Just as I started this post I found a video of it. It is worth watching all 4 parts on utube. URL at end.
A BBC Horizon programme on the subject of Evolution called “Are we still Evolving?” It was presented by Dr. Alice Roberts who also produced “The incredible Human Journey” a five part series which is excellent. It investigated derelict copper and arsenic mine in Devon, England. It produced so much arsenic at one point that it was the largest producer of it in the world for a time.
To this day much of the surrounding land is poisoned. However earthworms have been found there that although initially appear similar to the common or garden earthworm, are much paler in appearance. If a regular earthworm was to be placed in the soil here it would not survive. Natural Selection, in less than 170 years, had selected the traits necessary or favourable for them to cope with arsenic and to survive in such a hostile environment.
They were genetically analysed and it turned out that they are actually a different species. They are more genetically different to regular earthworms than humans are to mice.