FORTY YEARS OF GENECOLOGY
187
the variation of Cilia achilleaefolia (1 954) provides an interesting contrast with his study of G. capitata, discussed above. G. achilleaefolia
shows very great local variation from colony to colony throughout its
range. According to Grant, the local variants “do not group themselves
into broad geographical assemblages, as in most species including the
related G. capitata, but the broader subdivision in the species is rather
along ecological lines. The large-flowered races with dense heads occupy
sunny hillsides in grassland and oak savannah ; the small flowered races
with loose cymes occur in the shade of oak woodland or redwood forest;
and there are numerous transitional forms in the semi-shade of open oak
woods”. The situation here described is comparable with that encountered by Bradshaw in Agrostis tenuis, the pattern of differentiation
following closely upon local ecology so that sometimes continuous and
sometimes discontinuous variation is encountered.
Cook’s study (1962) of Eschscholzia californica, the Californian poppy,
is of interest since it combined a regional survey of thirty-one populations dispersed throughout the Californian range with a local survey
over a 35 mile transect. Some of the data concerning population variation appear to be based upon samples from the wild, so that there is no
guarantee that all variation recorded is genotypic; but in so far as it
concerned floral characteristics the probability is that direct environmental influence accounted,for little. Longevity was assessed in samples
under comparative cultivation. Each of the floral characteristics examined was found to vary gradually, sometimes independently and
sometimes in a parallel manner ; no discontinuities were encountered
suggesting the existence of regional races. Some of the clines observed
could be related in a general way to major ecological gradients, but the
evidence presented does not suggest that all necessarily do so. From the
local study, it was concluded that much of the variation, particularly in
such features as developmental periodicity, is closely related to habitat
conditions, so that neighbouring populations may differ abruptly if the
habitats are strongly differentiated, or intergrade, in a clinal manner,
when the habitats themselves intergrade.
G. OENECOLOQICAL DIFFERENTIATION : SOME GENERALIZATIONS
The foregoing examples illustrate most of the principal kinds of genecological differentiation known in higher plant species. The interpretation of the different patterns must necessarily depend upon an appreciation of the evolutionary situation each represents, and this in turn must
be based upon some understanding of the nature of the response of plant
populations to selection and the role of genetic systems in determining
that response, matters discussed in Section 11. So far as the observational evidence is concerned the following generalizations may be made.
187
the variation of Cilia achilleaefolia (1 954) provides an interesting contrast with his study of G. capitata, discussed above. G. achilleaefolia
shows very great local variation from colony to colony throughout its
range. According to Grant, the local variants “do not group themselves
into broad geographical assemblages, as in most species including the
related G. capitata, but the broader subdivision in the species is rather
along ecological lines. The large-flowered races with dense heads occupy
sunny hillsides in grassland and oak savannah ; the small flowered races
with loose cymes occur in the shade of oak woodland or redwood forest;
and there are numerous transitional forms in the semi-shade of open oak
woods”. The situation here described is comparable with that encountered by Bradshaw in Agrostis tenuis, the pattern of differentiation
following closely upon local ecology so that sometimes continuous and
sometimes discontinuous variation is encountered.
Cook’s study (1962) of Eschscholzia californica, the Californian poppy,
is of interest since it combined a regional survey of thirty-one populations dispersed throughout the Californian range with a local survey
over a 35 mile transect. Some of the data concerning population variation appear to be based upon samples from the wild, so that there is no
guarantee that all variation recorded is genotypic; but in so far as it
concerned floral characteristics the probability is that direct environmental influence accounted,for little. Longevity was assessed in samples
under comparative cultivation. Each of the floral characteristics examined was found to vary gradually, sometimes independently and
sometimes in a parallel manner ; no discontinuities were encountered
suggesting the existence of regional races. Some of the clines observed
could be related in a general way to major ecological gradients, but the
evidence presented does not suggest that all necessarily do so. From the
local study, it was concluded that much of the variation, particularly in
such features as developmental periodicity, is closely related to habitat
conditions, so that neighbouring populations may differ abruptly if the
habitats are strongly differentiated, or intergrade, in a clinal manner,
when the habitats themselves intergrade.
G. OENECOLOQICAL DIFFERENTIATION : SOME GENERALIZATIONS
The foregoing examples illustrate most of the principal kinds of genecological differentiation known in higher plant species. The interpretation of the different patterns must necessarily depend upon an appreciation of the evolutionary situation each represents, and this in turn must
be based upon some understanding of the nature of the response of plant
populations to selection and the role of genetic systems in determining
that response, matters discussed in Section 11. So far as the observational evidence is concerned the following generalizations may be made.