On the windswept archipelagos off the Scottish mainland, something unusual has been happening to the wren – plump, sturdy, and ordinarily no heavier than a couple pound coins. It has been quietly getting bigger.
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A new study, published in the Evolutionary Journal of the Linnean Society and conducted by researchers from the University of Birmingham, found that isolated island populations of Scottish wrens are evolving separately from their mainland counterparts – and the most striking difference is their size.
Mainland wrens typically weigh between 7-10 grams, but wrens inhabiting the remote Scottish islands Shetland and St Kilda have been recorded at 13-16 grams.
According to the scientists, the largest wrens from St Kilda are more than twice the size of the smallest wrens on the mainland, which means they’re among the top 25 per cent known cases of avian island gigantism.
Island gigantism occurs when species living in isolation on remote islands become larger than their mainland relatives – like the giant tortoises of the Galapagos. It’s a well-documented, but still not fully understood, phenomenon.
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A window into evolution
Islands host between 20 and 30 per cent of Earth’s known species. Isolation and lower predation pressure create ideal conditions for evolution to branch off in unusual directions – making them natural laboratories for biologists.
The researchers involved in this study compared the island birds with mainland counterparts using body measurements, recordings of song calls and genome sequencing, providing one of the most detailed investigations yet into the biology behind island syndromes.
They wanted to find out whether animals that evolve the same physical traits are doing so by changing the same parts of their DNA, or by finding different genetic solutions to arrive at the same result – which is not just relevant in this case, but in the broader study of convergent evolution.
Analysis revealed that, despite their physical similarities, the Shetland and St Kilda wrens are genetically quite distinct from one another. They arrived at the same result – bigger bodies, different songs – by travelling different genetic routes.
Wrens from Fair Isle and the Outer Hebrides, by contrast, remain more genetically similar to their mainland counterparts.
Scientists call this parallel evolution – a similar ancestral population reached each island separately and independently evolved in the same general direction. This suggests that island environments push species towards the same outcomes, but the underlying genetic machinery can differ each time.
"Our research suggests that islands with similar environments can produce similar evolutionary outcomes using different genetic pathways," says the study's co-author Will Smith, from the University of Nottingham.
"The wrens of Scotland provide us with a powerful case study to understand the mechanisms by which island biodiversity is generated 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," explains the study's lead author Michał Jezierski, a biogeographer from the University of Birmingham.
"Their genetic distinctiveness is so high, that it is likely they are on their way to becoming new species."
According to the researchers, their findings support the idea that island environments shape evolution in predictable ways – but that doesn’t mean they’re easy to understand and study. Scientists still don’t fully know why island syndromes occur, or how specific traits helps species adapt – but the Scottish wrens could become a model for future research into the process.
Read the full findings here.
Top image: Shetland wren (Troglodytes troglodytes zetlandicus) calling on a mossy stone against blue sky, Shetland Islands. Credit: Dgwildlife/Getty Images
