Only a few days after writing about Darwin’s famous Galápagos finches and how the Galápagos daisy had parallelled their evolutionary radiation, another group of island birds provides an even more spectacular example of adaptive radiation.
This time the setting is the Hawaiian archipelago, and the birds are the Hawaiian honeycreepers — an extraordinary collection of finches that evolved an assortment of beaks, diets and ways of life from a shared ancestral population.
More than 60 species of Hawaiian honeycreeper are known, ranging from thick-billed seed-eaters and nectar-feeders to birds with long, curved beaks for probing flowers or extracting insects from beneath bark. Some became so anatomically different that, before their evolutionary relationships were understood, naturalists found it difficult to believe that they could all be related.
But related they are. According to research published in the Proceedings of the National Academy of Sciences (PNAS) by Michael G. Campana and colleagues at the Smithsonian’s National Zoo and Conservation Biology Institute and National Museum of Natural History, the honeycreepers form a single adaptive radiation whose history includes not only repeated branching into new species but also genetic exchange between some of those emerging lineages.
The researchers constructed the first comprehensive phylogenomic reconstruction to include every honeycreeper species known to have survived until European contact with the Hawaiian Islands. Their analysis covered all 17 surviving species, 18 species that have since become extinct and two additional species known only from fossil bones.
This was possible because modern genomic techniques allowed the team to recover DNA not only from living birds but also from historic museum skins and tiny fragments of fossil bone. Far from being dusty repositories of obsolete curiosities, natural-history collections have become genetic archives from which the evolutionary history of extinct organisms can increasingly be reconstructed.
The resulting family tree shows that most honeycreeper lineages arose during a particularly rapid burst of diversification between about 3.5 million and 2.5 million years ago, around the time the island of Oʻahu emerged through volcanic activity. Each new island created unoccupied habitats and ecological opportunities. Colonising populations became geographically isolated, adapted to different food sources and habitats, and gradually diverged into distinct species.
However, their history was not a simple succession of branches separating permanently from one another. The paper describes the radiation as “reticulated” because some branches subsequently exchanged genes through hybridisation and introgression. The researchers found evidence of genetic mixing between the now-extinct ʻōʻū and Lānaʻi hookbill, while related ʻamakihi populations occupying different islands continue to exchange genetic material. Evolutionary history is therefore sometimes better represented as a branching network than as a perfectly bifurcating tree.
For creationists, this presents several familiar difficulties. There is no evidence that the different honeycreepers were independently created for their respective ecological roles. Instead, their genomes record common ancestry, geographic isolation, descent with modification, adaptation and occasional hybridisation — precisely the processes predicted by evolutionary biology.
Nor would it help to protest that the honeycreepers are “still finches”. Evolution does not predict that descendants cease to belong to their ancestral group: birds remain dinosaurs, humans remain apes and mammals, and Hawaiian honeycreepers remain finches. What changed was the diversity within that lineage as inherited variation, mutation, recombination, natural selection, isolation and gene flow generated more than 60 specialised species from one ancestral population.
Sadly, the research also documents evolution in reverse: not the formation of biological diversity but its destruction. Only 17 honeycreeper species survive, and many of those face extinction from habitat loss, introduced predators, avian malaria and climate change. The reconstructed family tree therefore reveals both the remarkable creative power of evolutionary processes over millions of years and how rapidly human activity can prune away the diversity they produced.
How Volcanoes Built an Evolutionary Laboratory — and Why the Honeycreeper Family Tree Became a Network. The Hawaiian evolutionary conveyor beltThe paper in PNAS was accompanied by a Smithsonian institutional news release:
The Hawaiian Islands were not formed simultaneously. They were created sequentially as the Pacific tectonic plate moved north-westwards over a comparatively stationary volcanic hotspot. Once an island was carried away from the hotspot, its volcanoes became inactive and the island gradually eroded and subsided, while a new volcanic island arose to the south-east.
Consequently, the islands become progressively younger towards the south-east:Genetic evidence indicates that the finch-like ancestor of the Hawaiian honeycreepers probably reached the archipelago from Asia about 5.7 million years ago, close to the time when the oldest of the present high islands were forming. Its descendants therefore encountered not a completed island chain but a continually changing landscape in which new islands, mountains, forests and ecological opportunities appeared over time.
Island or island group Approximate geological age Kauaʻi and Niʻihau About 5 million years Oʻahu About 3–4 million years The former Maui Nui complex Mostly less than 2 million years Hawaiʻi Less than 1 million years and still volcanically active
Each newly formed island could be colonised by a small number of birds from an older island. Geographic separation then restricted interbreeding with the original population. Founder effects, mutation, genetic drift and natural selection caused the separated populations to diverge as they adapted to different foods, habitats and ecological niches.
The process can be summarised as follows:The new genomic study indicates that most honeycreeper lineages originated during an especially rapid period of diversification between about 3.5 million and 2.5 million years ago, broadly coinciding with the emergence of Oʻahu. The appearance of another substantial island apparently provided both new ecological opportunities and new geographical barriers, accelerating colonisation, isolation and speciation.
Stage Evolutionary consequence A new volcanic island forms Unoccupied habitats and food sources become available A few birds colonise it A small founder population becomes geographically isolated Populations exploit different resources Natural selection favours different beaks, behaviours and feeding methods Differences accumulate Populations become increasingly distinct and may form new species Birds disperse between islands New colonisations begin and previously separated lineages may meet again
Why the family tree became a network
A conventional evolutionary tree shows an ancestral population dividing into two lineages, each of which may subsequently divide again. This is a useful representation, but it can give the misleading impression that separated branches can never exchange genes again.
During a rapid adaptive radiation, newly formed species may remain genetically similar enough to produce fertile hybrids. If those hybrids reproduce with members of either parental species, genes can pass from one lineage into another. This movement of genetic material across species boundaries is called introgression.
Because such gene flow reconnects branches that had previously separated, the result is known as reticulate evolution, from the Latin reticulum, meaning a net. The evolutionary history is still predominantly branching, but some of its branches are joined by genetic bridges.
The researchers detected evidence of genetic mixing between the now-extinct ʻōʻū and Lānaʻi hookbill, two birds that looked remarkably different. They also found that closely related ʻamakihi populations occupying separate islands continue to exchange genetic material.
Hybridisation does not mean that the species are imaginary or that speciation has not occurred. Species boundaries can be incomplete or permeable, especially during the early stages of divergence. The lineages may remain distinguishable in their appearance, ecology and overall genomes while still exchanging a limited number of genes.
Introgression can also contribute to evolution by transferring useful variants into another population. A gene that evolved in one lineage may help a related lineage adapt to a new food source, habitat, pathogen or climate. Evolutionary novelty can therefore arise not only through mutations within a lineage but also through the recombination and redistribution of variation between related lineages.
The Hawaiian honeycreeper radiation was consequently produced by an interaction between geology and biology. Volcanism repeatedly created islands; dispersal brought founder populations to them; isolation allowed those populations to diverge; natural selection adapted them to different niches; and occasional hybridisation transferred genetic material between some of the resulting species.
This is not the history of separately created, immutable “kinds”. It is a detailed record of common ancestry, divergence and genetic exchange unfolding across a changing volcanic landscape over millions of years.
Smithsonian Scientists Resolve the Hawaiian Honeycreeper Family Tree
Family Tree Includes All Species Known at the Time of European Arrival to the Hawaiian Islands
Smithsonian researchers have identified the genetic relationships among all the Hawaiian honeycreeper species that were known to exist—and have subsequently faced major losses—since the arrival of Europeans to the islands. The study, published today in the Proceedings of the National Academy of Sciences by scientists at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) and the National Museum of Natural History, places extinct and extant species of this famously diverse group of birds in their evolutionary context, informing diversity loss studies and potential conservation priorities.
`Akiapōlā`au, Hemignathus monroi.Photo credit: Jeffrey Jack, CC0.
More than 60 Hawaiian honeycreeper species evolved from a single ancestor on the Hawaiian Islands across a relatively small time window, making them a prime example of adaptive radiation, the rapid diversification of one species into many new ones. When a species diversifies this quickly, it can be challenging to build a comprehensive family tree and accurately measure subsequent biodiversity loss. This problem is coming to a head today as only 17 Hawaiian honeycreeper species are still alive, with most facing extinction due to habitat loss, introduced diseases, invasive predators and climate change. To address this challenge, the Smithsonian team used genetic methods to resolve the relationships among the 17 living species, plus an additional 18 that existed when Europeans first arrived at the Hawaiian Islands in 1778 and two that are only known from fossil bones.
The findings are bittersweet because while we have a much better understanding of the relationships between the Hawaiian honeycreepers and the role hybridization played in their evolution, they also show we have lost more Hawaiian honeycreeper lineages than we previously knew.
Michael G. Campana, lead author.
Center for Conservation Genomics
National Zoo and Conservation Biology Institute
Smithsonian Institution, Washington, DC. USA.
The research team presents the first comprehensive map of the Hawaiian honeycreeper radiation. They found that most species originated during a “big bang” of diversification that took place approximately 2.5–3.5 million years ago, around the same time that the island of O’ahu formed. The team speculates this period facilitated a burst of diversification among honeycreepers because the new island afforded them unoccupied habitats with room to form multiple, distinct populations that then contributed their new genetic material back to other lineages on older islands. While breeding between two animals of different species is often a genetic dead end, the team confirmed genetic mixing between two honeycreeper species, ‘ō‘ū (Psittirostra psittacea) and Lāna‘i hookbill (Dysmorodrepanis munroi), and that a group of closely related honeycreepers called ‘amakihi (Chlorodrepanis spp.), which occupied different islands, continue to share genetic material today.
Researchers were able to gain a fuller understanding of the honeycreeper radiation by using minimally invasive techniques to collect samples from extant and extinct species held within museum collections in the United States and Europe. The work demonstrates the crucial nature of museum and paleontological specimens for genomics research.
I marvel at the amount of genetic data that my colleagues recovered from tiny bits of epidermis cut from old museum specimens of extinct honeycreepers. They even succeeded at getting genetic data out of small fossil bones of the honeycreepers. This really highlights how improvements in genetic sequencing have transformed traditional natural history museum collections into vast repositories of historical and comparative genetic information. This is especially true for birds and mammals, which were often preserved in the form of dried skins and bones, and we are now good at getting DNA from those types of specimens.
Helen F. James, co-author
Department of Vertebrate Zoology
National Museum of Natural History
Washington, DC. USA.
As the team looks ahead, they are focused on investigating the impacts, evolution and control of introduced diseases that have decimated honeycreeper populations, including avipoxvirus and avian malaria. Specifically, they are seeking to understand how both the birds and the pathogens have evolved since the diseases’ introductions.
The genomic data we generated for this study should also be useful for understanding the morphological evolution of the honeycreepers, as well as to offer clues about how the genomic variation of some honeycreeper species may have influenced their ability to survive disease.
Robert C. Fleischer, senior author.
Center for Conservation Genomics
National Zoo and Conservation Biology Institute
Smithsonian Institution, Washington, DC. USA.
Publication:
What the researchers have reconstructed is not merely a collection of similar birds but an evolutionary history written simultaneously in their genomes and in the geology of the islands they inhabit. As new volcanic islands appeared, founder populations colonised them, became isolated, adapted to different ecological opportunities and diverged into new species, illustrating the close connection between environment and evolution, just as the theory predicts. When some of those populations subsequently met again, hybridisation allowed genes to cross the developing species boundaries, turning a simple branching tree into a partially interconnected network.
That reticulation is not a weakness in evolutionary theory or an awkward exception added to rescue it. It is one of the processes evolutionary biology predicts when closely related populations diverge rapidly while retaining some capacity to interbreed. Modern genomics has merely given scientists the means to detect those ancient episodes of gene flow and produce a more accurate account than was possible from anatomy alone.
Creationists may respond that the honeycreepers remained finches, as though evolution predicts that descendants should cease to belong to their ancestral group. In fact, their continued membership of the finch family is exactly what common descent predicts. The important fact is that one ancestral population generated more than 60 species with different beaks, diets, behaviours and ecological roles through mutation, recombination, natural selection, isolation and introgression. No boundary corresponding to a separately created “kind” appears anywhere in that history.
Nor is there any shortage of new genetic information. Mutations produced new variants, recombination assembled them into new combinations, selection preserved those that worked in particular environments, and hybridisation sometimes transferred useful variants between lineages. Given millions of years and a succession of new islands and ecological opportunities, ordinary biological processes produced an astonishing range of specialised forms without requiring either foresight or supernatural intervention.
There is, however, a sobering conclusion. Evolution generated this diversity over millions of years, but human activity has destroyed much of it within a few centuries. Museum skins and fossil bones now preserve genetic information from lineages that no longer exist, while many of the remaining 17 species face introduced disease, habitat loss and climate change. The honeycreeper family tree therefore records both the creative power of evolution and the alarming speed with which its products can be erased.
Once again, scientists had no difficulty explaining the evidence within the framework of evolution. The volcanic history supplied the changing stage; inheritance, isolation, selection and gene flow supplied the mechanisms; and the genomes of living and extinct birds supplied the record. The only element for which the researchers found neither evidence nor explanatory need was a supernatural creator manufacturing immutable species by magic.
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