FORTY YEARS O F GENECOLOUY
179
fined to certain extreme soil types occurring locally in otherwise normal
terrain, as for example over outcrops of serpentine rocks, or on the
spoil heaps derived from lead and other mineral workings. The sudden
changes of vegetation encountered with the transition from non-serpentine to serpentine soils is well documented for many parts of the world
(Whittaker, 1954; Walker, 1954). Frequently this is seen in- a complete
disappearance of intolerant species. I n some cases, a taxonomic species
is replaced abruptly by another, very closely related one; in others,
edaphic races tolerating serpentine conditions occur which are not
sufficiently well differentiated morphologically to have merited taxonomic recognition (Kruckeberg, 1954).
These edaphic ecotypes are usually quite sharply demarcated
physiologically from neighbouring populations on normal soils, reflecting the abrupt change in habitat. Kruckeberg’s study of 1950 is of
special interest, since he included observations on two species already
extensively studied genecologically, Cilia capitata (Grant, 1950 ; see p.
175) and Achillea borealis (Clausen et al., 1948). Within the subspecies
capitata of Cilia capitata, considered by Grant to constitute a reasonably
homogeneous race with a distribution related primarily to regional
climate, Kruckeberg detected local populations adapted to serpentine
soils and meriting recognition as edaphic ecotypes. Similarly, in the
inner Coast Range-Sierran foothill race of Achillea borealis subsp. californica, regarded by Clausen et al. as a climatic ecotype, serpentine
tolerant and intolerant edaphic ecotypes were distinguished. Kruckeberg (1954) comments on the Achillea situation as follows : “There are
thus edaphic subdivisions within climatic ones in this species, i.e. ecotypes within ecotypes. Moreover, since the geographical area covered by
the foothill climatic ecotype is very diverse lithologically and therefore
is a mosaic of different soil types, additional edaphic ecotypes may well
be expected. The superimposition of ecotype on ecotype at least suggests
that there exists a much more complex genotypical response to habitat
than is implied in consideration of single environmental factors.”
The differentiation of serpentine tolerant races in wide-ranging
species has its parallel in miniature in the emergence of local populations
tolerant of the toxic soils formed on the spoil heaps from lead mines
and other mineral workings (Bradshaw, 1952). Wilkins (1960a, b) has
initiated the study of lead tolerant populations of Festuca owina in
Britain. Three “types” were distinguished, in respect to tolerance, one
(wide-ranging) intolerant, and two others, restricted to lead-containing
soils, of medium and high tolerance respectively when tested for rooting
in lead nitrate solutions. Tolerance was not associated with any morphological features, so Wilkins has been reluctant to talk of “lead soil ecotypes”. Evidently there is nothing comparable to a homogeneous race
179
fined to certain extreme soil types occurring locally in otherwise normal
terrain, as for example over outcrops of serpentine rocks, or on the
spoil heaps derived from lead and other mineral workings. The sudden
changes of vegetation encountered with the transition from non-serpentine to serpentine soils is well documented for many parts of the world
(Whittaker, 1954; Walker, 1954). Frequently this is seen in- a complete
disappearance of intolerant species. I n some cases, a taxonomic species
is replaced abruptly by another, very closely related one; in others,
edaphic races tolerating serpentine conditions occur which are not
sufficiently well differentiated morphologically to have merited taxonomic recognition (Kruckeberg, 1954).
These edaphic ecotypes are usually quite sharply demarcated
physiologically from neighbouring populations on normal soils, reflecting the abrupt change in habitat. Kruckeberg’s study of 1950 is of
special interest, since he included observations on two species already
extensively studied genecologically, Cilia capitata (Grant, 1950 ; see p.
175) and Achillea borealis (Clausen et al., 1948). Within the subspecies
capitata of Cilia capitata, considered by Grant to constitute a reasonably
homogeneous race with a distribution related primarily to regional
climate, Kruckeberg detected local populations adapted to serpentine
soils and meriting recognition as edaphic ecotypes. Similarly, in the
inner Coast Range-Sierran foothill race of Achillea borealis subsp. californica, regarded by Clausen et al. as a climatic ecotype, serpentine
tolerant and intolerant edaphic ecotypes were distinguished. Kruckeberg (1954) comments on the Achillea situation as follows : “There are
thus edaphic subdivisions within climatic ones in this species, i.e. ecotypes within ecotypes. Moreover, since the geographical area covered by
the foothill climatic ecotype is very diverse lithologically and therefore
is a mosaic of different soil types, additional edaphic ecotypes may well
be expected. The superimposition of ecotype on ecotype at least suggests
that there exists a much more complex genotypical response to habitat
than is implied in consideration of single environmental factors.”
The differentiation of serpentine tolerant races in wide-ranging
species has its parallel in miniature in the emergence of local populations
tolerant of the toxic soils formed on the spoil heaps from lead mines
and other mineral workings (Bradshaw, 1952). Wilkins (1960a, b) has
initiated the study of lead tolerant populations of Festuca owina in
Britain. Three “types” were distinguished, in respect to tolerance, one
(wide-ranging) intolerant, and two others, restricted to lead-containing
soils, of medium and high tolerance respectively when tested for rooting
in lead nitrate solutions. Tolerance was not associated with any morphological features, so Wilkins has been reluctant to talk of “lead soil ecotypes”. Evidently there is nothing comparable to a homogeneous race