188
J. HESLOP-HARRISON
(a) The geographical distributional pattern of a species is a determinant
of the pattern of genecological differentiation. Where an area is continuous, spanning, say, an appreciable range of latitude, clinal variation
is likely to be found, particularly in long-lived species of stable communities; if there are discontinuities, regional races with distinct ecological tolerances may be encountered.
( b ) The total ecological range of a species is significant. I n a species
restricted to the modulations of a particular kind of habitat, any ecological variation will tend to be clinal. If a species spans a range of discrete habitats which are themselves sufficiently distinct, ecological races
of the nature of the ecotypes may be found, even when the habitats are
contiguous.
(c) Systems of continuous and discontinuous variation may be combined a t the same and at different leveIs. Thus :
(i) A species may show differentiation into major ecological r&es
associated with different types of habitat within its area, and within
one or more of the races there may at the same time be ecoclinal
variation adaptive to smoothly varying climatic factors.
(ii) Local variation of an “ecotypic” kind may occur within the
framework of a grand system of clinal variation extending throughout
the species area.
(iii) Conversely, within major ecological races local adaptive clines
may be found.
(iv) Hierarchical patterns of both ecotypes and ecoclines may exist :
thus a regional ecological race may itself be composed of a system of
more local ecotypes, and within a major ecocline local systems of
clinal adaptation may be present.
(v) In widely ranging species, clinal variation adaptive to different
factors may occur, giving in the extreme case, patterns of “intersecting clines”.
The existence of complexities such as those listed under (c) above
means that the results of any particular genecological study will tend to
be related to the scale on which it is conceived. Thus the minutae that
engage Bradshaw (1959, 1960) in his investigation of Agrostis tenuis in a
small area of central Wales would pass largely unnoticed in a study on
the geographical scale of that of Clausen et al. of Potentilla glandulosa,
(1940). This is not to suggest that identical methods would reveal
corresponding variational patterns in these two particular species, but
merely to point out that the results so far obtained with them are not
capable of direct comparison. We shall see that much can be deduced
about the probable patterns of genecological differentiation within a
species from a knowledge of its genetic system, distribution and general
ecology, and it would seem an essential now t o take these factors into
J. HESLOP-HARRISON
(a) The geographical distributional pattern of a species is a determinant
of the pattern of genecological differentiation. Where an area is continuous, spanning, say, an appreciable range of latitude, clinal variation
is likely to be found, particularly in long-lived species of stable communities; if there are discontinuities, regional races with distinct ecological tolerances may be encountered.
( b ) The total ecological range of a species is significant. I n a species
restricted to the modulations of a particular kind of habitat, any ecological variation will tend to be clinal. If a species spans a range of discrete habitats which are themselves sufficiently distinct, ecological races
of the nature of the ecotypes may be found, even when the habitats are
contiguous.
(c) Systems of continuous and discontinuous variation may be combined a t the same and at different leveIs. Thus :
(i) A species may show differentiation into major ecological r&es
associated with different types of habitat within its area, and within
one or more of the races there may at the same time be ecoclinal
variation adaptive to smoothly varying climatic factors.
(ii) Local variation of an “ecotypic” kind may occur within the
framework of a grand system of clinal variation extending throughout
the species area.
(iii) Conversely, within major ecological races local adaptive clines
may be found.
(iv) Hierarchical patterns of both ecotypes and ecoclines may exist :
thus a regional ecological race may itself be composed of a system of
more local ecotypes, and within a major ecocline local systems of
clinal adaptation may be present.
(v) In widely ranging species, clinal variation adaptive to different
factors may occur, giving in the extreme case, patterns of “intersecting clines”.
The existence of complexities such as those listed under (c) above
means that the results of any particular genecological study will tend to
be related to the scale on which it is conceived. Thus the minutae that
engage Bradshaw (1959, 1960) in his investigation of Agrostis tenuis in a
small area of central Wales would pass largely unnoticed in a study on
the geographical scale of that of Clausen et al. of Potentilla glandulosa,
(1940). This is not to suggest that identical methods would reveal
corresponding variational patterns in these two particular species, but
merely to point out that the results so far obtained with them are not
capable of direct comparison. We shall see that much can be deduced
about the probable patterns of genecological differentiation within a
species from a knowledge of its genetic system, distribution and general
ecology, and it would seem an essential now t o take these factors into