Complex mosaic of sexual dichromatism and monochromatism in Pacific robins results from both gains and losses of elaborate coloration

Abstract

Pacific robins exhibit one of the most complex range‐wide mosaics of sexual dichromatism and monochromatism. The evolutionary origins of this geographic mosaic remain poorly understood despite long‐standing interest from ornithologists, and its influential role in the development of Ernst Mayr's theories on speciation and the 'Biological Species Concept'. One factor limiting our understanding of the evolution of sexual plumage variation in Pacific robins is a lack of well‐resolved taxon boundaries and phylogenetic relationships in the group. Here, we use primarily historical museum specimens to obtain dense sampling of mtDNA, nuclear DNA, plumage color and morphometrics from all named taxa in the radiation in order to infer taxon boundaries and relationships. We use these data to test hypotheses about colonization history, plumage evolution and reduced island dichromatism. Our data show that the Pacific robin radiation comprises four distinct lineages, which warrant recognition as separate species – the previously recognized Norfolk robin P. multicolor and red‐capped robin P. goodenovii, and two new species we propose to name: ‘Solomon robin’ P. polymorpha Mayr, 1934 for the populations on Solomon and Bougainville Islands, and ‘Mayr's robin’ P. pusilla Peale, 1848 (in honor of Ernst Mayr's detailed work on the southwest Pacific robins) for the populations on Vanuatu, Fiji and Samoa. Our data suggest that the common ancestor of the entire Pacific robin radiation was most likely sexually dichromatic and that the radiation‐wide mosaic of sexual plumage color arose via repeated losses of elaborate plumage in males and gains of elaborate plumage in females on separate islands.

Population genomics of the island thrush elucidates one of earth’s great archipelagic radiations

“…S3). This is in line with dating estimates for other great speciators, which also radiated explosively during the Pleistocene (Moyle et al 2009; Irestedt et al 2013; Jønsson et al 2014; Andersen et al 2013, 2014, 2015; Pedersen et al 2018; Kearns et al 2020). The sequential branching pattern of the island thrush tree (Fig.…”

“…Together with the fact that sexual dichromatism in the island thrush is mostly weak (Peterson 2007), this is consistent with a general pattern that island populations are less sexually dimorphic than their congeneric mainland populations (Omland 1997; Badyaev and Hill 2003). The seemingly random appearance and disappearance of sexual dichromatism may be attributable to repeated founder effects (Omland 1997; Kearns et al 2020).…”

“…This dynamic makes great speciators an alluring model for investigating how lineage expansion and diversification shape global biodiversity patterns 4 (Moyle et al 2009;Jønsson et al 2014;Pepke et al 2019). Molecular phylogenetic studies (Moyle et al 2009;Irestedt et al 2013;Jønsson et al 2014;Andersen et al 2013Andersen et al , 2014Andersen et al , 2015Pedersen et al 2018;Kearns et al 2020) have confirmed that great speciators represent rapid and geographically complex lineage radiations. However, those same attributes, combined with limited genetic sampling, have impeded precise evolutionary reconstruction of these radiations (though see Gwee et al [2020] and Manthey et al [2020]).…”

Pressure for rapid and accurate mate recognition promotes avian‐perceived plumage sexual dichromatism in true thrushes (genus: Turdus)

“…Evidence in favour of this sex dichromatism‐based species recognition hypothesis in birds includes a positive comparative association of greater plumage sexual dichromatism with migratory behaviour and shorter breeding seasons across taxa (Badyaev & Hill, 2003 ), both of which reduce the amount of time available to search and find suitable mates and successfully breed. Additional support for this recognition hypothesis includes a consistent pattern of greater plumage sexual dichromatism and plumage colour elaboration in avian species that reside on mainland continents and have large geographic ranges in comparison with species that do not migrate, reside on islands and have limited breeding ranges (Badyaev & Ghalambor, 1998 ; Dale et al, 2015 ; Doutrelant et al, 2016 ; Figuerola & Green, 2000 ; Friedman et al, 2009 ; Kearns et al, 2020 ; Matysioková et al, 2017 ; Simpson et al, 2015 ; Tobias & Seddon, 2009 ).…”

Pressure for rapid and accurate mate recognition promotes avian-perceived plumage sexual dichromatism in true thrushes (genus:Turdus)

“…Evidence in favour of this recogni on hypothesis for sexual dichroma sm includes a posi ve associa on of greater plumage sexual dichroma sm with migratory behaviour and shorter breeding seasons [9]. Addi onal support for the recogni on hypothesis includes a consistent pattern of greater plumage sexual dichroma sm and plumage colour elabora on in avian species that reside on mainland con nents and have large geographic ranges in comparison to species that do not migrate, reside on islands, and have limited breeding ranges [10,[15][16][17][18][19][20][21][22][23].…”