Thursday 28 March 2019

New Bird Shows How Species are Maintained

Newly-identified Cream-eyed bulbul, previously thought to be a variant of the Cream-vented bulbul.

Credit: Subir Shakya, Louisiana State University
Department of Biological Sciences
New Bird Species Discovered by LSU Researchers

A very nice example of how speciation between closely-related species occupying the same or overlapping ranges is maintained, is illustrated by the discovery of a new species of bird hiding in plain sight in Borneo.

Over most of its range, the Cream-vented bulbul, Pycnonotus simplex, a rather drab olive-brown bird, has white eyes but on the island of Borneo it was thought to have occurred in two forms; the locally more common red-eyed form and the 'normal' white-eyed form.

Now, after painstaking analysis of the mitochondrial DNA (mtDNA) of both red and white-eyed bulbuls and other related species, a team of researchers from Louisiana State University (LSU) led by PhD student, Subir Shakya, has shown that the white-eyed form on Borneo is a newly-identified species, genetically distinct from the red-eyed Borneo form and the white-eyed form found elsewhere. They have named this new species, Pycnonotus pseudosimplex.

Their paper was published a few days ago in the Bulletin of the British Ornithologists’ Club.

Cream-vented Bulbul Pycnonotus simplex of Borneo was previously considered to be polymorphic in iris colour, having either red or white (creamy-yellow) irides. Mitochondrial DNA sequence comparisons, however, indicate that white- and red-eyed Bornean individuals are not closely related to one another. Instead, white-eyed birds are sister to Ashy-fronted Bulbul P. cinereifrons of Palawan Island, in the south-west Philippines, and red-eyed birds are sister to white-eyed P. simplex of the Thai-Malay Peninsula. Consequently, we elect to treat the white-eyed Bornean population as a distinct, previously overlooked species. In respect to plumage, white- and red-eyed individuals are almost identical, varying only slightly in the amount of yellow coloration in their feathers. The two taxa are sympatric at some localities, but white-eyed individuals are rarer and more consistently associated with mature forest than red-eyed birds.

In fact, what the team have named as the Cream-eyed bulbul is not closely related to Pycnonotus simplex at all, but is a sister species to the Ashy-fronted Bulbul, P. cinereifrons, of Palawan Island, in the south-west Philippines - an island from which Pycnonotus simplex is absent.

What we seem to have here is a mechanism for preventing hybridization which acts at the prezygotic level - in other words to prevent mating so that a zygote never forms, as opposed to the post-zygotic level which prevents an embryo forming to produce a viable hybrid. Barriers to hybridization at the prezygotic level are common in birds which could theoretically hybridize but don't due to various mechanism. These mechanisms are usually plumage and courtship displays couples with female sex selection, courtship rituals and songs or mating calls. These are all things that make birds such a diverse order often with striking plumage and song.

In this group of bulbuls, on the island of Borneo there are four different species, all very much alike in their overall plumage and in their song, so something else is acting as a barrier to hybridization. The four species are:
  • Cream-vented bulbul, P. simplex (red-eyed form).
  • Cream-eyed bulbul, P. pseudosimplex (cream eyes).
  • Red-eyed bulbul, P. brunneus (orange or two-toned iris (red at the edge and orange towards the centre)).
  • Spectacled bulbul, P. erythropthalmos (dark red iris surrounded by an eye-ring of yellow skin).

This is highly suggestive that eye colour plays a major part on maintaining speciation and is a fascinating and interesting example of what an evolutionary process can produce. In this case there was a diversity which was not even apparent to us until DNA analysis showed there to be a species of which we were unaware. To the four Bornean species of bulbul the difference between them was obvious.

It is impossible to see this rationally as the work of an intelligence. Even if there was a purpose in creating four different species of bulbul on one island, all with very similar plumage and song, why create them so they can interbreed and then have to create a system such as subtly different eye colours to prevent it? Invoking some explained and unexplainable mysterious purpose behind such an apparently stupid design renders ID utterly useless as an explanation.

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