Bios 
Toxic Tailings and Tolerant Grass 
Adapted to mine soils contaminated with heavy metals, 
a species of grass in Wales provides an opportunity to 
learn about natural selection — and perhaps land reclamation 
by Robert E. Cook 
The tailings of abandoned mines 
hardly seem a fitting place to watch 
evolution. Vestiges of former excava- 
tions, they lie like forgotten scars on 
the face of the landscape. Rain rapidly 
drains through the coarse, porous soil, 
which holds little organic matter and 
few mineral nutrients. Toxic concen- 
trations of heavy metals — zinc, cop- 
per, and lead — contaminate the sedi- 
ments, rendering the ground water 
poisonous for most plants. Careful 
searching across this largely barren 
surface, however, frequently turns up 
tufts and large patches of grass that 
apparently tolerate the toxic metals 
and thrive under the stress of drought 
and mineral depletion. The successful 
adaptation of these rugged plants to 
derelict mines is a testament to the 
power of natural selection and pro- 
vides a fine opportunity to study the 
process in operation. 
More than one hundred years ago, 
Charles Darwin hinged his theory of 
evolution on the natural selection of 
individuals. Through the differential 
survival and reproduction of variants 
appearing in each generation, traits 
that adapt an organism to a particular 
habitat can increase in frequency in 
future generations. A century after 
the publication of Origin of Species, 
British geneticist Anthony Bradshaw 
and his students at University College 
of North Wales in Bangor set out to 
piece together a detailed picture of 
rapid adaptation among plants to the 
presence of heavy metals. To do so, 
they focused on the significance of 
natural variation. 
Bradshaw and his students noticed 
that although pastures surrounding lo- 
cal mines were rich with species ap- 
parently unable to survive on the con- 
taminated mine soils, there were some 
species growing in both habitats. For 
example, Agrostis tenuis, a wind-pol- 
linated grass found in abundance in 
the pasture, forms sparse populations 
on mine soils. Two explanations of this 
phenomenon seem possible. On the 
one hand, perhaps certain species, 
such as Agrostis, possess a tolerance 
mechanism that enables them to ex- 
tend their range to the excavation site. 
Alternatively, most of the pasture 
plants of Agrostis may not be able to 
tolerate heavy metals, and the coloniz- 
ers of the mine may represent excep- 
tional individuals that have adapted to 
the metals. 
To determine which possibility is 
correct, researchers have developed an 
assay to measure the degree of tol- 
erance possessed by an individual 
plant. They grow selected tillers or 
seedlings of a test plant in two so- 
lutions: one, the control, supplies only 
the nutrients found in normal soil; the 
second also contains a toxic concen- 
tration of a particular heavy metal. 
In the second nutrient solution, 
nontolerant plants do not grow roots, 
but the roots of tolerant individuals 
rapidly proliferate. Hence the vigor 
of the root growth in the toxic me- 
dium, compared with the growth in 
the control medium, provides a re- 
liable index of tolerance. On this scale 
a rating of zero indicates a lack of 
tolerance, while a completely tolerant 
plant is rated ten. 
Bradshaw sampled plants of 
Agrostis from a local mine and the 
surrounding pasture and grew them 
in the metal solution and the control. 
The tolerant plants rapidly rooted to 
form vigorous tillers, but the roots of 
the pasture plants were stunted and 
the shoots did not grow. The mine 
populations were clearly different 
from those found in the pasture and 
had evolved heavy metal tolerance 
since the original excavations. More- 
over, although some mining operations 
date from the Middle Ages, many 
Agrostis populations have been dis- 
covered growing on mines less than 
one hundred years old; this evolution 
must be remarkably rapid. 
Because the tolerant and nontol- 
erant populations of Agrostis grow so 
close together, the explanation for 
such rapid evolution is more complex 
than simply quick adaptation to new 
conditions. Tolerant individuals of 
Agrostis grow well on mine soils in 
the presence of heavy metals and un- 
der harsh habitat conditions. Their off- 
spring share their tolerance, indicating 
a strong genetic basis for this adap- 
tation. In the pasture, nontolerant 
Agrostis survive nicely, competing 
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