The second component of variation (Fig. 4.6B-C) is unique in that it explains a large amount of variation present in both ancient and modern individuals. This corresponds to the primary split seen in fineSTRUCTURE analysis by Byrne et al. (submitted), which segregates Ireland and Britain into two distinct genetic islands, capturing
what is defined as an Anglo-Celtic cline. Western Ireland and southeastern Britain form the two extremes of this component, with the Scottish population bridging the gap between the two clusters. Strikingly, ancient samples also separate out along this axis, with Irish individuals from both the Early Bronze Age and Iron Age periods falling further towards modern Irish variation than their British and continental counterparts (Fig. 4.6B-C).
Irish Iron Age samples extend the entire range of Irish variation on PC2, suggesting substantial continuity with the modern population. Irish Early Bronze Age samples show a more constricted distribution closer to the center of the plot, but still exhibit a systemic shift towards Irish Iron Age and modern populations, particularly those from individualised burials. The most parsimonious explanation for such observations is direct continuity between the Chalcolithic/Early Bronze Age and modern period in Ireland, with much of the haplotypic variation explained by PC2 forming in the intervening millennia, in a similar manner as suggested for Wales in PC3. While migration may be partially responsible for this structure, it is worth noting that the Irish Iron Age and modern population typically extends away, rather than towards, any potential external sources of variation in the dataset, including a contemporary Iron Age population from Britain, the most likely source of migration into Ireland between the Bronze Age and Early Christian periods. However, several exceptional Irish Iron Age samples exist, returned to in later sections.
The homogenisation of British population structure through admixture
In contrast to the gentle gradient of ancient Irish variation, British and continental individuals show a more punctuated distribution along PC2 (Fig. 4.6B-C), forming two clear clusters at both ends of modern British variation.
Anglo-Saxons fall with southeastern English variation in this and all other PCs considered, alongside a Nordic Iron Age sample, reflecting the large genetic contribution of Germanic migrations to this part of the island (Leslie et al. 2015; Schiffels et al. 2016). Iron Age Britons comprise another tight grouping at the opposite end of British variation, emphasising the admixed nature of the modern population (Leslie et al. 2015; Martiniano et al. 2016; Schiffels et al. 2016). Early snapshots of continental introgression events may be represented by two samples that fall midway between the two groups, one from an Anglo-Saxon context (O3), which was reported as admixed in the original study (Schiffels et al. 2016), and the second from a Roman British population (6DT23), another member of which was demonstrated to be of likely Middle Eastern origin (Martiniano et al. 2016). Notably, no Irish Iron Age samples are seen to fall into this region of the PC space.
The compression of Iron Age British haplotypic variation close to the zero coordinate, relative to that of Ireland, suggests that PC2 may not effectively explain the majority of diversity present within this group, possibly due to their lack of representation within the larger admixed modern British cohort. In this respect, PC2 is perhaps best considered as explaining the distribution of Irish-related haplotypic variation in both modern and ancient individuals, which acts as somewhat of an imperfect proxy for Celtic ancestry in the neighbouring island of Britain, counterbalancing the Anglo-Saxon input. We caution that such a phenomenon may cause similar placement of individuals for unrelated demographic reasons.
For example, the placement of Northern Irish and Scottish individuals between the two islands is proposed to be the result of numerous migrations in both directions, including the Gaelicisation of Scotland circa 600 AD and the later Ulster plantations (Byrne et al. submitted). It is notable that no PC segregates Scotland from the rest of the dataset, suggesting the modern population has been mainly borne from admixture, rather than isolation, the reverse of what is proposed for Wales. Indeed, the more muted and systematic shift towards Irish variation of Welsh populations, whose diversity is better captured in PC3 and PC6, may represent more ancient shared Celtic ancestry between the groups. The tight clustering of three German Late Neolithic and Bronze Age individuals at the edge of ancient Irish variation, alongside the Iron Age British population, could also be due to a similar effect of older shared ancestry. Such an interpretation may find some temporal grounding in the differential placement of a Nordic Late Neolithic individual further towards the Germanic extreme of the plot.
Originally Posted by Tooting Carmen
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