The Establishment of Irish Haplotypic Diversity
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).
Originally Posted by Septentrion
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