There are two types of evolutionary change from the perspective of where the variation that is the starting point for all evolution ‘comes from’. First, there is the type of change that I described in the previous chapter, in which the relevant variation already exists within the evolving population, in the form of what we call the ‘standing variation’. This is exemplified by the variation that we observe between individuals within any human population in characters such as height, shape, strength, and so on. Second, there is the type of change that requires a new mutation to come along. An example of this is when newly conspicuous pale moths became vulnerable to predators against the blackened tree trunks of the industrial revolution, but had to await the appearance of a dark ‘melanic’ mutant moth before evolving towards a novel form of camouflage.

You might well want to question the wisdom of the ‘first’/ ‘second’ order that I have just used. There is certainly a logic in reversing this order. When a population of any species first appears in any geographical region, it often does so by the immigration of a few individuals from elsewhere, followed by a period of rapid growth in the number of their progeny because of the relative lack of competition for resources in their new-found home. Such a population will have an unusually restricted amount of standing variation because the number of founding individuals was so low. Effectively, the population is ‘inbred’. Its evolutionary potential may be quite small. For many potentially adaptive evolutionary changes to occur, it is ‘waiting for mutation’.