Migration and Colonization in Human Microevolution (review)

Migration and Colonization in Human Microevolution, by Alan G. Fix. Cambridge Series in Biological and Evolutionary Anthropology 24. Cambridge, UK: Cambridge University Press, 1999. 236 pp.

Migration is one of the main forces shaping the genetic diversity of human populations. Thus, understanding the causes, patterns, and effects of migrations is fundamental for interpreting the evolutionary history of our species. However, when trying to explain the spatial distribution of gene frequencies in human populations, disentangling the effects of history versus geography remains a difficult challenge. Interpreting the trees so commonly used in the anthropological literature as a sequence of historical branching events is not based on any realistic assumption, particularly in human populations, where migration has had an important evolutionary role. Satisfactory approaches have not yet been found to analyze the more realistic "network" pattern of human evolution, in part because of our lack of knowledge of the migration events in our past. With regard to the present, there are still many gaps in our understanding of how different migration models affect the spatial pattern of gene frequencies in human populations. In his latest book, Migration and Colonization in Human Microevolution, Alan G. Fix aims to fill some of these gaps, exploring the effects of migration primarily from a microevolutionary perspective. Although the central point of the book is migration in a restricted spatial and temporal scale, in the final chapter attention shifts to the role of migration in the long-term evolutionary history of our species.

The brief first chapter ("The Study of Migration") introduces different approaches to studying migration, with varying emphasis depending on the discipline (biological or social sciences), finally focusing on the integrative perspective offered by anthropological genetics. The difficulty in finding a migration model that is universally applicable to human populations is emphasized in the second chapter ("The Anthropology of Human Migration"). No single model can be invoked to understand the role of migration throughout our evolutionary history. Differences in population density, mobility, subsistence economy, and population structure determine widely variable migration patterns among human populations. The author describes the migration patterns in ten current human societies, which serve as examples for use in subsequent chapters of the book. These populations show a wide range of demographic, cultural, and ecological characteristics. Throughout the book, special attention has been devoted to the Semai, an Austroasiatic population living in Central Malaysia, where the author has carried out extensive fieldwork.

In the third chapter ("Population Genetics Models and Human Migration"), the author introduces the classic genetic models that have been used to understand migration patterns in human populations. They include the island model, the isolation-by-distance model, the stepping-stone model, the migration matrix model, and the neighborhood knowledge model. The methods are not presented in much statistical detail; instead, attention has been focused on the problems derived from the simplifying and unrealistic assumptions of the different theoretical approaches, and the consequences in terms of their applicability to actual human populations. Some examples are shown using data on the populations previously described in chapter 2. Emphasis has also been placed on important variables usually not considered in the classic genetic models, and how they can have an important impact on the expectations of the theoretical models. These variables include timing and units of migration, population size and migration rate, and spatial and geographical aspects of migration, A particularly detailed discussion focuses on the structure of migration (random migration vs. kin-structured migration), which can have a profound effect in terms of the genetic variation observed among groups interchanging migrants. Arguably, there are major differences with respect to the importance of kin-structured migration among human populations. However, it provides an interesting example of the importance of a careful analysis of the pattern of migration in order to understand the spatial variation in gene frequencies observed in human populations.

After introducing in chapter 3 the classic theoretical models of migration and the problems associated with their unrealistic assumptions, the author focuses the fourth...