Introduction

Mutations alter the genotype by changing the nucleotide sequence in DNA, but natural selection operates on the phenotype, which is largely dependent on the particular proteins made by an organism. A study of the structure and sequence of individual proteins can therefore be useful in furthering the understanding of evolution in two important ways. Firstly, by comparing the structures of a specific protein, e.g. cytochrome c, that occur in different species, it is possible to establish or confirm phylogenetic relationships amongst organisms, and to build up phylogenetic trees involving phyla, classes, orders, etc. Secondly, a number of proteins are found to exist in more than one closely related form, e.g. ovalbumin A and B; these are known as polymorphisms. A study of the distribution of different polymorphic forms within a population of a given species, together with a knowledge of their dominance relationships, can be used to explain their more recent history. These two areas are interrelated, and in this chapter a number of proteins are examined, some of which have been primarily of importance in establishing and confirming phylogenetic relationships, e.g. the haem proteins, and others have been more useful in determining relationships between breeds of the domestic fowl, e.g. egg-white proteins.