This chapter opens with a discussion of the species concept. All too often the processes of evolution are covered without explaining the nature of species, the basic units of evolution. While indicating that species are real entities, I also try to give some idea as to the fluid nature of species—the fact that species themselves change, that they change in relation to one another, and that they may give rise, after a transitional period, to new species. Examining the idea of the frequency of alleles within a species sets the stage for the quantification of the processes of evolution.
The four major processes of evolution are then examined and the topics of this and the previous chapter are all brought together by looking at the example of sickle cell anemia. The discussion of this disease also provides a perfect example of what is meant by a holistic study.
