FORTY YEARS OF QENECOLOGY
161
commonly genetically based rather than dependent upon the direct
modification of individuals. The experimental methods employed - involving mostly sampling from wild populations and comparison under
standard conditions of cultivation - have been reviewed and discussed
many times, and need not be described further here. The results of
Turesson’s studies were satisfactorily gonclusive. To them may now be
added a mass of evidence accumulated subsequently by others, so that
it may be accepted as an established fact that plant species in general do
show genetically-based ecological differentiation.
If this be generally conceded, it cannot be said that there has been as
satisfactory an agreement on the nature of the patterns of infraspecific
ecological differentiation in the higher plants. Turesson’s method of
population sampling and comparative cultivation led him to the view
that species commonly constituted a mosaic of populations each adapted
to characteristic habitats, each habitat-form being more or less distinct
- that is to say, separated by a variational discontinuity - from other
(e.g. Turesson, 1936). This conclusion led in turn to the concept of the
ecotype, now so fimly identified with Turesson’s work that genecology
itself has been called “the doctrine of the ecotype” (Faegri, 1937). Perhaps the most specific statement of the doctrine appears in Turesson’s
second paper (192210): “The mass of genetically distinct forms which
make up the Linnaean species do not distribute themselves indiscriminately over an area comprising different types of localities, but, on the
contrary, are found in nature to be grouped into different types, each
confined to a definite habitat. Further, these ‘ecotypes’ do not originate
through sporadic variation preserved by chance isolation; they are, on
the contrary, to be considered as products arising through the sorting
and controlling effect of the habitat-factors upon the heterogeneous
species-population. ”
According to Faegri (1937), Langlet (1934) was the first to point out
that, since most of the major habitat factors vary spatially in a continuous and not “stepped” manner, graded rather than discontinuous
variation is to be expected in a wide-ranging species as a consequence of
adaptation to habitat. There can be no doubt that Turesson’s methods,
involving as they generally did sampling from rather remote populations
occupying markedly different habitats, were not of a type likely to detect continuous spatial variation did it exist. More recent studies using
adequate sampling methods have shown that adaptative variation may
be either continuous or discontinuous, and it may now be said that the
important questidns concern not the existence of different patterns of
variation but the forces acting within species to generate them, matters
considered at length in Section 11. Nevertheless, the lively discussion of
the relative significance of continuous and discontinuous variation has
161
commonly genetically based rather than dependent upon the direct
modification of individuals. The experimental methods employed - involving mostly sampling from wild populations and comparison under
standard conditions of cultivation - have been reviewed and discussed
many times, and need not be described further here. The results of
Turesson’s studies were satisfactorily gonclusive. To them may now be
added a mass of evidence accumulated subsequently by others, so that
it may be accepted as an established fact that plant species in general do
show genetically-based ecological differentiation.
If this be generally conceded, it cannot be said that there has been as
satisfactory an agreement on the nature of the patterns of infraspecific
ecological differentiation in the higher plants. Turesson’s method of
population sampling and comparative cultivation led him to the view
that species commonly constituted a mosaic of populations each adapted
to characteristic habitats, each habitat-form being more or less distinct
- that is to say, separated by a variational discontinuity - from other
(e.g. Turesson, 1936). This conclusion led in turn to the concept of the
ecotype, now so fimly identified with Turesson’s work that genecology
itself has been called “the doctrine of the ecotype” (Faegri, 1937). Perhaps the most specific statement of the doctrine appears in Turesson’s
second paper (192210): “The mass of genetically distinct forms which
make up the Linnaean species do not distribute themselves indiscriminately over an area comprising different types of localities, but, on the
contrary, are found in nature to be grouped into different types, each
confined to a definite habitat. Further, these ‘ecotypes’ do not originate
through sporadic variation preserved by chance isolation; they are, on
the contrary, to be considered as products arising through the sorting
and controlling effect of the habitat-factors upon the heterogeneous
species-population. ”
According to Faegri (1937), Langlet (1934) was the first to point out
that, since most of the major habitat factors vary spatially in a continuous and not “stepped” manner, graded rather than discontinuous
variation is to be expected in a wide-ranging species as a consequence of
adaptation to habitat. There can be no doubt that Turesson’s methods,
involving as they generally did sampling from rather remote populations
occupying markedly different habitats, were not of a type likely to detect continuous spatial variation did it exist. More recent studies using
adequate sampling methods have shown that adaptative variation may
be either continuous or discontinuous, and it may now be said that the
important questidns concern not the existence of different patterns of
variation but the forces acting within species to generate them, matters
considered at length in Section 11. Nevertheless, the lively discussion of
the relative significance of continuous and discontinuous variation has