The great speciator paradox: what silvereye genomics reveal about island evolution

New research, published in the journal Molecular Ecology, has provided insights into how islands promote diversification of the silvereye species complex.

Picture this: a small bird with a distinctive white ring around its eye manages to colonise dozens of islands scattered across the vast Pacific Ocean, yet on each island it evolves into something slightly different. This is the story of the silvereye (Zosterops lateralis), and our new research published today in Molecular Ecology reveals how these small birds can help us understand one of the most classic evolutionary puzzles.

Capricorn silvereye (Zosterops lateralis chlorocephalus) on Heron Island, April 2024. Photo credit: Dr. Andrea Estandía.

The great speciator paradox

This paradox challenges one of the core assumptions in evolutionary biology: that high dispersal prevents populations from becoming genetically distinct. If individuals frequently move between islands, gene flow should homogenise populations and slow the process of divergence. Yet great speciators like the silvereye defy this logic. Understanding how they achieve both long-range dispersal and rapid local differentiation is key to resolving broader questions about how new species form, especially in island systems where biodiversity tends to evolve quickly and in unique ways.

The silvereye is a perfect example of this paradox. This small songbird, part of the Zosteropidae family, has managed to establish populations across the Australian continent and numerous Pacific islands, from New Caledonia to French Polynesia. Along the way, it has diversified into at least 16 recognised subspecies, some differing dramatically in size, colouration, and behaviour.

Continental vs island effects

To solve this puzzle, we needed to compare how the same species evolves in different environments. Using whole-genome sequencing, we analysed 114 silvereyes from 12 subspecies across both continental Australia and Pacific islands, creating the most comprehensive genetic picture of these birds ever assembled.

We found that on the Australian continent, despite being separated by distances of over 2,000 kilometers and facing major barriers like the Nullarbor Plain, silvereye populations showed little genetic differentiation and high connectivity.

But on islands, even populations separated by relatively short water gaps showed dramatic genetic and morphological differences. Islands less than 50 kilometres apart harboured silvereyes that were as genetically distinct as continental populations separated by thousands of kilometers.

The island syndrome

Our findings support a classic evolutionary hypothesis: silvereyes undergo a fundamental behavioural shift when they colonise islands. The same dispersal ability that allows them to cross hundreds of kilometers of open ocean becomes a liability once they establish on an island. Over-water dispersal becomes risky business when your new home is a small island surrounded by the ocean.

Natural selection appears to favour individuals who stay put rather than risk long-distance dispersal that might end in the middle of the Pacific. This behavioural shift from "coloniser" to "resident" creates the perfect conditions for rapid evolution and subspecies formation on islands.

Reconstructing the silvereye evolutionary history

Using multiple phylogenetic approaches, we also reconstructed the evolutionary history of silvereyes, revealing that they began their Pacific expansion approximately 1.5 million years ago during a period of increased island formation in the region. This timing was crucial as it allowed silvereyes to colonise new islands as they emerged, taking advantage of fresh ecological opportunities before competition arrived.

However, even with whole-genome data, some relationships remained frustratingly unclear. The rapid pace of silvereye evolution, combined with ongoing gene flow between some populations, obscures their true evolutionary relationships.

Our silvereye study provides crucial insights into how biodiversity is generated on islands worldwide. The pattern we've documented—high dispersal followed by dispersal reduction—may be a key mechanism driving the extraordinary diversity of island life.

This research was conducted in collaboration with researchers from the University of Oxford (United Kingdom), Queensland Museum Kurilpa, the University of the Sunshine Coast (Australia), Griffith University (Australia), and Otago University (New Zealand). The full study is published in Molecular Ecology and represents decades of fieldwork across some of the most remote islands in the Pacific Ocean.

Reference: Estandía, A., Merino Recalde, N., Sendell-Price, A.T., Potvin, D.A., Goulding, W., Robertson, B.C. and Clegg, S. (2025), Islands promote diversification of the silvereye species complex: A phylogenomic analysis of a great speciator. Molecular Ecology 34(14). https://doi.org/10.1111/mec.17830