210
J. HESLOP-HARRISON
flowering time; plants in the drier, open sites regularly reached anthesis
before those in the moister, shadier localities. Here, of course, the possibility of some genetical differentiation for flowering time in the
population is not excluded, but the circumstances as discussed by
Ehrendorfer do suggest that the environmental control of phenology
may have imposed some restriction on free gene exchange between the
sub-populations even during the iktial differentiation.
The implication of observations such as these is that the mere occupancy of a heterogeneous habitat can fragment a plant population
into partly isolated breeding groups by breaking down the synchroneity
of flowering. Since each distinct habitat will have its own particular
effect, the sub-populations in each will have a characteristic flowering
period. The plants in each type of habitat will thus remain panmictic,
but will be genetically isolated from others in different habitats to the
extent that the flowering periods fail to overlap. This is an irrefutable
example of ecological isolation, not to be discounted on the basis of any
of the arguments of Mayr (1947).
E. MONOTOPIC AND POLYTOPIC ORIGIN AND THE RETENTION
O F RACIAL IDENTITY
A distinction can be drawn at least on theoretical grounds between
mosaic patterns of ecological races where each race has had a monotopic
origin, and the superficially similar patterns which have arisen through
the polytopic origin of like ecotypes in response to corresponding selective factors in different parts of a species range. It is to be expected that
races that have had single independent origins will normally be distinguished by several correlated differentiae, both adaptive and nonadaptive. On the other hand, only the adaptive characteristics are
likely to be shared in common between the various populations of an
ecotype which has originated polytopically, and the populations may
differ among themselves in respect to non-adaptive features. A classical
case of presumed polytopic origin is that of the sand-dune ecotype of
Hieracium umbellatum described by Turesson (1922b). This occurs in
sand-dune habitats widely scattered around the periphery of the species
area in Sweden. The different populations share in common various
adaptive features such as a capacity for rapid shoot regeneration, but
they differ in minor leaf characteristics, each showing a resemblance in "
these characteristics with neighbouring inland populations.
The inhomogeneity of ecotypic populations of polytopic origin is
likely to be evident also in the genetic basis of their adaptation. Since
similar phenotypes can be established by different combinations of polygenes, there is no reason to suppose that identical selective pressures in
remote sites will necessarily fix the same group of genotypes. In con-
J. HESLOP-HARRISON
flowering time; plants in the drier, open sites regularly reached anthesis
before those in the moister, shadier localities. Here, of course, the possibility of some genetical differentiation for flowering time in the
population is not excluded, but the circumstances as discussed by
Ehrendorfer do suggest that the environmental control of phenology
may have imposed some restriction on free gene exchange between the
sub-populations even during the iktial differentiation.
The implication of observations such as these is that the mere occupancy of a heterogeneous habitat can fragment a plant population
into partly isolated breeding groups by breaking down the synchroneity
of flowering. Since each distinct habitat will have its own particular
effect, the sub-populations in each will have a characteristic flowering
period. The plants in each type of habitat will thus remain panmictic,
but will be genetically isolated from others in different habitats to the
extent that the flowering periods fail to overlap. This is an irrefutable
example of ecological isolation, not to be discounted on the basis of any
of the arguments of Mayr (1947).
E. MONOTOPIC AND POLYTOPIC ORIGIN AND THE RETENTION
O F RACIAL IDENTITY
A distinction can be drawn at least on theoretical grounds between
mosaic patterns of ecological races where each race has had a monotopic
origin, and the superficially similar patterns which have arisen through
the polytopic origin of like ecotypes in response to corresponding selective factors in different parts of a species range. It is to be expected that
races that have had single independent origins will normally be distinguished by several correlated differentiae, both adaptive and nonadaptive. On the other hand, only the adaptive characteristics are
likely to be shared in common between the various populations of an
ecotype which has originated polytopically, and the populations may
differ among themselves in respect to non-adaptive features. A classical
case of presumed polytopic origin is that of the sand-dune ecotype of
Hieracium umbellatum described by Turesson (1922b). This occurs in
sand-dune habitats widely scattered around the periphery of the species
area in Sweden. The different populations share in common various
adaptive features such as a capacity for rapid shoot regeneration, but
they differ in minor leaf characteristics, each showing a resemblance in "
these characteristics with neighbouring inland populations.
The inhomogeneity of ecotypic populations of polytopic origin is
likely to be evident also in the genetic basis of their adaptation. Since
similar phenotypes can be established by different combinations of polygenes, there is no reason to suppose that identical selective pressures in
remote sites will necessarily fix the same group of genotypes. In con-