FORTY YEARS O F GTENECOLOGY
22 1
the general problem of ecotype evolution. The adaptation of these races
must be to the whole environment of the serpentine, yet the dominant
factor in this, according to Kruckeberg, is the calcium deficiency. Selection pressure has presumably been greatest therefore for the property of
accumulating calcium preferentially ; when this was acquired, the invading populations would become exposed to selection for tolerance to
the other conditions in which the serpentine habitat differs less radically
from the normal. The inter-locking nature of the various factors becomes
apparent when it is appreciated that certain peculiarities of the serpentine habitat arise just because some species - including shade-forming
trees - are excluded from it by their intolerance of calcium deficiency,
A single major factor is here seen to generate a complex of selective
pressures secondarily.
The demonstration of adaptive properties in races occurring on extreme soil types raises the further question of whether adaptation has
been bought at the cost of the ability to survive in normal soils. I n the
case of lead tolerant fescue races, Wilkins (1960b) records a higher mortality among tolerant biotypes in cultivation under conditions of low
competition. This observation does indeed suggest that selection for the
ability to endure high lead concentrations in the soil has led to theplants’
becoming dependent in some way on lead-rich soils, possibly, as Wilkins
suggests, even by establishing a requirement for lead itself. Kruckeberg’s
observations (1954) on serpentine endemic species seem to show that
their exclusion from normal soils is entirely in consequence of competition. I n culture on calcium-replenished soils free from the suppressive
effects of weedy annuals, serpentine endemic Streptanthus species
thrived; subject to competition, survival was poor. It is perhaps not
justifiable to argue directly from this to the case of serpentine ecotypes,
and it may be that the adaptation of these has involved some sacrifice.
McMillan (1956a) records that a “strain” of Agrostis halliii from a serpentine soil performed very substantially better, as judged by height and
dry weight production, when cultured on Serpentine soil than when
grown on a control soil; this may indicate that here adaptation has involved specialization. But the example of Achillea borealis, in which
some biotypes from normal soils are recorded by Kruckeberg (1950) as
showing tolerance to serpentine conditions, proves that the capacity to
survive on calcium-deficient soil does not necessarily put a plant to
selective disadvantage in an otherwise unadapted population on a
normal soil.
The two cases of edaphic adaptation discussed above relating to the
tolerance of heavy metals and serpentine soils are unusual in that the
selective factors are obviously severe and the circumstances in which
they are encountered rather rare. It is obviously important to know
H2
C.E.R.
22 1
the general problem of ecotype evolution. The adaptation of these races
must be to the whole environment of the serpentine, yet the dominant
factor in this, according to Kruckeberg, is the calcium deficiency. Selection pressure has presumably been greatest therefore for the property of
accumulating calcium preferentially ; when this was acquired, the invading populations would become exposed to selection for tolerance to
the other conditions in which the serpentine habitat differs less radically
from the normal. The inter-locking nature of the various factors becomes
apparent when it is appreciated that certain peculiarities of the serpentine habitat arise just because some species - including shade-forming
trees - are excluded from it by their intolerance of calcium deficiency,
A single major factor is here seen to generate a complex of selective
pressures secondarily.
The demonstration of adaptive properties in races occurring on extreme soil types raises the further question of whether adaptation has
been bought at the cost of the ability to survive in normal soils. I n the
case of lead tolerant fescue races, Wilkins (1960b) records a higher mortality among tolerant biotypes in cultivation under conditions of low
competition. This observation does indeed suggest that selection for the
ability to endure high lead concentrations in the soil has led to theplants’
becoming dependent in some way on lead-rich soils, possibly, as Wilkins
suggests, even by establishing a requirement for lead itself. Kruckeberg’s
observations (1954) on serpentine endemic species seem to show that
their exclusion from normal soils is entirely in consequence of competition. I n culture on calcium-replenished soils free from the suppressive
effects of weedy annuals, serpentine endemic Streptanthus species
thrived; subject to competition, survival was poor. It is perhaps not
justifiable to argue directly from this to the case of serpentine ecotypes,
and it may be that the adaptation of these has involved some sacrifice.
McMillan (1956a) records that a “strain” of Agrostis halliii from a serpentine soil performed very substantially better, as judged by height and
dry weight production, when cultured on Serpentine soil than when
grown on a control soil; this may indicate that here adaptation has involved specialization. But the example of Achillea borealis, in which
some biotypes from normal soils are recorded by Kruckeberg (1950) as
showing tolerance to serpentine conditions, proves that the capacity to
survive on calcium-deficient soil does not necessarily put a plant to
selective disadvantage in an otherwise unadapted population on a
normal soil.
The two cases of edaphic adaptation discussed above relating to the
tolerance of heavy metals and serpentine soils are unusual in that the
selective factors are obviously severe and the circumstances in which
they are encountered rather rare. It is obviously important to know
H2
C.E.R.