Population divergence in humans
Differentiation among populations of humans is a subject of natural interest and significant progress has been made in recent years in understanding the spatial and temporal course of human microevolution since humans left their place of origin in east Africa approximately 150 000 years ago.
Fig. 8. Geographic distribution of the four catgories of human ethnicity.
Recently, a large number of DNA samples have been collected from human populations around the world. The analysis of these samples, combined with linguistic, fossil, and archaeological evidence, has allowed the history of the spread of humans to be reconstructed. This reconstruction can be viewed interactively at the Journey of Mankind: The Peopling of the World: http://www.bradshawfoundation.com/journey/
Genetic variation in human populations
Because human populations have diverged recently relative to evolutionary time scales, there are relatively few genetic differences among human populations. However, there are several noteworthy examples where populations differ in allele frequencies for genes that have geographically variable selective pressures. These include variation in the sickle-cell trait with malaria incidence, ABO blood types and endemic disease, skin pigmentation and vitamin D deficiency risk, and lactose tolerance associated with cattle farming and pastoralism.
In other cases, genetic variation is essentially random and not related to natural selection. Single tandem repeat or STR (a.k.a. microsatellite) mutations occur at a high rate and therefore accumulate on a relatively short time scale. When these mutations occur on the Y chromosome or in the DNA of mitochondria, they are particularly valuable as neutral genetic markers for historical reconstruction. The system is relatively simple because Y chromosomes are inherited exclusively through fathers and mitochondria are inherited exclusively through mothers and therefore neither undergoes genetic recombination as commonly occurs in autosomal chromosomes. Therefore, each mitochondrion or Y chromosome provides a full set of markers that are carried together and can be analyzed statistically. This makes them useful in criminal forensics, genetic genealogy, and in studying human population dispersal.
One easily observable type of human genetic variation is differences in allele frequencies of "taster" genes in human populations. PTC (phenylthiocarbamide) is a harmless substance which some individuals find very bitter, and others do not taste at all. For a small fraction of individuals PTC has a taste other than bitter. Saldanha (1958) and Saldanha and Beçak (1959) showed that PTC alleles varied greatly among human populations. Relatively large differences were observed between participants from east Asia and participants in other parts of the world. Historically, PTC tasting has been considered to be a neutral trait. However, there has also been speculation as to whether there is a selective advantage to being a taster and whether one's taster phenotype influences food preferences, particularly foods from the plant family Brassicaceae (which includes broccoli and Brussels sprouts). Because it is an easily measured phenotype, we have been collecting PTC taster and ethnicity data in the BSCI 111b course for several years and will analyze them as an exercise.
PTC Taste Test information
References
Saldanha, P.H. 1958. Taste thresholds for phenylthiourea among Japanese. Annals of Human Genetics 22:380-384.
Saldanha, P.H. and W. Beçak. 1959. Taste thresholds for phenylthiourea among Ashkenazic Jews. Science 129(3342):150-151. http://dx.doi.org/10.1126/science.129.3342.150
Wooding, S. 2006. Phenylthiocarbamide: A 75-year adventure in genetics and natural selection. Genetics 172(4):2015-2023. http://www.genetics.org/cgi/content/long/172/4/2015
Interesting optional reading:
Tepper, B.J. 2008. Nutritional implications of genetic taste variation: the role of PROP sensitivity and other taste phenotypes. Annual Review of Nutrition 28:367-388. http://dx.doi.org/10.1146/annurev.nutr.28.061807.155458
In particular, note the possible relationship between bitter taste receptors and alcoholism.
