A study combining whole genome sequencing, body measurements and song analysis has found that Shetland and St Kilda shearwaters are almost twice as large as their British relatives. The two populations arrived at their similar appearance through different genetic pathways – a clear example of parallel evolution.
The grebe is one of the smallest songbirds in Britain. A typical grebe in England weighs just seven to ten grams. However, much larger grebes live on the remote islands of Scotland: on St Kilda they weigh between 13 and 16 grams, and the largest individuals there are more than twice as heavy as grebes on the British mainland.
A new study shows that this is not just a random difference in size. The populations of the Shetland and St Kilda gannets have also become different from their relatives in Britain in body structure, plumage color, song and genome. According to the researchers, the degree of difference may indicate that they are in the midst of a process that may eventually lead to their recognition as independent species. The study was published in the journal Evolutionary Journal of the Linnean Societysystem. (University of Birmingham)
Four populations, four evolutionary paths
The researchers examined four subspecies of the distinct shrike, troglodyte wrens, each of whom lives on a different island or group of islands in Scotland: Shetland Isles (T. t. zetlandicus), Gidron Pierre Eyal (T. t. fridariensis), Outer Hebrides (T. t. hebridensis(and St Kilda)T. t. hirtensis).
All of these populations are geographically isolated, although exposed to somewhat similar climatic and environmental conditions. The isolation has limited gene flow between them and the British population of stilts, allowing each to evolve in slightly different directions. (University of Birmingham)
The researchers combined several methods: they sequenced the full genomes of 32 jays, measured live birds and 117 stuffed birds from museum collections, analyzed weight data from 1,311 jays recorded as part of the British imprinting program, and examined recordings of jays’ song. The combination made it possible to compare populations genetically, physically, and vocally, rather than relying on just one trait.
Giant islands – but on the scale of Gidron
The phenomenon in which small animals become larger after their population becomes isolated on an island is called "island gigantism." The most famous examples are the giant tortoises of the Galapagos Islands and the extinct dodo of Mauritius. In the case of the grebes, they are still tiny birds, but the relative change in size is extraordinary.
After the researchers took into account the effect of latitude – animals living in colder regions tend to be larger – it turned out that the Shetland and St Kilda guillemots were still larger than expected. Both populations are in the top quarter of island gigantism cases recorded in birds worldwide. In contrast, the Pierre Isle guillemots are not larger than expected for the latitude in which they live.
Other differences were found in body structure. Shetland shearwaters have a longer, deeper bill and relatively longer legs, while St Kilda shearwaters have a wider bill, longer wings and a longer tail. However, the study did not find a clear decrease in investment in flight organs compared to walking organs – a trait that appears in some island birds and even leads to a loss of flight ability in extreme cases.
Same solution, different genes
One of the important findings of the study is that Shetland and St Kilda ganders are similar in size, but are not particularly close populations. Each became large independently, after its ancestors probably arrived on the islands from Britain.
This is a case of parallel evolution: populations that started from a similar starting point were exposed to similar conditions and developed similar traits independently. However, the study shows that the similar external outcome does not require the use of the same genetic changes.
Of the regions of the genome that were most different from British shorthairs, only about 3.6% were shared by the Shetland and St Kilda populations. In other words, most of the genetic differences that accumulated in each population were found in other parts of the genome.
The researchers suggest that much of the genetic variation is due to the “founder effect” and genetic drift. When a small number of individuals establish a population on an island, they carry with them only a fraction of the genetic diversity of the original population. In a small population, the frequency of genetic changes can also rise or fall randomly. However, for the two populations to independently develop large bodies and similar traits, natural selection is likely to have played a role.
The singing has also changed.
Differences between bird populations are not measured only in genes and body structure. Many birds use song to identify mates and defend territory, so changes in song patterns may contribute to the separation of populations.
The researchers found that the song of the Scottish warblers was different from that of the warblers in Britain and Ireland. The songs of the St Kilda warblers were shorter, had a narrower frequency range and a higher delivery rate. The presence of buzzing sounds and differences in the number of trills and croaks also separated the island populations from the mainland population.
A change in song may reduce the chances that birds from different populations will recognize each other as suitable mates. Together with low gene flow, differences in appearance and genetic uniqueness, this may be a sign of the process of differentiation – the gradual formation of new species.
However, the researchers did not conclude that Shetland and St Kilda shearwaters have already become separate species. Such a declaration would require a broader examination of the extent of their reproductive isolation, behavior, and evolutionary history. The findings suggest a possible ongoing process, rather than a formal change in scientific classification. (University of Birmingham)
A natural laboratory for understanding evolution
The islands where the grebes live were only freed from ice cover after the glaciers retreated at the end of the last ice age. Therefore, the current populations could have formed no earlier than about 18–25 years ago—a relatively short time in evolutionary terms. During this time, some of them had time to develop significant differences in size, song, color, and genome.
"We found that all four Scottish subspecies are genetically distinct from mainland British warblers, with the Shetland and St Kilda warblers particularly distinct in both appearance and song," said Dr Michał Jężarski of the University of Birmingham, lead author of the study. He said their genetic distinctiveness was so great that it was possible they were on the verge of becoming new species. (University of Birmingham)
The study demonstrates that evolution does not have to use the same genetic pathway to reach a similar result. Similar environmental conditions can shape animals in parallel directions, even when the result is hidden by a different combination of mutations, genetic drift, and natural selection.
Questions and Answers
What is island giants?
An evolutionary phenomenon in which small animals become larger after their population becomes isolated on an island.
How big are the St Kilda crags?
Their weight ranges from 13 to 16 grams, compared to about seven to ten grams for typical English gerbils.
Are St Kilda and Shetland herons already new species?
No. They are still defined as subspecies of the distinct gerbil, but the genetic, physical, and vocal differences indicate the possibility of continued differentiation.
What is parallel evolution?
A situation in which closely related populations of similar origin independently develop similar traits in response to similar environmental conditions.
Why are the two populations not genetically identical?
Most genetic changes have accumulated in different regions of the genome, probably due to a combination of the founder effect, genetic drift, and natural selection.
For the scientific article: Michał T. Jezierski, Jenny C. Dunn, Carolina RF Chagas and William J. Smith, "Parallel evolution of island syndromes coincides with limited parallel genetic differentiation in a passerine bird", Evolutionary Journal of the Linnean Society, May 28, 2026, DOI: 10.1093/evolinean/kzag008.
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