Blackwell Publishing

The idea of a species - How do we decide between the species concepts?


In some circumstances, selection can produce divergence despite gene flow.

Bradshaw studied the grass Agrostis tenuis (opposite) on and around spoil-tips in the UK. Spoil-tips are deposited from metal mining and contain high concentrations of poisonous heavy metals. Few plants can grow on them.

Some plants have colonized them, however, and the grass A. tenuis  has colonized the spoil-tips by means of genetic variants that are able to grow where the concentration of heavy metals is high; around a spoil-tip, therefore, there is one class of genotypes growing on the tip itself, and another class in the surrounding area. Natural selection works strongly against the seeds of the surrounding forms when they land on the spoil-tip: they are poisoned. There is also selection against the resistant forms off the spoil-tips; there is probably some cost in possessing a detoxification mechanism.

In this case, divergence is favored by natural selection: the reproductive species concept predicts that gene flow will be reduced (otherwise divergence could not have taken place) but in fact gene flow is large. Pollen blows in clouds over the edges of the spoil-tips and interbreeding between the genotypes is extensive. In this case, selection has been strong enough to overcome gene flow.

Like most other examples of populations that have diverged in the face of gene flow, the conditions on a spoil tip are exceptional. This is what we should expect: if gene flow and selection come into conflict there is selection to alter reproductive behavior and speciation will occur.

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