The phylogenetic tree among harps seals can be used to construct a time-linearized model in which all branch "tips" [left] come out contemporaneous, as expected for seals collected in the modern era. The model requires a statistically constant "Molecular Clock" of molecular evolution calibrated against known geological or biological events. The bars around each internal branch indicate its 95% confidence interval.

    Time linearization permits the genetic evidence to be fitted to temporal geological evidence, in this case temperature variation over the last 200,000 years, derived from the Vostok Ice Core. Cold glacial periods (blue) sequester water in the ice, and lower sea level, such that coastal populations of species such as seals remain connected with each other. Warming inter-glacials (red) melt the ice and raise sea levels, such that continuous coastal populations become separated as islands. Observed that the Sangamon inter-glacial warming [red peak] ca 120 ~ 130 KYBP coincides with the separation of the younger western lineages (ABC) from the older eastern lineages (DEF). Western populations may have migrated from the East along the coast of Greenland, and become isolated from Eastern populations when seas levels fell gain.. None of the observed harp seal genetic divergences occurred in the most recent inter-glacial, the Holocene climatic optimum. The genetic data in combination with the geological evidence suggest that population structure in Harp Seals is old, dating back as much as 200 KYBP.