Introduction
Theoretical predictions (MacArthur & Wilson, 1967) and empirical data
(Crottini et al. , 2012; Economo et al. , 2015) suggest that
islands are rarely a source of diversity for continents. This broad
pattern is thought to be underpinned by varying combinations of the
typically lower diversity of species on islands (Matthews et al. ,
2019), tendency for insular taxa to lose adaptations for dispersal or
competition (Bellemain & Ricklefs, 2008), and putatively higher rates
of extinction in less stable island systems (Whittaker et al. ,
2017). Because biotic communities on larger continents are often more
species-rich they are also predicted to generate higher numbers of
outwards colonists (MacArthur & Wilson, 1967) and act as a strong
ecological filter against upstream colonisation (White et al. ,
2021). However, running counter to these broad predictions, phylogenetic
analyses and ancestral state reconstructions have demonstrated that some
continental lineages have insular origins (upstream colonisation or
reverse colonisation) (Carine et al. , 2004; Filardi & Moyle,
2005; Bocek & Bocak, 2019), especially in the Caribbean region
(Nicholson et al. , 2005; Bellemain & Ricklefs, 2008).
Understanding where, when and how islands contribute diversity to
continents has the potential to provide broader insights into the
factors that shape biotic dispersal and colonisation (Bellemain &
Ricklefs, 2008).
The Indo-Australian Archipelago (IAA) spans from the Sundaland
continental promontory to New Guinea, and is sometime also referred to
as Malesia (as opposed to Melanesia). This region provides a prominent
example of putative upstream colonisation with subsequent radiation in
the global radiation of Oscine songbirds, which comprises of over 5000
extant species, all hypothesised to be descended from ancestors which
occurred in islands of the IAA (Jønsson et al. , 2010; Moyleet al. , 2016). Upstream colonisation of south-east Asia from the
IAA has also been recently inferred in two insect lineages (Letschet al. , 2020; Bank, Cumming, & Bradler, 2021). The original
geographic source of these upstream colonisations has been variably
linked to terranes accreted to the northern edge of the Australian
continent, often referred to as ‘the proto-Papuan Archipelago’ (Jønssonet al. , 2010), or the islands of “Wallacea” (Moyle et
al. , 2016) (see more discussion of the geology of these regions below).
These results emphasise the potential significance of the islands of
eastern IAA as a source for diversity on nearby continents. However, the
dynamics of insular diversification and island-continent dispersal
across this region remains poorly understood.
Pigeons and doves (Columbiformes) are a moderately diverse global
radiation of fruit and seed-eating birds (~350 nominal
species), that also show a strong signature of insular diversification
(Cibois et al. , 2014, 2017) and upstream colonisation (Lapiedraet al. , 2021). The islands of the IAA, and especially New Guinea
and surrounds, are a world centre of pigeon ecological and lineage
diversity, and were even more diverse pre-human settlement across the
Pacific (Steadman, 2006; Steadman & Takano, 2020). Conversely, the
oldest known Columbiform fossils are from early Miocene deposits in
Australia (Worthy 2012) and New Zealand (Worthy et al. 2009), and on
this basis it has been suggested that pigeons may have originated in
Australia, or nearby islands (Low, 2014). To better understand the
temporal and spatial dynamics of pigeon dispersal and diversification
between islands and continents in the IAA, we assembled a supermatrix
dated phylogeny for the pigeons, and from this we estimate, 1) the
timing and geographic foci of insular pigeon diversification, with
particular focus on the potential role of island arc terranes in the IAA
in the early radiation of the group, and 2) estimate the relative
frequency and direction of dispersal events between major island groups,
and nearby continents.