Two populations of all black Monarcha Flycatchers might have independent origins.
In 1942, the German ornithologist Ernst Mayr published his seminal book on speciation: Systematics and the Origin of Species: from the Viewpoint of a Zoologist. In this book (which is still a nice read today), he argues that geographic isolation is the main driver of speciation. To support his claims, he discusses the subspecies of the Monarcha castaneiventris flycatcher that resides on the Solomon Islands (located east of Papua New Guinea). In this blogpost, I will focus on two subspecies: megarhynchus and ugiensis.
Introducing the Islands
The first subspecies (megarhynchus) is found on the large island of Makira. These birds have chestnut bellies and iridescent blue-back upper parts. The second subspecies (ugiensis) is distributed on nearby satellite islands: Ugi and Three Sisters in the north and Santa Anda and Santa Catalina in the southeast. In contrast to the birds from Makira, this subspecies is entirely blue-black. A recent study in Molecular Ecology provides a genomic perspective on the evolution of this species complex.
Hybridization or Independent Origins?
Elizabeth Cooper and Albert Uy used over 70,000 genetic markers (SNPs; Single Nucleotide Polymorphisms) to unravel the origin of the blue-black subspecies. Surprisingly, both melanic populations, which Ernst Mayr considered as a single subspecies, do not cluster together. The birds from the southeastern islands are more closely related to the chestnut-bellied individuals from Makira. This pattern can be explained in two ways: hybridization or independent origins.
To solve this riddle, Cooper and Uy performed isolation with migration analyses. The model with the highest likelihood pointed to migration between the large island of Makira and the satellite islands, but not between the satellite islands themselves. This results weakened a role for hybridization. It thus seems that the melanic populations have independent origins. This hypothesis is supported by the observation that different mutations underlie the black plumage color in each population, as shown in a previous study by Albert Uy and colleagues.
From a taxonomic point of view, the melanic populations should be considered separate subspecies. It turns out Ernst Mayr was wrong here. And it’s not the first time he made a mistake. In his 1963 book Animal Species and Evolution, Mayr stated that “the available evidence contradicts the assumption that hybridization plays a major evolutionary role.”
Cooper, E.A. and J.A.C. Uy, Genomic evidence for convergent evolution of a key trait underlying divergence in island birds. Molecular Ecology, 2017. 26(14): p. 3760-3774.
Mayr, E., Systematics in the origin of species : from the viewpoint of a zoologist. 1942, New York: Harvard University Press.
Mayr, E., Animal species and evolution. 1963, Cambridge: Belknap Press of Harvard University Press. xiv, 797 p.
Uy, J.A.C., et al., Mutations in different pigmentation genes are associated with parallel melanism in island flycatchers. Proceedings of the Royal Society B-Biological Sciences, 2016. 283(1834): p. 20160731.
This paper has been added to the Monarchidae page.