How climate change drives bird speciation on different continents.
Climate change is a hot topic these days (no pun intended). Many researchers are interested in the impact of current alterations in climate on bird populations. However, the evolutionary history of several bird species has been shaped by past climate changes, specifically during the Plio-Pleistocene period (between roughly 3 and 1 million years ago). Today, I will discuss two papers that focused on this period.
First, let’s travel to South America where a small hummingbird flies frantically through the upland forests of Amazonia. This species, the Straight-billed Hermit (Phaethornis bourcieri) is currently divided into two subspecies: boucieri and major. But the validity of this taxonomic treatment is not fully elucidated yet. Therefore, Lucas E. Araujo-Silva and his colleagues set out unravel the evolutionary history of this small bird.
The construction of an evolutionary tree for this species revealed several groups. The subspecies major clustered with two other species, the Needle-billed Hermit (P. philippii) and the Koepcke’s Hermit (P. koepckeae), and should thus be regarded as separate species. The other subspecies (boucieri) is divided into several cryptic lineages that originated during the Pleistocene. These lineages might also represent different species as there was no evidence of gene flow between them.
The split between both subspecies occurred at the end of the Pliocene (about 4 million years ago), coinciding with the time when the Amazon river originated. Divergence within the groups happened around the transition from Pliocene to Pleistocene (about 3 to 1 million years ago) when the Tapajos River formed and the landscape between the Negro and the Branco rivers changed. These findings show the importance of Amazonian rivers in the diversification of bird species.
Now, let’s cross the Atlantic Ocean to have a look on the African continent. Here, Jerry Huntley and Gary Voelker studied the evolutionary history of Crombecs (genus Sylvietta), a group of nine species that are spread across tropical and arid regions of Africa.
The genetic analysis of this group revealed a southern origin (South Africa and Zambia) with a split into two groups some 6 million years ago. The first group consists of four species that are adapted to arid habitats, whereas the second group is comprised of three forest species and two arid-adapted species. Similar to the Hummingbirds discussed above climatic changes during the Pliocene and Pleistocene probably instigated the diversification of these species.
A Dry Corridor
Around 2.3 million years ago the retraction of tropical forests from East Africa resulted in a corridor that connected arid regions in the north and the south. This might have driven the expansion and evolution of the arid-adapted species of the first group, such as Red-capped Crombec (S. ruficapilla) and Somali Crombec (S. isabellina).
The two arid-adapted species from the second group, Philippa’s Crombec (S. philippae) and Northern Crombec (S. brachyura), probably shifted from forest to arid habitats during the contraction of tropical forests. These results show how the fragmentation of Plio-Pleistocene forest in Africa impacted the evolution of birds.
Different, but the same
These two studies indicate how the same climatic event (the Pliocene-Pleistocene transition) led to different processes – origin of rivers vs. expansion of arid regions – that ultimately resulted in a similar outcome: the diversification of bird species.
Araújo‐Silva, LE, LS Miranda, L Carneiro, A Aleixo (2017) Phylogeography and diversification of an Amazonian understory hummingbird: paraphyly and evidence for widespread cryptic speciation in the Plio‐Pleistocene. Ibis, 159:778-791. http://onlinelibrary.wiley.com/doi/10.1111/ibi.12500/abstract
Huntley, JW, G Voelker (2017) A tale of the nearly tail‐less: the effects of Plio‐Pleistocene climate change on the diversification of the African avian genus Sylvietta. Zoologica Scripta, 46:523-535. http://onlinelibrary.wiley.com/doi/10.1111/zsc.12240/full