168
J. HESLOP-HARRISON
E. GENECOLOGICAL TECHNIQUE
Phenotypic inter-population variation in a plant species may arise
from three principal sources : (i) the direct plastic modification of individuals ; (ii) genetical divergence in consequence of selection, and (iii)
fortuitous genetical divergence resulting in varioui ways from sampling
errors - e.g. drift in small populations, or the establishment of deviating colonies from small numbers of founders. The variation arising
from (i) and (ii) may be expected to show correlation with habitat
factors; (iii) will be random with respect to habitat. The essential problem of genecology is to devise ways to distinguish and study variation
arising from (ii). For the sake of clarity, we will refer to this variation as
genecological digeerentiation, following Harberd (1 957). The problem of
distinguishing genecological differentiation will be seen to have two
parts; it requires the separation of adaptive from random inter-population variation, and the separation of the adaptive variation, in turn,
into genetic and non-genetic components.
The difference between the requirements of genecological and taxonomic surveys is worthy of note. From the taxonomic viewpoint, ell
variation may be regarded as grist for the mill, except that the purist
would reject the “modification” as a candidate for nomenclatural
recognition. It is of no taxonomic significance whether two subspecies
have diverged under selective pressures or merely in consequence of
some fortuitous change in gene frequency in the course of an ancient
migration; it is the difference that matters, not its source. In the genecological survey, on the other hand, information about regional variation is useless per se ; it must have the support of other types of evidence
before any evaluation of the adaptive differentiation of the species can
be attempted.
This evidence can be sought in two general ways. The direct approach
is to attempt to demonstrate adaptive responses experimentally, the
aim being to discover whether or not populations in different parts of the
species range do vary inter se in their ability to cope with diverse environmental variables, each being best equipped to deal with its own
local circumstances. This essentially physiological approach to the investigation of genecological differentiation is discussed in Section 111.
The alternative, indirect, approach is to seek correlations between
“plant type” and “habitat type”. If these are consistently found, they
may be taken to indicate adaptive divergence without any knowledge of
the physiological meaning of the differences observed, since the only
reasonable explanation for their existence is the differential effect of
selection in the various habitats, save for the possibility of conditioning
mentioned on p. 215.
J. HESLOP-HARRISON
E. GENECOLOGICAL TECHNIQUE
Phenotypic inter-population variation in a plant species may arise
from three principal sources : (i) the direct plastic modification of individuals ; (ii) genetical divergence in consequence of selection, and (iii)
fortuitous genetical divergence resulting in varioui ways from sampling
errors - e.g. drift in small populations, or the establishment of deviating colonies from small numbers of founders. The variation arising
from (i) and (ii) may be expected to show correlation with habitat
factors; (iii) will be random with respect to habitat. The essential problem of genecology is to devise ways to distinguish and study variation
arising from (ii). For the sake of clarity, we will refer to this variation as
genecological digeerentiation, following Harberd (1 957). The problem of
distinguishing genecological differentiation will be seen to have two
parts; it requires the separation of adaptive from random inter-population variation, and the separation of the adaptive variation, in turn,
into genetic and non-genetic components.
The difference between the requirements of genecological and taxonomic surveys is worthy of note. From the taxonomic viewpoint, ell
variation may be regarded as grist for the mill, except that the purist
would reject the “modification” as a candidate for nomenclatural
recognition. It is of no taxonomic significance whether two subspecies
have diverged under selective pressures or merely in consequence of
some fortuitous change in gene frequency in the course of an ancient
migration; it is the difference that matters, not its source. In the genecological survey, on the other hand, information about regional variation is useless per se ; it must have the support of other types of evidence
before any evaluation of the adaptive differentiation of the species can
be attempted.
This evidence can be sought in two general ways. The direct approach
is to attempt to demonstrate adaptive responses experimentally, the
aim being to discover whether or not populations in different parts of the
species range do vary inter se in their ability to cope with diverse environmental variables, each being best equipped to deal with its own
local circumstances. This essentially physiological approach to the investigation of genecological differentiation is discussed in Section 111.
The alternative, indirect, approach is to seek correlations between
“plant type” and “habitat type”. If these are consistently found, they
may be taken to indicate adaptive divergence without any knowledge of
the physiological meaning of the differences observed, since the only
reasonable explanation for their existence is the differential effect of
selection in the various habitats, save for the possibility of conditioning
mentioned on p. 215.