FORTY YEARS OF QENECOLOGY
209
cally adapted recombinants, perhaps not immediately available. “his
must very frequently be the situation during the migration of a plant
species into new territory, and it is one of a good deal more than theoretical interest considering that the north temperate floras have undergone
massive oscillatory migrations during glacial and post-glacial times.
It may be that the circumstance mentioned above (p. 201) is significant under these conditions, namely that during a period of migration a
free release of genetic variation might be favoured in the van. This
would arise if the intraspecific competition were heavy enough under
the conditions of a closed community to restrict very severely the range
of phenotypes reaching reproductive maturity in the territory already
occupied. In a thinly colonized marginal zone, some of the intense selective factors active in the closed community would be relaxed, with the
consequent survival of new phenotypes. To the extent that these carried
new gene combinations, they might be expected to enhance the segregation range in the pioneer belt. This availability of fresh recombinants
would necessarily favour migration into novel habitats and the establishment of adapted races there, provided only that any adaptive gene
complexes assembled were not immediately broken up by contamination from elsewhere. Mayr’s (1954) argument that peripheral populations
would be inhibited from adapting successfully to new environments by
the disruptive effect of gene migration from the interior of the species
range has been contested by Thoday and Boam (1959), who on the basis
of the experiments described above state that there is no reason in
principle why a locally adapted population should not be formed even if
one-way gene flow were so great that all progeny were hybrids. Evidently this particular facet of genecological differentiation would merit
more observational and experimental study.
There is another factor that is likely to be significant in promoting the
differentiation of plant populations under disruptive selection, namely
the effect of the immediate environment on phenology. I n the British
Ecological Society’s series of transplant experiments, summarized by
Marsden-Jones and Turrill (1938), some effect of soil type on date of
flowering was observed in half of the species tested. Thus in 1932 a
sample of Plantago major cultivated on “clay” reached maximum
flowering by 19 June, but a corresponding sample of plants of the same
parentage cultivated on “sand” in an adjacent plot did not reach a
flowering peak until 15 July. Aspect may similarly produce phenological
differences. Thus Mitra (personal account) working at Saskatchewan
found up to a fortnight delay in the flowering of Stipa species on the
north as compared with the south faces of an artificial east-west mound
of 10’ slope. Ehrendorfer, in the study mentioned of Galiumpumilum (p.
186) { 1953) observed distinct differences over the small area studied in
209
cally adapted recombinants, perhaps not immediately available. “his
must very frequently be the situation during the migration of a plant
species into new territory, and it is one of a good deal more than theoretical interest considering that the north temperate floras have undergone
massive oscillatory migrations during glacial and post-glacial times.
It may be that the circumstance mentioned above (p. 201) is significant under these conditions, namely that during a period of migration a
free release of genetic variation might be favoured in the van. This
would arise if the intraspecific competition were heavy enough under
the conditions of a closed community to restrict very severely the range
of phenotypes reaching reproductive maturity in the territory already
occupied. In a thinly colonized marginal zone, some of the intense selective factors active in the closed community would be relaxed, with the
consequent survival of new phenotypes. To the extent that these carried
new gene combinations, they might be expected to enhance the segregation range in the pioneer belt. This availability of fresh recombinants
would necessarily favour migration into novel habitats and the establishment of adapted races there, provided only that any adaptive gene
complexes assembled were not immediately broken up by contamination from elsewhere. Mayr’s (1954) argument that peripheral populations
would be inhibited from adapting successfully to new environments by
the disruptive effect of gene migration from the interior of the species
range has been contested by Thoday and Boam (1959), who on the basis
of the experiments described above state that there is no reason in
principle why a locally adapted population should not be formed even if
one-way gene flow were so great that all progeny were hybrids. Evidently this particular facet of genecological differentiation would merit
more observational and experimental study.
There is another factor that is likely to be significant in promoting the
differentiation of plant populations under disruptive selection, namely
the effect of the immediate environment on phenology. I n the British
Ecological Society’s series of transplant experiments, summarized by
Marsden-Jones and Turrill (1938), some effect of soil type on date of
flowering was observed in half of the species tested. Thus in 1932 a
sample of Plantago major cultivated on “clay” reached maximum
flowering by 19 June, but a corresponding sample of plants of the same
parentage cultivated on “sand” in an adjacent plot did not reach a
flowering peak until 15 July. Aspect may similarly produce phenological
differences. Thus Mitra (personal account) working at Saskatchewan
found up to a fortnight delay in the flowering of Stipa species on the
north as compared with the south faces of an artificial east-west mound
of 10’ slope. Ehrendorfer, in the study mentioned of Galiumpumilum (p.
186) { 1953) observed distinct differences over the small area studied in