Heterozygosity levels and historical demography
Current level of intraspecific heterozygosity is a relevant parameter to determine the adaptive capacity of a population (or species) (Ørsted, Hoffmann, Sverrisdóttir, Nielsen, & Kristensen, 2019). Since the Balearic shearwater is categorised as Critically Endangered by the IUCN, we could naively expect low heterozygosity levels in the species when compared to other Procellariiformes. However, the fossil record suggests that the Balearic shearwater had a very large population (>30.000 pairs) until the arrival of human settlers in the Balearic Islands (Alcover, J.A., Bover, P., Seguí, 1991), which hunted shearwaters (Ramis, 2018) and introduced invasive mammals that also predated on them (Pinya & Carretero, 2011). In line with Genovart, Oro, Juste, & Bertorelle, (2007) results using mtDNA markers, we observed relatively high genome-wide heterozygosity levels, suggesting that the very recent demographic decline in the species is not yet visible in its genetic diversity.
Regarding the historical demography, our PSMC’ analysis shows an increase in N e in the Balearic shearwater from around 1 Mya to later expand to reach high population sizes, until around 150,000 ya, when it suddenly suffered a sharp decline, resulting in lower N e values maintained until 10,000 years ago. Since current PSMC’ analysis is based on the analysis of a single genome, we could not reliably infer more recent events (Schiffels & Wang, 2020). Pimiento et al., 2017 had shown that the Plio-Pleistocene eustatic variations resulted in a loss of neritic zones as sea level regressed, this may represent a loss of coastal habitat availability, which added to other oceanographic alterations (changes in ocean circulation or productivity) may have been the drivers of great population losses in marine megafauna, including seabirds. In the case of the Balearic shearwater, the PSMC’ analysis shows an abrupt decay ofN e associated with a long period of low sea level during the Penultimate Glacial Period (~194-135 kya)  which may have resulted in an important loss of neritic zones.
Here, we have observed a negative correlation between heterozygosity and body size within the Procellariiformes, where small-bodied species (O. oceanicus , F. grallaria , H. tethys and P. urinatrix ) have higher heterozygosities than large-bodied species (F. glacialis , C. borealis , P. mauretanicus andT. chlororhynchos ). Although controversial, contrasting heterozygosity levels between species with different body-sizes has also been reported in different species including Procellariiformes (Estandia et al. 2021; and references therein). Since body-size also correlates with population size and with other life-history traits, current data does not allow to determine the biological meaning of such correlation effect (Estandía et al., 2021; Mackintosh et al., 2019).
In view of the critical population declines affecting the Balearic shearwater populations, understanding its impacts on current genetic diversity of the species and among colonies will be crucial to assess the conservation status of the Balearic shearwater. Future ongoing research, using a more powerful population genomics approach, will allow to reconstruct the most recent demographic history of the species and to test the fossil-based hypotheses of a recent loss of population due to human colonization of the island, as well as why heterozygosity values have not decayed.