Migration and Colonization in Human Microevolution.

Alan G. Fix. Cambridge University Press, Cambridge. 1999. Pp. 236. Price £40.00, hardback. ISBN 0 521 59206 2.

During the twentieth century various biological disciplines tried to reconstruct and understand the origin and past history of modern humans. For anthropologists and human geneticists the study of microevolution, i.e. the factors affecting the variation of gene frequency patterns, has been the major field of investigation. In the early thirties, Fisher developed a systematic theory of evolution by natural selection and predicted that in a large population even slight selective differentials could replace a less advantageous twentieth with a more favoured one. At the same time Wright also developed a general theory of evolution and his argument was based on the fact that a species is composed of many small and nearly isolated subpopulations and within most there exist some boundaries to breeding. The size of these isolates is very important in the evolutionary process and assuming the population is randomly mating and there is no selection or mutation, such subdivisions will show genetic differentiation as a result of chance processes. When the isolation is partial the rate of divergence will depend on the amount of migration or gene flow. Gene flow between subpopulations retards the process of genetic differentiation. It is obvious therefore that in addition to conventional genetic factors, an understanding of demography, ecology, environment of the natural habitat, social behaviours and all other factors which promote migration and colonization are very important for understanding present day population structure. Since extensive information was available from historical records of the social and demographic structure of the human population, this led to the development of many classical models of population structure. More recently computer-intensive simulation methods have been developed which allow the study of migration and its effects through time, during the microevolution of humans.

The other vital achievement of the late 1900s was a revolution in the development of methodologies for genome analysis. There was a sudden explosion of new molecular markers, which helped in the reconstruction of the history of human migration and in testing explanations for demic diffusion. The spread of Homo sapiens sapiens to occupy the New World, colonization of Oceania and expansion through Europe via demic diffusion of agriculture were initially investigated using classical markers (blood groups, enzymes and proteins, human leucocyte antigens and immunoglobulin allotypes), but later confirmed by the study of autosomal (STRs) and sex-related mitochondrial and Y chromosome markers. With all these new inventions and the sequential expansion of population genetics in mind, Alan Fix has attempted to trace the role of migration in human populations by taking examples of well investigated populations which vary extensively in population density, land occupied and social integration.

The book includes six chapters. In the first migration is examined through causal models based on the overlapping interest in migration among different biological disciplines. As the book uses detailed data on human migration from anthropological and historical sources this chapter defines the basic terminology used in later ones. Chapter 2 is very interestingly designed. It selects populations from different parts of the world, representing diverse social and environmental conditions, with very variable densities. For low population density, extensive land use and family groups it includes the populations of Yolgnu (Australia), Kung-San (South Africa) and Aka pygmies (Central Africa), while for low to moderate density, extensive agriculture and local kin groups it selects the populations Vaupes and Yanomama (Lowland Amazonia), Semai-Senoi (Malaysia) and Gainj-Kalam (New Guinea). For high density, intensive agriculturist and local groups and castes, the populations covered are the Basques (Spain), Oxfordshire populations (England) and Uttar Pradesh (India). No perfect correlation was observed between migration pattern and the continuum of increasing population density, intensity of land use and socio-cultural integration; however, all these factors have been shown to affect mobility and marriage patterns or gene flow.

The next two chapters deal with highly computer-intensive methods for analysis of human migration. Chapter 3 includes classic population genetic models (the island, isolation by distance, stepping-stone, migration matrix and neighbourhood-knowledge models) of migration and population structure. The merits of these models are examined by using basic variables (life-cycle timing, unit of migration, kin structure, population size, geography and distance) identified from populations studied in the previous chapter. In Chapter 4 more complex computer simulation models explore further the consequences of these variables. Chapter 5 focuses attention on the large-scale migrations within continental boundaries and colonization of the vast territories of the New World and Oceania. Several controversies are addressed concerning the origin and spread of Homo sapiens and the variation in genetic diversity observed for different types of classical and molecular markers among different continental populations. It is suggested that migration may explain some existing controversial issues. Chapter 6 draws concluding epilogues from various sections dealt with in the foregoing chapters. There is also a detailed list of references and a satisfactory subject index.

This book has several interesting features. It is written in simple language and avoids extensive mathematical equations. As a result, is likely to be popular among readers from a large number of biological disciplines. For those working with the analysis of human diversity it provides an important introduction to the use of extensive datasets on classical and molecular haplotype markers in the resolution of the microevolutionary debate concerning our species. In addition to the anthropogeneticist this book is highly recommended for all biological research workers who are interested in understanding the role of migration in evolution.