Understanding how new species evolve

In evolutionary biology, a ring species is formed when populations expand around a geographical barrier, eventually forming two distinct species that are unable to interbreed. It occurs as traits within the connected populations diverge with geographic distance, until the two now-distinct forms meet around the barrier to close the ring.

The EU-funded NGS-RINGSPS project used a ring species as a living evolutionary model to investigate how a single species splits into two new daughter species. NGS-RINGSPS looked at a songbird, the greenish warbler, which inhabits a ring around the Tibetan Plateau, with two distinct, non-interbreeding forms co-existing in central Siberia.

Researchers have long hypothesised that populations diverged when an ancestral species in the south expanded northwards along two pathways either side of the inhospitable plateau. As the greenish warbler's song became more complex from south to north, the birds became reproductively isolated as they could not recognise each other's calls.

NGS-RINGSPS analysed thousands of genetic sequences from greenish warblers to provide further insight into this 'evolutionary divergence' hypothesis. It indeed found evidence for ancient periods of geographical isolation followed by more recent secondary contact (where the two forms meet to close the ring).

Surprisingly, although the two Siberian forms were supposed to be reproductively isolated by the time they established contact, genetic evidence indicates that they interbred. In another twist, genetic analysis revealed that the geographically distinct forms evolved similar traits in isolation, a process known as parallel evolution.

In identifying specific genomic regions that diverged in populations either side of the ring, NGS-RINGSPS has provided compelling evidence for how adaptation and speciation came about. This information will be extremely useful for helping evolutionary biologists understand how genes contribute to physical variation in species.