In this book I have described the results of my study of the population ecology and genetics of the Arctic Skuas on Fair Isle. When a population ecologist studies an organism, he asks questions, such as the following, about its population size and numbers:

1. (i) Are its populations stable, increasing, or decreasing?

2. (ii) Can future population changes be predicted?

3. (iii) How are population numbers regulated – for example, is regulation density-dependent?

Seeing a polymorphism, a population geneticist asks such questions as:

1. (i) What are the genetics of the polymorphism?

2. (ii) Is the polymorphism stable, or is it still evolving?

3. (iii) How are the gene frequencies spatially distributed – is their distribution uniform, or are there clines from one area to another?

4. (iv) What selective forces are acting on the phenotypes – natural selection, sexual selection, or both – and are they sufficient to ‘protect’ the polymorphism against extinction of alleles?

I hoped I might answer such questions when I set out to study the Arctic Skua and its striking polymorphism in plumage. In previous chapters – chapter 2 on population ecology, chapter 5 on genetics and chapters 6, 8 and 10 on natural and sexual selection, I have given some answers. Some answers are tentative: some – on the population changes and the natural and sexual selection of the population – are bold and decisive. Now, in this last chapter, I can omit the details of analysis, summarize the results, and draw general conclusions about the evolutionary forces that maintain the polymorphism.