THE ROLE OF GENETIC DRIFT IN THE DIFFERENTIATION OF ICELANDIC AND NORWEGIAN CATTLE.

Abstract

Gene frequency differences between populations can arise from (1) random genetic drift, the accumulation of sampling variance over several generations, (2) selective forces acting differentially on the populations, (3) differential migration from a population with different gene frequencies, and/or (4) distinct mutation histories, resulting in the chance occurrence of a particular allele in one population, but not in others in the absence of migration equalizing such effect. If differential gene flow from outside has not occurred and no unique mutations are present, the problem of deciding between drift and selection as the cause of observed differences still remains difficult. The effects of selective forces can be predicted only if the selection coefficients are known, an exceedingly rare circumstance, but the expected magnitude of difference due to random genetic drift can be predicted on the basis of demographic data which are relatively easy to obtain. However, little is known about the historical demography of most species, man being the major exception. Another species for which both genetic and demographic data are available is domestic cattle. The genetic data on cattle comes mostly from blood typing. Cattle blood typing has a long history (Stormont, 1962), and is routinely done in many countries as the best method of parentage control. The specific demographic (“bosographic”) data we have analyzed come from Iceland and Norway. Comparable genetic data are also available on these breeds, providing a uniquely suitable material for comparing the expected genetic change due to drift with observed gene frequency differences: (1) they have been separated from each other, and probably all other breeds as well, for about 1000 years, a sizable length of time; (2) they are very likely to share a common origin in Norway; (3) there are some historical demographic data available on both Icelandic and Norwegian breeds, and (4) there are good genetic data available on the present day populations. With this material we shall study whether random genetic drift alone can account for the observed differences among Norse cattle breeds.