In human population genetics, Y-Chromosome haplogroups define the major lineages of direct paternal (male) lines back to a shared common ancestor in Africa. Men in the same haplogroup share a set of differences, or markers, on their Y-Chromosome, which distinguish them from men in other haplogroups. These UEPs, or markers used to define haplogroups, are SNP mutations. Y-Chromosome Haplogroups all form "family trees" or "phylogenies", with both branches or sub-clades diverging from a common haplogroup ancestor, and also with all haplogroups themselves linked into one family tree which traces back ultimately to the most recent shared male line ancestor of all men alive today, called in popular science Y Chromosome Adam.