Two nectar-feeding birds from Hawai’i, the kioea (brown-streaked, in middle) and an o’o species (lower left), looked so much like nectar specialists from the western Pacific (two species on right) that taxonomists put them all in the same honeyeater family, the Meliphagidae. All the Hawaiian birds are unfortunately extinct, but DNA evidence shows that their resemblance resulted from convergent evolution, because the Hawaiian birds were actually much closer to birds from the Americas, such as Bohemian Waxwings (upper left).
Image: John Anderton.
Every once in awhile, I will read a scientific paper that astonishes and delights me so much that I can hardly wait to tell you all about it. Such is the situation with a newly published paper about the Hawai’ian Honeyeaters. In short, due to the remarkable power of convergent evolution, Hawai’ian Honeyeaters have thoroughly deceived taxonomists and ornithologists as to their true origin and identity for more than 200 years.
Like an artist molding a lump of clay into a specific shape to meet her demands, so nature operates through natural selection to take advantage of naturally-occurring genetic variation throughout the generations to alter the physiology, morphology and behavior of living beings to meet ecological demands. Bold mutations can result in dramatic changes, while subtle genetic variations result in smaller, sometimes hidden, shifts — but only if these changes are adaptive, only if they provide the owner of those genes with a special “edge” that allows them to attract more and fitter mates, and to produce more and fitter offspring who then pass on these adaptive genes to future generations.
“It’s like we had this animal we always thought was a dog, and it’s turned out to be a mongoose,” reports one of my scientific colleagues, Robert Fleischer, of the Smithsonian Institution in Washington, D.C. Fleischer was the lead author of this study.
The Hawai’ian birds’ striking resemblance to the Honeyeaters of New Guinea and Australia demonstrates how evolution within different lineages can cause dissimilar species to converge on similar body plans and forms to meet the demands of similar “jobs”.
Figure 1. Tongues of Meliphagids, Moho, and Relatives of Moho and Chaetoptila
Shown are illustrations of tongues of meliphagids (A-D), two species of Moho (E and F), and relatives of Moho and Chaetoptila on the basis of our results (G and H). This suggests convergence of the tongues of Moho from ancestral tongues like (G) and (H) to tongues like (A)-(D). The following are shown: (A), Meliphaga fasciogularis; (B) Myzomela sclateri; (C), Anthornis melanura; (D), Philemon buceroides; (E), Moho nobilis; (F), Moho braccatus; (G), Dulus dominicus; and (H), Phainopepla nitens. Illustrations (A), (C), and (E) are from Dorst ; (B) is from Scharnke ; (D) and (H) are from Beecher ; (F) is from Gadow ; and (G) is from Gardner . Tongue illustrations are reproduced with permission from the British Ornithologists’ Union, American Ornithologists’ Union, Société Ornithologique de France, and the Journal of Ornithology.
The Honeyeaters are — were — all classified into the same taxonomic family, Meliphagidae, based on several shared characters. First, they specialize in consuming a sugary diet of nectar so they have long tubular or semi-tubular tongues with a brush on the tip, which evolved for gathering nectar from flowers (figure 1); their beaks curve downwards (figure 2); they have an operculum over their nares to protect their nasal cavity from pollen; and all these species function in ecologically important roles as pollinators and as dispersers of seeds. Other characters that these birds share are long legs and strong, perching feet; and remarkable similarities in plumage color and pattern (figure 2), behavior — and even in song.
Figure 2. Illustrations of Three of the Five Species of Hawaiian ”Honeyeaters” and Three Representative Meliphagid Honeyeaters
The three Hawaiian taxa represent the three primary morphological types found in Hawaiian ”honeyeaters” (Mohoidae: [A], Moho nobilis; [C], Chaetoptila angustipluma; and [E], Moho braccatus). The three meliphagids include one from New Zealand ([B], Prosthemadera novaeseelandiae), one from Australia ([D], Anthochaera carunculata), and one from Samoa ([F], Gymnomyza samoensis). Paintings are by John Anderton and are used by the authors with permission. [larger view].
“The similarities between these two groups of nectar-feeding birds in bill and tongue structure, plumage and behavior result not from relatedness, but from the process of convergent evolution — the evolution of similar traits in distantly related taxa because of common selective pressures,” Fleischer points out.
To do this work, the team sequenced and pieced together as much as 1923 basepairs (bp) of nuclear (chromosomal) DNA and 717 bp of mitochondrial DNA from multiple specimens of Hawai’ian “Honeyeaters” in the genera, Moho and Chaetoptila. When they analyzed their data alongside DNA data for other Meliphagids, they discovered that the Hawai’ian birds formed a cohesive group, but astonishingly, the Hawai’ian group was nowhere near the Honeyeaters of New Guinea and Australia (figure 3);
Figure 3. Phylogeny Reconstructions for Hawaiian Mohoids and Outgroups with Different Data Partitions
(A) Section of a ML tree constructed from up to 717 nucleotide sites of mtDNA sequence for the five species of Mohoidae and 43 additional songbird taxa. The tree shows strong support for monophyly of the Mohoidae and also supports placement of the Mohoidae within the waxwing and silky flycatcher clade and Passerida. Relationships among species within the Mohoidae are not well resolved. Bayesian posterior probabilities and ML bootstrap support values are provided at relevant nodes.
(B) Schematic of a phylogenetic tree constructed with Bayesian inference from 190 sequences of up to 1544 bp of the RAG-1 gene [9, 19]. Taxa are merged into triangles indicating major, supported clades that generally match the topology found by Barker et al.  with a larger data set. Sequences from Moho nobilis and Moho bishopi fall within the red clade, rather than, as expected, within the basal honeyeater clade (dark blue); the expanded clade shown at upper right reveals the position of these taxa within the clade containing waxwings, silky flycatchers, and the palm chat. This tree includes only the two Moho species for which more than 1000 bp of RAG-1 sequence was obtainable. Shorter RAG-1 sequences of Chaetoptila angustipluma and Moho apicalis are nearly identical to these sequences of Moho in sections of overlap, and thus support these results.
(C) Maximum likelihood phylogram constructed from analysis of up to 421 nucleotide sites of b-fibrinogen introns 5 and 7 combined. At nodes are Bayesian posterior probabilities and ML bootstrap values (100 repetitions). The sequence data set for this tree was limited to outgroup species for which sequences of both genes were available, but analyses with considerably larger numbers of taxa (115 and 189 sequences) for each gene separately produced the same results. [larger view].
Surprisingly, the Hawai’ian “Honeyeaters'” closest mainland relatives are the silky flycatchers (the waxwings and Phainopepla) of North America! This finding is in agreement with a controversial prediction made by the famous ornithologist and island biogeographer, Ernst Mayr; that all or nearly all endemic Hawai’ian birds were of North American origin.
Fleischer and his colleagues also used their DNA data to estimate that the Moho and the silky flycatchers diverged between roughly 14 and 17 million years ago. This is a significant period of time for evolution to occur, so the strong resemblance between the Meliphagid Honeyeaters of New Guinea and Australia with those “Honeyeaters” of Hawai’i is plausible, even while it remains remarkable. Further, their DNA-based estimate of the divergence time suggests that the “Honeyeaters” are the oldest lineage found on the Hawai’ian island chain — a time that also coincides nicely with the estimated arrival of the first bird-pollinated plants.
As a result of the deep divergence between the two “Honeyeater” groups, Fleischer and his colleagues proposed that the five Hawai’ian Honeyeaters be placed into their own taxonomic group. They also proposed that this group be elevated to the status of family — a very rare occurrence in modern times. They christened the Hawai’ian “Honeyeaters” the Mohoidae — “Moho” in honor of the most species-rich genus, Moho, and “-idae”, a Latin suffix that denotes a taxonomic family.
“This was something that we were not expecting at all,” remarked a surprised Fleischer. “It’s a great example of how much we can learn about systematics and evolution by applying new technologies like ancient DNA analysis to old museum specimens.”
Why do I think this paper is amazing? First, this work brilliantly reveals the astonishing power of convergent evolution, which made disparate bird groups look, sound and behave so similarly that no one ever had any reason to question their taxonomic placement as each other’s closest relatives in the avian tree of life. Indeed, without the DNA data, no one would ever have been able to recognize or correctly identify the closest relatives of the Mohoidae at all. Second, these data show that the avian colonization of the Hawai’ian islands occurred from the mainland of North America, as Mayr hypothezised — not from the islands of the western Pacific, as many taxonomists erroneously thought, based on the deceptively similar AustraloPapuan Honeyeaters.
According to Fleischer, all five Mohoidae were medium-sized songbirds with slender, slightly downward curved bills with unique scroll-edged and fringed tongues, making them very specialized nectar-feeding birds. They inhabited undisturbed forests on most of the larger Hawaiian islands.
Tragically, all of the members of this newly described family are extinct. The Kioea disappeared first, in 1850, and then the four O’os soon followed that species into oblivion until only the lonely Kauai O’o, Moho braccatus, remained in the Alakai Swamp [short film of this species]. That species was last seen in 1987. All five species of Mohoidae disappeared due to human factors: destruction of their habitats, hunting for feather art, the introduction of predators such as rats, mongoose, pigs and domestic cats, and the introduction of avian diseases, particularly malaria, to the islands.
Thus, even before it was named, the newly christened Mohoidae became the only bird family to become extinct within the last hundred years.
R Fleischer, H James, S Olson (2008). Convergent Evolution of Hawaiian and Australo-Pacific Honeyeaters from Distant Songbird Ancestors. Current Biology DOI: 10.1016/j.cub.2008.10.051.