Chapter 13 - Population Genetics

Summary

INTRODUCTION

Mendel's investigation of heredity and the major interest in heredity in the nineteenth century arose from two related problems: how to breed improved crops and domestic animals, and how to understand the nature and origin of species. What these problems have in common is that they are concerned with populations rather than individuals. Most population concepts involve a hierarchical classification in which units are defined in terms of the extent to which individuals can be identified by common characteristics. The definition of a population for demographic purposes can be made entirely on arbitrary grounds. It can involve social, ecological or genetic criteria.

Population genetics is the study of allele and genotype frequency distribution and change under the influence of four main evolutionary forces: mutation, genetic drift, gene flow and natural selection. These forces can result in different allele frequencies in different populations and one of the forces can dominate at one time in the evolution of a population, while others can have a major effect at another. For example, endemic exposure to pathogenic infectious organisms would select individuals with increased survival potential (determined by their phenotype, which in turn is partly determined by their genotype) and lead to an increase in the frequency of advantageous alleles during the time that the selective pressure is active. A population must be defined before an allele frequency within it can be determined.

Human populations are made up of the old, the middle-aged and the young, those who have reproduced and those who have not. Each mature member of a population has some chance of reproducing with any other mature individual in the population. The magnitude of that chance depends on factors such as proximity, sub-culture, age, physical attributes, education and socio-economic status. The fraction of a generation that produces the next generation is called the effective population (N_e), even though non-breeding individuals certainly play important supporting roles.

GENETIC VARIATION

It is obvious that all individuals of the same gender in any human population are not the same; they do not look the same or react in the same way. These differences are due to their life stage, the environment in which they have developed and their genetic make-up.