There are two conditions that are necessary for evolution to occur. First, the characteristics of an organism must vary in the population, and that variation must be related to differences in survival or reproductive success. Second, the variation must also have a genetic basis, at least in part. As a consequence, evolution changes the gene frequencies of populations. In Part 1, we noted that Darwin made a strong argument that natural selection was the main force driving evolution. However, the gene frequencies in populations can also be changed by other forces, such as mutation, migration, and even chance, and so we need to assess the importance of these factors on the evolution of populations.

The main purpose of the following eight chapters is to make a quantitative assessment of the various factors that affect the gene frequencies of populations. How do we measure the allelic and genotypic frequencies in populations, and how are they affected by sexual reproduction (Chapter 6)? How does genetic variation arise in populations and how is it maintained (Chapter 7)? How are gene frequencies in populations affected by mutation (Chapter 7), chance (Chapter 8), migration (Chapter 9) and selection (Chapters 10 to 12)? What are the relative strengths of these factors and how do they interact with one another (Chapter 13)? Thus, we will try to make an objective assessment of Darwin's theory of evolution by natural selection to see if it is supported by the theory of population genetics.