A Shetland Wren, Troglodytes troglodytes zetlandicus, Kergord, Mainland, Shetland
Photo credit: Dr Michał Jezierski
Creationists will continually demand evidence for evolution being observed, then, when the evidence is provided, immediately insist that science should adopt their childish parody of evolution, in which one species turns into an unrelated species in a single miraculous event. That is not evolution as any biologist understands it. In fact, if such a thing were ever observed, it would falsify the Theory of Evolution, not confirm it.
By demanding evidence for something no scientist has ever claimed happens, creationists imagine they are somehow refuting science, or at least providing a plausible anti-Darwin argument for people who do not understand the science.
So this example of evolution in living populations will almost inevitably be dismissed by creationists using that same disingenuous tactic. It is evidence for the evolution of island gigantism in isolated populations of the wren, Troglodytes troglodytes, on Scottish islands. And, to rub salt in creationists' wounds, it is not merely a single isolated example, but multiple examples of gigantism evolving in island environments — an example of parallel evolution in response to similar environmental pressures acting on different local populations.
In other words, this is not some local curiosity that can be waved away as a one-off oddity, but the predictable result of isolation, restricted gene flow and similar island conditions acting on related populations. The evidence has just been published, open access, in the Evolutionary Journal of the Linnean Society by researchers led by the University of Birmingham.
The researchers, led by Dr Michał Jezierski, examined four subspecies of island wren, each isolated on a specific Scottish island or archipelago — Shetland, Fair Isle, the Outer Hebrides and St Kilda. Each of these subspecies is geographically isolated, yet exposed to broadly similar island environments, and each differs significantly from the wrens found throughout mainland Britain and continental Europe.
The study showed that the wrens of St Kilda and Shetland show little evidence of interbreeding with the mainland population. These two populations have evolved spectacular island gigantism: a wren from England will typically weigh about 7–10 grams, while a St Kilda wren weighs about 13–16 grams. The largest St Kilda wrens are therefore more than twice the weight of the smallest mainland wrens, and their genetic distinctiveness is so marked that the researchers say they may be on the way to becoming separate species.
Importantly, the genomic evidence shows that the Shetland and St Kilda wrens are genetically distinct from each other, despite having evolved similar enlarged body sizes. In other words, the same broad evolutionary outcome has arisen independently in separate island populations, rather than being inherited from a single already-giant ancestor. That is exactly what evolutionary biology predicts: related populations, isolated in similar environments, can be shaped in similar directions by similar selection pressures, even when the detailed genetic route differs.
Island Gigantism - Evolution in Isolation. Island gigantism is part of what biologists call the “island rule”: when mainland animals become isolated on islands, small species often evolve larger bodies, while large species often evolve smaller ones. This is not magic, and it is not a sudden transformation. It is natural selection, genetic drift, founder effects, restricted gene flow and altered ecological pressures acting over many generations.The publication in the Evolutionary Journal of the Linnean Society is accompanied by a press release by the University of Birmingham:
Islands are unusual evolutionary laboratories. They often lack the full range of mainland predators, competitors and parasites. They may also have empty ecological niches that mainland animals cannot exploit because those niches are already occupied. In those circumstances, a small animal that would normally survive by hiding, fleeing or reproducing quickly may instead benefit from becoming larger, stronger, more competitive, longer-lived, or better able to survive periods of food shortage.
Examples include:
- Giant tortoises - The giant tortoises of the Galápagos and Aldabra are among the best-known examples of island giants, although their history is more complicated than a simple small-to-large transformation. Their large size probably helped tortoises colonise remote oceanic islands in the first place, but island conditions then allowed giant forms to persist and diversify.
- Flores giant rat - The island of Flores, Indonesia, has produced unusually large rodents, including the living Flores giant rat, Papagomys armandvillei. Fossil deposits at Liang Bua, the same cave system associated with Homo floresiensis, also contain evidence of large and giant murine rodents, showing how island ecosystems can produce extreme size shifts in small mammals.
- New Zealand giant wētā - New Zealand’s giant wētā are large, flightless insects that evolved in an island environment where mammals were absent before human arrival. In effect, some insects were able to occupy ecological space that, on continental landmasses, would more often be occupied by small mammals.
- Lord Howe Island stick insect - The Lord Howe Island stick insect, Dryococelus australis, sometimes called the “tree lobster”, is a large, flightless island insect. It was thought to have been exterminated after rats reached Lord Howe Island, but a tiny surviving population was later found on Ball’s Pyramid, a nearby volcanic sea stack.
- New Zealand moa - The extinct moa were flightless birds that evolved in New Zealand, where there were no native land mammals apart from bats. In the absence of mammalian herbivores, these birds became the dominant large browsing animals in many New Zealand ecosystems.
- Madagascar elephant birds - Madagascar’s extinct elephant birds were another spectacular example of island bird gigantism. These huge, flightless birds evolved in isolation on Madagascar and, like the moa, show how islands can produce evolutionary outcomes very different from those on continents.
- Canary Island giant lizards - Several lizards in the genus Gallotia evolved large body sizes in the Canary Islands. Some forms survive today, while others are extinct, their history illustrating both the creative power of island evolution and the vulnerability of island endemics once humans and introduced predators arrive.
A useful caution is the Komodo dragon. It is often presented as a simple example of island gigantism, but fossil evidence suggests that giant monitor lizards had already evolved elsewhere in the Australasian region before the modern Komodo dragon became restricted to Indonesian islands. So, even here, the science is more interesting than the slogan.
The important point is that island gigantism is not a one-off anomaly. It is a repeated evolutionary pattern, seen in mammals, birds, reptiles and invertebrates, whenever isolation changes the selective pressures acting on a population. The Scottish island wrens are simply a living, local example of the same broad evolutionary principle.
Source notes:
The “island rule” is supported by large comparative studies of island vertebrates, though its strength varies between groups. [1] Giant tortoises are classic island giants, but Galápagos tortoise research notes that large size may have aided oceanic colonisation rather than always evolving only after island arrival. [2] The Flores rodents, Lord Howe Island stick insect and Komodo dragon caveat are supported by the Liang Bua/Flores literature, IUCN/CSIRO conservation material, and fossil work on giant varanids. [3]
New research helps scientists unlock evolution of gigantism in Scottish island wrens
Island birds could be the key for researchers to better understand the evolutionary paths that lead to ‘island syndromes’.
A new study of British Wrens has provided new insights into the inner workings of ‘island syndromes’, according to research led by the University of Birmingham.
The paper, published in the Evolutionary Journal of the Linnean Society, reveals that different subspecies of island Wrens are evolving independently, with the team finding particularly strong evidence of ‘island gigantism’ in two of the studied populations.
Researchers examined four subspecies of island Wrens, each found on a specific island or archipelago in Scotland – Shetland, Fair Isle, the Outer Hebrides, and St Kilda. Each of these subspecies is geographically isolated (but exposed to broadly similar environments on different Scottish islands), and each differs significantly from the Wren subspecies found across mainland Great Britain and continental Europe.
Island gigantism is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relative. This is most famously exemplified by the giant tortoises of the Galapagos, and the extinct Dodo of Mauritius; both of which far exceed the sizes of their continental ancestors.
Led by Dr Michał Jezierski, the study’s findings represent one of the most in-depth explorations of the population-level processes giving rise to island syndromes. These evolutionary phenomena, detected across disparate types of animals and plants across Earth’s islands, involve a suite of evolutionary changes in island species including; island gigantism, longer lifespans, slower rate of reproduction and, in birds, a tendency towards lower flight ability.
Key findings from the study include:
- Wren populations on Shetland and St Kilda show minimal evidence of interbreeding with their cousins from mainland Britain.
- These two subspecies have evolved spectacular island gigantism; a Wren from England will weigh 7-10g on average, whereas on St Kilda they range from 13-16g.
- The largest St Kilda Wrens are more than twice the size of the smallest on mainland Great Britain, putting them within the top 25% of cases of island gigantism in birds worldwide.
We found that all four Scottish Wren subspecies are genetically distinct from the Wrens of mainland Britain; with the Wrens of Shetland and St Kilda being especially distinct in both appearance and song. Their genetic distinctiveness is so high, that it is likely they are on their way to becoming new species.
Dr Michał T. Jezierski, lead author.
School of Geography, Earth and Environmental Sciences
University of Birmingham
Birmingham, UK.
By comparing these four Scottish island subspecies with mainland Wrens, using body measurements, song recordings, and whole genome sequencing, researchers were able to explore the differences in biology between the different island populations in more detail than has previously been possible, allowing them to further investigate the ways in which island syndromes evolve.
A Wren from mainland Britain will weigh 7-10g on average, whereas on St Kilda they range from 13-16g.
Islands host 20-30% of species worldwide and are famous for their unusual wildlife – from Madagascan lemurs to Komodo Dragons. Similar conditions found across the world’s islands, including lower predation and competition than on adjacent mainlands, are driven by their inherent isolation. Although island syndromes may be found across much of our planet’s biodiversity, their underlying mechanisms are poorly understood.
Our research suggests that islands with similar environments can produce similar evolutionary outcomes using different genetic pathways. The Wrens of Scotland provide us with a powerful case study to understand the mechanisms by which island biodiversity is generated worldwide.
William J. Smith, co-author School of Life Sciences
University of Nottingham
Nottingham, UK.
Our genomic data indicates that Shetland and St Kilda Wrens are genetically distinct from each other, despite their similarities in physical appearance. This means that their island gigantism is a case of ‘parallel evolution’, where a similar original population (probably colonists from the British mainland) made it to each island archipelago, and then independently evolved to become island giants.
Dr Michał T Jezierski.
Shared traits, different genetics
Whole genome comparisons showed that each island population is genetically distinct and largely isolated: while the Wrens of Shetland and St Kilda are physically similar, the regions of their genomes that show the most differences from mainland Wrens are largely independent from each other.
By contrast, Wrens from Fair Isle and the Outer Hebrides are more similar to those on the British mainland: highlighting that island evolution does not proceed in the same way, even within a relatively small geographic region, with each population representing a related, but largely independent, evolutionary unit.
Our genomic data indicates that Shetland and St Kilda Wrens are genetically distinct from each other, despite their similarities in physical appearance. This means that their island gigantism is a case of ‘parallel evolution’, where a similar original population (probably colonists from the British mainland) made it to each island archipelago, and then independently evolved to become island giants. In the process, their songs also became very different from those of ‘mainland’ British birds.
Dr Michał T Jezierski.
Understanding the ‘micro’ processes that lead to ‘macro’ patterns
The ‘giant’ size of the Wrens has evolved alongside other island associated traits, including distinctive songs and subtle differences in plumage and body proportions, supporting the idea that island environments consistently shape evolution in predictable directions.
The ‘why’ of island syndromes remains a mystery, and researchers do not yet fully know how changes in body size, or other island syndromes, represent adaptations to the special ecological conditions on islands. However, these latest findings amongst Wren populations may provide a great case study with which Birmingham researchers can further investigate this field of evolutionary science.
Publication:
So, once again, creationists are left with the evidence they keep demanding, but never seem willing to accept when it is placed in front of them. Here are living populations of the same species, isolated on different islands, diverging genetically and morphologically from their mainland relatives, and doing so in broadly similar ways under broadly similar environmental pressures. That is not a belief, an assumption, or a philosophical preference; it is measured, testable, observable evolutionary biology.
Of course, no wren has obligingly turned into a duck, a mouse, or a cabbage in a single generation, so creationists who rely on that infantile caricature of evolution will pretend nothing of interest has happened. But biology was never obliged to conform to creationist misrepresentation. Evolution proceeds by accumulated change in populations over time, shaped by mutation, selection, drift, gene flow, isolation and ecological opportunity. The Scottish island wrens show exactly that process in action.
What makes this example especially awkward for creationists is that it is not merely a single population becoming unusual in isolation. Similar enlarged forms have evolved independently in different Scottish island populations. That is parallel evolution: repeated, predictable evolutionary change under similar conditions. It is precisely the kind of pattern we should expect if populations evolve naturally, and precisely the kind of pattern that makes no sense if every species was separately manufactured in its present form a few thousand years ago.
And so, yet again, science explains the evidence while creationism is left explaining it away. The wrens of St Kilda, Shetland, Fair Isle and the Outer Hebrides are not anomalies to be ignored; they are another small but elegant demonstration that evolution is not merely something inferred from fossils and genomes, but a continuing natural process, still shaping living populations today.
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