ABSTRACT
Consider a population of a certain species that occupies a particular range. The population is distributed fairly evenly throughout the range and utilizes the whole of it. Then something happens to fragment the range. Perhaps a network of roads is made through it, or parts of it are ploughed for agriculture or afforested, or rivers intersecting the range become so polluted that individuals drinking from them or trying to swim across them are killed. Whatever the cause, and human activities of one kind or another are nowadays the most frequent, the effect is to divide the population into several groups. These are isolated from one another by barriers they cannot cross.
