anderson (1949) picturesquely calls “hybridization of the habitat” may entail a new release of genetic variation, and a new starting
point for adaptive differentiation (Anderson, 1949,1953).
F. PLASTICITY, GENETIC ASSIMILATION AND CONDITIONINQ
In consequence of their growth by apical meristems, higher plants
possess high potential phenotypic plasticity. The continuous serial production of fresh organs means that a capacity t o respond developmentally to environmental influences is present throughout life, not, as
in the higher animal, only during an early embryonic period.
It is apparent that these responses may or may not be adaptive.
Thoday (1953) has suggested the term “developmental flexibility” for
the truly adaptive property of generating phenotypes functioning
satisfactorily in a range of environments. Mere morphological plasticity
is not necessarily evidence of developmental flexibility. For example,
the passive response to a factor establishing, say, a particular growth
form, does not necessarily mean that a plant is better adapted for the
environment in which that factor is present than it would be if it produced some other growth form under the same conditions. Nevertheless,
there is good reason to suppose that the plasticity of higher plants does
often have adaptive values, within certain limits.
It has long been supposed that the wind forms of plants are adaptive,
and direct supporting evidence for this view is now available from the
work of Whitehead and Luti (1962) and Whitehead (1962, 1963a, b).
In maize and sunflower, exposure to wind speeds approximating those
found in mountain regions induced anatomical and morphological
changes which were demonstrably advantageous with respect to overall
water economy. The phenotype became more xeromorphic, and the
change was harmoniously related to the incidence of the adverse conditions. Whitehead points out that neither species is notably plastic, yet
there is no doubt that they are equipped with a developmental flexibility which must extend their potential ecological tolerance ranges
quite substantially.
The work of Bjorkman and Holmgren (1963), discussed at length in a
later section (p. 224 et seq.), provides some evidence of the adaptive
value of variation in leaf morphology and physiology caused by differing
levels of insolation. As part of these experiments, leaves of the different
H
C.E.R.
213
Mather’s phrase, fitness for existing environments is not acquired at the
cost of a narrow specialization which sacrifices the flexibility offered by
a large gene pool. The change of circumstance which renders the original
close adaptation to previously existing habitats valueless may simultaneously permit the survival of hybrid derivatives that otherwise
would have been eliminated in favour of the strictly adapted types.
What Anderson (1949) picturesquely calls “hybridization of the habitat” may entail a new release of genetic variation, and a new starting
point for adaptive differentiation (Anderson, 1949,1953).
F. PLASTICITY, GENETIC ASSIMILATION AND CONDITIONINQ
In consequence of their growth by apical meristems, higher plants
possess high potential phenotypic plasticity. The continuous serial production of fresh organs means that a capacity t o respond developmentally to environmental influences is present throughout life, not, as
in the higher animal, only during an early embryonic period.
It is apparent that these responses may or may not be adaptive.
Thoday (1953) has suggested the term “developmental flexibility” for
the truly adaptive property of generating phenotypes functioning
satisfactorily in a range of environments. Mere morphological plasticity
is not necessarily evidence of developmental flexibility. For example,
the passive response to a factor establishing, say, a particular growth
form, does not necessarily mean that a plant is better adapted for the
environment in which that factor is present than it would be if it produced some other growth form under the same conditions. Nevertheless,
there is good reason to suppose that the plasticity of higher plants does
often have adaptive values, within certain limits.
It has long been supposed that the wind forms of plants are adaptive,
and direct supporting evidence for this view is now available from the
work of Whitehead and Luti (1962) and Whitehead (1962, 1963a, b).
In maize and sunflower, exposure to wind speeds approximating those
found in mountain regions induced anatomical and morphological
changes which were demonstrably advantageous with respect to overall
water economy. The phenotype became more xeromorphic, and the
change was harmoniously related to the incidence of the adverse conditions. Whitehead points out that neither species is notably plastic, yet
there is no doubt that they are equipped with a developmental flexibility which must extend their potential ecological tolerance ranges
quite substantially.
The work of Bjorkman and Holmgren (1963), discussed at length in a
later section (p. 224 et seq.), provides some evidence of the adaptive
value of variation in leaf morphology and physiology caused by differing
levels of insolation. As part of these experiments, leaves of the different
H
C.E.R.