Why is there so much genetic diversity? In spite of the huge amounts of polymorphism found in most populations, in most cases we have no idea. I am interested in the relationship between habitat structure and visible variation in snail shell colour and banding pattern. There is a clear fit between the two, and experiments with a paint that fades at a known rate when exposed to the sun shows that animals of different phenotype choose to spend more, or less, time in sunny patches of the habitat. I have extended this work to natural populations of fruit flies, using eye mutants whose appearance depends on the temperature experienced during development, and am now involved in a Europe-wide survey of snail populations to see if there has been any change in gene frequency over the past fifty years and more in response to the changes in climate that have taken place. I have recently been looking at genetic differences in pain reception in populations from different climates and exposure to thermal stress.