The herring gull complex refers to a group of large gulls that were formerly considered subspecies of the herring gull (Larus argentatus) but are now often recognized as distinct species. This group includes the European herring gull (L. argentatus), American herring gull (L. smithsonianus), Vega gull (L. vegae), and East Siberian gull (L. vegae) among others. The distribution of these gulls around the Arctic and North Atlantic oceans was classically cited as an example of a ring species, with gene flow occurring between adjacent populations but not between the endpoints of the ring. However, recent research has called into question whether the herring gull complex truly represents a ring species.
What is a ring species?
A ring species refers to a continuous chain of interbreeding populations that circles around a geographic barrier. In a ring species, the two end populations are reproductively isolated due to accumulation of genetic and/or behavioral differences, despite the continuity of interbreeding populations between them. The classic example is the herring gull complex, with a ring distribution around the Arctic. Gene flow was thought to occur between adjacent populations, but the two endpoints (European herring gull and American herring gull) were considered separate species that do not interbreed. Ring species were considered important demonstrations of how speciation could occur through accumulation of small changes around a ring, even when endpoints retained the ability to interbreed with adjacent populations.
Evidence against ring species status
Recent genetic studies have challenged the view of the herring gull complex as a ring species in several ways:
- Limited gene flow between some adjacent populations around the ring
- Genetic differentiation between endpoints less than between some adjacent populations
- Interbreeding and hybridization occurring between endpoints (European and American herring gulls)
These findings disrupt the continuity of interbreeding required for a ring species. There are breaks in gene flow around the ring, and reproductive isolation is incomplete between endpoints. This suggests speciation occurred not through gradual divergence around a ring but more rapidly in allopatry between discontinuous populations. The complex may better be described as a circumpolar series of related gull populations, not a ring species.
Evidence for and against ring species status
Here is a more detailed look at the key evidence for and against the herring gull complex being a ring species:
Evidence supporting ring species
- Circumpolar distribution around the Arctic, suggestive of gradual divergence
- Phenotypic differences between endpoints consistent with subspecies or incipient species
- Appearance of clinal variation in traits like size and plumage color
Evidence against ring species
- Genetic distances between some adjacent populations as high or higher than between endpoints
- Limited gene flow between multiple pairs of adjacent populations
Population 1 Population 2 Fst (genetic distance) Iceland gull European herring gull 0.15 Vega gull Birula’s gull 0.67 American herring gull East Siberian gull 0.37 - Hybridization and interbreeding between European and American herring gulls where they come into contact
This genetic evidence shows reproductive isolation is incomplete between the ring endpoints and that gene flow has been disrupted between many adjacent populations. This contradicts the continuous interbreeding required for a ring species.
Explanation for conflicting evidence
So why does some evidence still seem to support the herring gull complex as a ring species? There are a few factors that help explain this conflicting perspective:
Incomplete sampling around the ring
Many of the gaps in gene flow detected are between little-studied or sampled populations (e.g. Birula’s gull). More thorough sampling may reveal additional breaks in gene flow not detected yet.
Differing evolutionary histories of traits
While genetic divergence shows reproductive isolation, phenotypic traits like size and plumage may still show clinal patterns of gradual variation around the ring. These traits evolved at different rates and may still appear consistent with a ring species.
Ongoing hybridization and introgression
Limited interbreeding between endpoints can blur the genetic distinction between them, giving a superficial appearance of continuous gene flow. More in-depth genomic analysis is required to reveal past isolation.
Sympatric populations not sampled
Studies have focused on sampling allopatric populations rather than looking for reproductive isolation between sympatric forms, which is critical for confirming ring species status.
Conclusion
While aspects of variation in the herring gull complex are consistent with a ring species, current evidence suggests reproductive isolation and gaps in gene flow contradict this model. The complex likely reflects a series of related gull populations that colonized the Arctic and diverged into distinct species, probably through multiple routes of allopatric and parapatric speciation across the circumpolar region. More thorough sampling and genomic analysis will help further unravel the evolutionary history of this group. The herring gull complex remains an important system for studying speciation, but should not uncritically be assumed to represent a true ring species.
