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involve the tolerance mechanism, 
which seems to enable the plant to 
sequester the toxic metals in an in- 
nocuous complex in the cell walls. We 
do know that without neighbors, tol- 
erant plants are perfectly capable of 
growth on pasture soils. Competition 
with normal plants completely sup- 
presses them in pastures, however, and 
garden experiments with mixtures of 
the two inevitably lead to the domi- 
nance of the pasture plants. The tol- 
erant individuals seem unable to 
corner their share of light, nutrients, 
and water. Plants found on mines gen- 
erally differ from nontolerant speci- 
mens in many ways, however, so this 
competitive inferiority may derive 
from biological characters other than 
the tolerance mechanism. One study 
of zinc-tolerant Anthoxanthum odor - 
alum, sweet vernal grass, found that 
such individuals are smaller in stature 
than nontolerant plants, have smaller 
leaves, and are able to grow more ef- 
fectively in soils with low levels of 
nutrients. They also flower earlier in 
the season and self-pollinate at a high 
frequency. Such reproductive charac- 
teristics tend to isolate the mine pop- 
ulation from the flow of foreign pollen, 
which would otherwise lower the aver- 
age tolerance among progeny. This in- 
cipient reproductive isolation of an in- 
terbreeding group of plants is the first 
step on the path to speciation. Perhaps 
the evolution of what are now two 
populations of Agrostis tenuis will in 
time result in the formation of two 
independent species. 
The metal tolerance story has a 
pleasing epilogue in the real world 
of land reclamation. Bradshaw has 
turned his attention to the use of tol- 
erant populations in the revegetation 
of contaminated soils. Derelict mines 
are more than an eyesore; they are 
also subject to severe gully and sheet 
erosion from surface drainage and 
wind. The flooding of neighboring 
streams and the contamination of 
ground water redistributes toxic spoils 
into surrounding pasture and agricul- 
tural land, sterilizing the vegetation 
and endangering grazing livestock. 
In 1969 Bradshaw and his students 
began experiments at a number of 
sites. Preliminary testing of the soils 
revealed that phosphorus is a major 
nutrient limiting the establishment of 
plants over the whole surface. At each 
site they laid out a set of small half- 
yard plots differing in topography and 
applied a series of fertilizer treatments 
to facilitate initial growth of seedlings. 
They harvested seeds of tolerant pop- 
ulations of Agrostis tenuis and several 
other tolerant species from contami- 
nated mines and sowed them at high 
densities, along with commercial, non- 
tolerant seeds that served as controls. 
When growth was assessed at the end 
of a year, tolerant plants consistently 
outyielded nontolerant plants. Ferti- 
lizer applications were essential, how- 
ever, and the pH of the soil determined 
which tolerant species were most suc- 
cessful: A. tenuis, for example, prefers 
strongly acidic conditions. 
These experimental populations of 
tolerant plants proved remarkably per- 
sistent, maintaining vigorous growth 
for more than nine years. The con- 
ventional process of reclamation in- 
volves the surface addition of a layer 
of sewage sludge or topsoil, followed 
by the sowing of commercial varieties 
of grass. Bradshaw has calculated that 
the cost of using metal-tolerant pop- 
ulations with appropriate fertilizer 
treatments may be less than one-sixth 
the cost of the traditional method. 
Like the best of biomedical science, 
Bradshaw’s study of heavy metal tol- 
erance is a paradigm of rigorous re- 
search and a prescription for increas- 
ing our well-being. But by focusing 
on the meaning of variation, evolu- 
tionary biology differs from medical 
biochemistry and physiology, where 
variation is often considered an un- 
fortunate experimental error. Biomed- 
ical investigators tend to concentrate 
on the commonness of events, on the 
average pattern seen beneath the 
glass. Students of ecology and evo- 
lution, by contrast, are concerned with 
the rich pageant of diversity in nature. 
Far from an annoyance to be mini- 
mized by meticulous control, natural 
variation is recognized as essential to 
answering the question, Why are there 
so many kinds of living things? 
The answer can have practical ap- 
plications. Today, the National Seed 
Development Organization, for exam- 
ple, is selling cultivars of metal-tol- 
erant Agrostis tenuis ready for use 
in reclamation. In a day when each 
dollar of the U.S. government’s budget 
for pure science is subjected to intense 
scrutiny and each week reveals a new 
chemical dump in someone’s back- 
yard, we may look to Bradshaw’s work 
for inspiration. Creative, rigorous sci- 
ence need not always be expensive, 
inexplicably abstract, or irrelevant. 
Robert E. Cook is associate professor 
of biology at Harvard University. 
38 
