212
J. HESLOP-HARRISON
tion does occur where the parents grow in close proximity, but the
hybrids, being ill-adapted to the available habitats, fail to survive in the
wild. The limitations of hybrids to intermediate habitats or ecotones has
been excellently demonstrated by Briggs (1962) for the Ranunculi of the
lappaceus group (p. 176). Here there seems no doubt that the restriction
of gene flow depends almost entirely upon selective elimination of
hybrids, even though the characteristic habitats may alternate over
distances of only a few metres.
The same principle is evident on a larger scale with the ecotypic
subspecies of Potentilla glandulosa. Clausen and Hiesey (1 958a) comment
on the situation as follows : “Undoubtedly, at the edges of the distribution of the four subspecies of P. glandulosa a certain amount of hybridization is constantly taking place. Natural selection, however,
limits the extent of genic infusion among the subspecies. In regions
where-equilibrium has thus been reached, the subspecies are able to
maintain their identity.” There are other factors acting t o limit gene
exchange between the P. gtandulosu subspecies ; reJtexa and hunseni, for
example, differ in flowering time by as much as a month. Moreover, all
of the subspecies except nevadensis are self-compatible and thus probably extensively inbred (Baker, 1953). Nevertheless, the importance of
ecological isolation in preserving their independence is beyond question.
The significance of isolation dependent upon the flower fidelity of
pollinators in entomophilous groups has been emphasized especially by
Grant ( 1949). Minor differences in floral characteristics among ecological
races of genecologically differentiated species may contribute to maintaining isolation through this so-called ethological means. The specialization of pollinators certainly forms an effective isolating mechanism
with the two species of Melandrium, M . rubrum and M . album (Baker,
1947). The ecotypic subspecies of Dactylorchis incarnata (p. 178) are
barely distinguishable in vegetative features , and are yet sharply
differentiated in flower colour and patterning (Heslop-Harrison, 1956).
There is no direct evidence, but the possibility is open that this provides
some measure of ethological isolation and has facilitated the independent migration of these races through north-western Europe (HeslopHarrison, 1958).
Ecological isolation and isolation dependent upon the behaviour of
pollen vectors both arise from genetical properties of the populations’
concerned, and all intermediates exist between these comparatively
mild barriers and the ultimate disharmony which results in hybrid
sterility or inviability. Ecological and to a lesser extent ethological
isolation differ, however, from isolating mechanisms dependent ,upon
partial or complete intersterility in not being irremoveable once established. This means that evolutionary fluidity is preserved; to extend
J. HESLOP-HARRISON
tion does occur where the parents grow in close proximity, but the
hybrids, being ill-adapted to the available habitats, fail to survive in the
wild. The limitations of hybrids to intermediate habitats or ecotones has
been excellently demonstrated by Briggs (1962) for the Ranunculi of the
lappaceus group (p. 176). Here there seems no doubt that the restriction
of gene flow depends almost entirely upon selective elimination of
hybrids, even though the characteristic habitats may alternate over
distances of only a few metres.
The same principle is evident on a larger scale with the ecotypic
subspecies of Potentilla glandulosa. Clausen and Hiesey (1 958a) comment
on the situation as follows : “Undoubtedly, at the edges of the distribution of the four subspecies of P. glandulosa a certain amount of hybridization is constantly taking place. Natural selection, however,
limits the extent of genic infusion among the subspecies. In regions
where-equilibrium has thus been reached, the subspecies are able to
maintain their identity.” There are other factors acting t o limit gene
exchange between the P. gtandulosu subspecies ; reJtexa and hunseni, for
example, differ in flowering time by as much as a month. Moreover, all
of the subspecies except nevadensis are self-compatible and thus probably extensively inbred (Baker, 1953). Nevertheless, the importance of
ecological isolation in preserving their independence is beyond question.
The significance of isolation dependent upon the flower fidelity of
pollinators in entomophilous groups has been emphasized especially by
Grant ( 1949). Minor differences in floral characteristics among ecological
races of genecologically differentiated species may contribute to maintaining isolation through this so-called ethological means. The specialization of pollinators certainly forms an effective isolating mechanism
with the two species of Melandrium, M . rubrum and M . album (Baker,
1947). The ecotypic subspecies of Dactylorchis incarnata (p. 178) are
barely distinguishable in vegetative features , and are yet sharply
differentiated in flower colour and patterning (Heslop-Harrison, 1956).
There is no direct evidence, but the possibility is open that this provides
some measure of ethological isolation and has facilitated the independent migration of these races through north-western Europe (HeslopHarrison, 1958).
Ecological isolation and isolation dependent upon the behaviour of
pollen vectors both arise from genetical properties of the populations’
concerned, and all intermediates exist between these comparatively
mild barriers and the ultimate disharmony which results in hybrid
sterility or inviability. Ecological and to a lesser extent ethological
isolation differ, however, from isolating mechanisms dependent ,upon
partial or complete intersterility in not being irremoveable once established. This means that evolutionary fluidity is preserved; to extend