The perils of inferring hybridisation from population structure
patterns
Our genome-wide data revealed a population structure closely matching
the two nominal species, Hogna insularum, and H.
maderiana , but with a third component, formed by individuals from Porto
Santo, first identified as H. insularum, of ambiguous
interpretation. This pattern was consistently observed across both
non-model-based (DAPC) and model-based clustering methods
(structure and fineradstructure) (Figure 2). DAPC and
fineradstructure analyses identified three genetic clusters,
with slightly higher co-ancestry within each cluster than among them,
suggesting a certain degree of genetic similarity across the three
genetic clusters. While in general agreement, the structure
analysis further supported this genetic continuity. At K=3 (as suggested
by Evanno’s criterion), we observed varying levels of genetic similarity
among the groups with similar dominant components across them, but
different secondary components for those individuals outside Porto
Santo. At K=2, individuals of H. insularum from Porto Santo
exhibited an equal ancestry proportion of 50% from each species,
suggesting ongoing hybridisation between them. At this point, it is
essential to note that determining the correct number of genetic
clusters using Evanno and Pritchard methods in STRUCTURE is challenging,
as these approaches can either underestimate or overestimate the number
of clusters, particularly in populations with complex evolutionary
histories (Janes et al.,
2017). In the case of the Hogna species complex, the
identification of K=2 suggests that the H. insularum lineage from
Porto Santo may result from continued hybridisation between H.
insularum and H. maderiana in Porto Santo. Surprisingly, none of
the samples collected from Porto Santo belonged to the H.
insularum Madeira, Desertas, and Bugio (MDB) (Figure 2), which
challenges the putative scenario of a potential hybrid zone between the
two nominal species co-occurring in Porto Santo.