"When the going gets tough, species start merging. Lake-dwelling fish species that once lived separately began interbreeding when pollution forced them together. Ultimately most of the lakes' remarkable diversity has been lost – and the same could be happening in threatened habitats around the world. After the last ice age, whitefish (Coregonus) in Europe's Alpine lakes split into several species, each with a specialised appearance and lifestyle." (Quoted from article)