The Natural History Reader in Evolution

By Niles Eldredge | Go to book overview

5
Toxic Tailings and Tolerant Grass

ROBERT E. COOK

The tailings of abandoned mines hardly seem a fitting place to watch evolution. Vestiges of former excavations, 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 concentrations of heavy metals-- zinc, copper, and lead--contaminate the sediments, 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 provides 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 local mines were rich with species apparently unable to survive on the contaminated mine soils, there were some species growing in both habitats. For example, Agrostis tenuis, a wind-pollinated grass found in abundance in the pasture, forms sparse populations on mine sods. Two explanations of this phenomenon seem possible. On the one hand, perhaps certain species, such as Agrostis, possess a tolerance

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