| Summary: | Isolation and adaptation to new environments are important steps for reproductive isolation and consequently speciation. Seabirds have low phenotypic variation along their ranges in the absence of clear geographic or environmental barriers to dispersal. Despite the lacking visible phenotypic differences, the number of taxa for the gentoo penguin ( Pygoscelis papua , Forster 1781) in the Southern Ocean has been under debate for the last decade, ranging from one to six different taxa. Here, we provide several lines of evidence from genomics, ecology, morphological data, and a complete systematic review that supports four distinctive gentoo penguin species, including the description of a new species. We also provide future niche projections for each of these species. Gentoo penguin genomes (n = 64) recover four main lineages: the northern gentoo (from South America), the southern gentoo (Antarctic Peninsula and maritime Antarctica, south of the Antarctic Polar Front, APF), the southeastern gentoo (from Kerguelen Islands), and the eastern gentoo (colonies located at lower latitudes north of the APF). Our analysis of selection across the genome recovered between 42 and 101 genes under selection for each of the four species, demonstrating that the four species are experiencing differing selective pressures that have caused them to diverge adaptively. The function of these genes affects traits that include reproduction, thermoregulation, osmoregulation, feed efficiency, and morphological variation. Morphological data were taken from museum individuals of all lineages, including from South Georgia gentoos, which have previously been considered a distinct taxon. Multivariate morphological comparisons of all pairs of lineages showed that the northern, southern, southeastern, and South Georgia gentoo penguins are morphologically distinct from each other (p < 0.05 for all pairwise comparisons), while the eastern lineage is intermediate in size and overlaps in morphospace with other lineages. This result also suggests ...
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