Mid-Cenozoic insular radiation in the IAA
Temporal and spatial patterns of insular diversification within the IAA
are poorly understood, and have at times been contentious (Jønssonet al. , 2010; Moyle et al. , 2016). A key challenge is that
while geological reconstructions suggest the position of key geological
terranes, it is often unclear if these were above or below water (Caoet al. , 2017). A further issue for biogeographic analysis is that
the contemporary proximity of geological features differs greatly from
paleogeographic configurations. Differing geological, biogeographic and
cultural delineations of key regions have potentially further confounded
analyses and comparison, most importantly New Guinea is not always
considered part of Melanesia (Mayr & Diamond, 2001) and sometimes even
lumped together with Australia as a single landmass of Sahul.
Our analyses emphasise that islands in the east of the IAA, here
considered to be part of the broader Melanesian region, have been a
hotspot of pigeon evolution and speciation since the Oligocene (Fig. S2,
S7). Deep (mid-Cenozoic) insular origins have now been inferred for
several radiations centred on Melanesia, strongly indicating that key
terrane complexes (Vitiaz Arc and Sepik Arc) and/or the proto-Papuan
region have likely been shaping diversity across the IAA since the
mid-Cenozoic (Aggerbeck et al. , 2014; Oliver et al. ,
2018a; Bank et al. , 2021) (Table S8). Wallacea to the west also
appears to be a potentially important source of upstream colonists in
pigeons, although this smaller region shows younger and weaker signals
of lineage accumulation and endemic cladogenesis (Fig 2B,C) and
geological evidence suggest more recent arrival and uplift of key
geological features (Zahirovic et al. , 2016). To the north-west
the Philippine pigeon fauna (Fig. S7B) appears to be relatively young
and derived when compared to that of Melanesia or Wallacea. However, in
light of the extreme geological dynamism of the region and the mobility
of pigeons, we suggest that further discussion of the extent to which
Oligo-Miocene insular diversification of pigeons may be linked to
specific geological features within Melanesia - such as the Caroline,
Sepik or Melanesian Arcs - remains speculative.
While the importance of Melanesian islands in the early diversification
of pigeons is strongly supported, the ultimate source from which this
region was colonised is obscured by the long stem lineage, the apparent
rapid radiation at the base of the Columbiformes and the sparse fossil
record. This contrasts with most other old insular radiations in the
IAA, for which biogeographic analyses typically point clearly to either
Australian or Asian origins (Table S8). One intriguing potential
scenario for pigeons is South America; with westward dispersal to island
arcs, leading to populations on quasi-continental fragments (Melanesia,
Australia/Zealandia and Philippines) and later dispersal to Old World.
This is speculative, but we suggest is as consistent with the current
information on distribution, diversity, phylogeny, dispersal ability,
biogeography and fossil record of pigeons as any other inference. Other
Pacific lineages also show evidence of deep ‘out-of-South America’
origins (Malone, Reynoso, & Buckley, 2017), showing that eastward
migration across the Pacific is possible, if not common.
The rapid early radiation of pigeons around the mid-Cenozoic also
mirrors the estimated timing of initial diversification in two lizard
radiations similarly centred on island arcs in the IAA (Oliver et
al. , 2018a; Slavenko et al. , 2022). This concordance may reflect
the timeframe when island arcs were especially conducive to biotic
colonisation and diversification – for example the putative
Philippine-Caroline-Vitiaz arcs (Zahirovic et al. , 2016). An
alternative or complementary explanation is a shared legacy of global
climatic shifts linked to changes in southern ocean circulation patterns
(Zachos et al. , 2001). Analyses of plant (Nge et al. ,
2020) and reptile (Oliver & Hugall, 2017) lineage diversification
patterns in Australia, patterns of lineage diversity in mammal faunas
globally (Stadler, 2011), and substantial turnover in marine and
terrestrial fossil records (McGowran et al. , 2004; Sun et
al. , 2014) all suggest the early Oligocene as a time of profound biotic
turnover.