(sometimes called sister taxa) Clades that are each other’s closest relatives. On a phylogeny, sister groups occur anytime a single ancestral lineage gives rise to two daughter lineages: the daughter lineages are sister groups, and since they arose from the same ancestor at the same time, sister groups are always the same age. Sister groups may differ widely in diversity level: one clade may be comprised of a single species, while its sister group may be comprised of 100 species.