Haplogroup R --- Description from the International Society of Genetic Genealogy's website

Y-DNA haplogroup R is mainly represented in two lineages. Lineage R1a is thought to have originated in the Eurasian Steppes north of the Black and Caspian Seas. It is associated with the Kurgan culture, known for the domestication of the horse (approximately 3000 B.C.E.). This lineage is currently found in central and western Asia, India, and in Slavic populations of Eastern Europe. A well-known individual of the R1a lineage is Somerled founder of Clan Donald. Lineage R1b originated prior to the end of the last ice age where it was concentrated in refugia in southern Europe and Iberia and is the most common in European populations. It is especially common in the west of Ireland where it approaches 100% of the population. This haplogroup contains the Atlantic modal STR haplotype.