Evolution over 3.85 billion years has imbued the natural world with bewildering biological diversity, a high degree of embedded resilience and the capacity to adapt. This has been through the process of natural selection, in which of the genes of the most fit to the diversity of habitat and niches within the natural world are preferentially passed on to the following generation. Loss of genetic diversity, homogenising the stock and replacing local adaptations and distinctiveness, is as much of a conservation problem as threats to species themselves. The chemical composition of Britain's fresh waters varies naturally. Water running off hard, largely insoluble rock tends to contain lower concentrations of dissolved and suspended substances, creating nutrient-poor streams and pools. In some catchments, impermeable geology means that little water is stored underground. Consequently, rainfall results in rapidly rising spate flows, with water levels then declining as rapidly.