Divergence in the face of high dispersal capabilities is a documented but poorly understood phenomenon. The white-tailed eagle (Haliaeetus albicilla) has a large geographic dispersal capability and should theoretically be able to maintain genetic homogeneity across its dispersal range. However, following analysis of the genomic variation of white-tailed eagles, from both historical and contemporary samples, clear signatures of ancient biogeographic substructure across Europe and the North-East Atlantic is observed. The greatest genomic differentiation was observed between island (Greenland and Iceland) and mainland (Denmark, Norway and Estonia) populations. The two island populations share a common ancestry from a single mainland population, distinct from the other sampled mainland populations, and despite the potential for high connectivity between Iceland and Greenland they are well separated from each other and are characterized by inbreeding and little variation. Temporal differences also highlight a pattern of regional populations persisting despite the potential for admixture. All sampled populations generally showed a decline in effective population size over time, which may have been shaped by four historical events: I) isolation of refugia during the last glacial period 110-115,000 years ago, II) population divergence following the colonization of the deglaciated areas ~10,000 years ago, III) human population expansion, which led to the settlement in Iceland ~1,100 years ago, and IV) human persecution and exposure to toxic pollutants during the last two centuries.

Using whole genome shotgun sequences from 92 white-tailed eagles (Haliaeetus albicilla) sampled from Greenland, Iceland, Norway, Denmark, Estonia, and Turkey between 1885-1950 and after 1990, we investigate the genomic variation within countries over time, and between countries. Clear genetic differentiation is observed between samples from the different countries, with the largest differences between the island and mainland populations, and indications that the island populations share the most recent ancestry with the Norwegian population. We find signs of strong inbreeding in the island populations. Further, temporal differences are observed in some populations, for example, replacement of the Danish gene pool following its population’s extinction in the early 20th century, as well as a change in the genetic diversity of the Icelandic population following a severe bottleneck during the last century, all of which could warrant a further conservation effort in Iceland. More generally, all populations show a decline in effective population size, which may have been shaped by I) distinct refugia during the last glacial period, II) population divergence following the colonization of the deglaciated areas ~10,000 years ago, III) human population expansion and e.g., settlement in Iceland ~1,100 years ago, and IV) human persecution and toxic pollutants during the last two centuries.

Retracing pathways of historical species introductions is fundamental to understanding the factors involved in the successful colonization and spread, centuries after a species’ establishment in an introduced range. Numerous plants are thought to have been introduced to regions outside their native ranges by European voyagers and early colonists making transoceanic journeys; however, records are scare to document this. We use genotyping-by-sequencing and genotype-likelihood methods on the selfing, global weed, Plantago major, collected from 50 populations worldwide to test hypotheses that the plant was brought to new regions during colonial times. We further investigate how patterns in genomic diversity facilitate the success of this global weed. Although genomic differentiation among populations is found to be low, we identify six unique ecotypes showing very little sign of admixture. Three of the most prevalent of these ecotypes present in the native range gave rise to introduced populations in the Americas, Africa, Australia and New Zealand, indicating that more than one successful ecotype colonized and spread. The distribution of ecotypes is found to have links to colonial history, and ecotypes are further found to be restricted by latitude. Dispersal of multiple successful ecotypes and prior adaptation in the native range to latitudinally dependent environmental factors (such as climate) are likely reasons for the success of this prolific, global weed. Genomic signatures can provide new perspectives on the drivers behind the historic introductions and the successful colonization of introduced species in an era of global change.