Although many people think evolution is a slow process, it can in fact happen extremely fast. There are thousands of examples of evolution in action, from the famous peppered moths that turned black to camouflage themselves on soot-covered trees to the ever-growing numbers of antibiotic-resistant bacteria.

In most cases of contemporary evolution, the genetic changes involved have never been identified. The mutation that turned the first few peppered moths black in about 1819 was identified only earlier this year, for instance.

With DNA sequencing getting ever cheaper, biologists in the US have now been able to sequence the genomes of nearly 400 individual Atlantic killifish (Fundulus heteroclitus), a small fish also known as the mummichog that lives in estuaries along the east coast. They compared the genomes of killifish in four highly polluted areas with those from four unspoilt sites.