Risso's dolphin Grampus griseus.
Credit: Nicola Hodgins
Risso's dolphin, Grampus griseus
© Nicola Hodgins.
According to newly published research in the Journal of the Marine Biological Association of the United Kingdom (JMBA), Risso’s dolphins, Grampus griseus, around the British Isles appear to form a single, genetically connected population. At the same time, their mitochondrial DNA distinguishes them from dolphins sampled elsewhere in the Atlantic and Mediterranean, suggesting that the British population has experienced relatively little genetic exchange with more distant populations.
The research was conducted by an international team whose joint first authors were Nicola K. Hodgins, a visiting researcher at the University of Exeter, UK, and Ing Chen of National Taiwan Normal University, with Michael Krützen of the University of Zurich as corresponding author.
At first sight, describing the British dolphins as both a single population and genetically distinct might seem contradictory, but the two findings concern different geographical scales. Within British waters, the dolphins appear to interbreed freely enough to prevent significant regional genetic divisions from developing. Compared with populations farther afield, however, reduced gene flow has allowed the British population to acquire a distinctive genetic signature.
This is not evidence that the British dolphins have become a separate species, nor that they are inevitably on their way to becoming one. It does, however, illustrate the population-level processes from which allopatric divergence can begin. When a small colonising population becomes geographically or behaviourally separated from its source population, the founder effect and genetic drift can alter its genetic composition. If the new environment imposes different selection pressures, natural selection may drive further divergence.
The British dolphins have relatively low genetic diversity, and all six mitochondrial haplotypes identified by the researchers were unique to the British Isles. These haplotypes formed a single evolutionary lineage closely related to those found in the Azores. The researchers suggest that the population may descend from a relatively recent, possibly one-off colonisation event after the retreat of the British–Irish Ice Sheet made the region accessible about 16,000 years ago.
Population divergence is not the same thing as completed speciation. Before barriers to interbreeding have evolved, separated populations may remain fully capable of exchanging genes. If they come back into contact, renewed gene flow may halt or partly reverse their divergence; if separation persists, the differences may continue to accumulate until reproductive isolation eventually develops. There is no predetermined point at which this must happen, and no sharp boundary that can always be identified while the process is under way.
Something broadly comparable appears to have characterised the evolution of Homo sapiens, although over much greater distances and involving several human lineages. Populations within Africa repeatedly became separated and later reconnected, while expanding populations of Homo sapiens subsequently interbred with Neanderthals, Denisovans and probably other archaic humans in Eurasia. Human evolution consequently resembles a branching network with occasional reconnections rather than a succession of completely isolated forms.
The team analysed DNA obtained from 72 Risso’s dolphins stranded on British coasts between 1992 and 2016. Fifty-two individuals were included in the analysis of 16 nuclear microsatellite markers, while mitochondrial DNA sequences were obtained from 60. The nuclear and mitochondrial results both revealed low genetic diversity and no significant geographical division among dolphins sampled from eastern, western and southern British waters.
Because dolphins from every neighbouring part of the Atlantic were not sampled, however, the researchers cannot yet define the geographical boundary of this British population or determine how much it mixes with dolphins from places such as Norway, the Celtic Sea, the south-western English Channel and the Bay of Biscay. More extensive sampling and whole-genome analysis will be needed to resolve those questions.
Nor does the absence of detectable genetic divisions exclude behavioural or cultural subdivisions within the British population. Photo-identification studies have found fewer than 3% of individual dolphins matched between the western and northern coasts of Scotland. This does not mean that the two groups were only 3% genetically alike; it means that very few of the same identifiable animals were photographed in both regions, suggesting strong preferences for particular areas despite continuing gene flow across the wider population.
There are several excellent examples. One qualification is needed, however: the traditional *Larus* “ring species” is no longer regarded as a simple ring. Genetic evidence revealed a more complicated history of divergence and later gene flow, with no support for the supposed closure of a circumpolar ring ([Liebers, de Knijff & Helbig, 2004](https://pmc.ncbi.nlm.nih.gov/articles/PMC1691675/)). The *Parus major* complex is also taxonomically unsettled, with some former subspecies now treated as separate species by some authorities ([Kvist et al., 2003](https://pmc.ncbi.nlm.nih.gov/articles/PMC1691391/)).
Population Divergence^ Speciation as a Continuum. Species are not genetically uniform entities. Populations can become geographically, behaviourally or ecologically distinct while retaining some capacity to exchange genes. Such populations may be described as ecotypes, host races or subspecies, depending upon the nature and extent of their differences.This nested pattern—local genetic connectivity, regional differentiation, founder effects, drift, incomplete lineage sorting and the possibility of renewed contact—is precisely the dynamic and historically contingent pattern predicted by evolutionary biology. It bears no resemblance to the magical creation of discrete, immutable ‘kinds’ a few thousand years ago.
Nor is complete inability to hybridise an absolute requirement for recognition as separate species: even well-established species occasionally exchange genes. The more important question is whether the populations remain on independent evolutionary trajectories despite that gene flow.
- Common bottlenose dolphins — Tursiops truncatus
Coastal and offshore bottlenose dolphins in the North Atlantic occupy different habitats, eat different prey and show pronounced genetic differentiation, despite the absence of any physical barrier preventing them from meeting. Feeding specialisation, natal-site fidelity and socially transmitted behaviour appear to restrict gene flow. They are generally retained within the same species, although some populations may be approaching independent evolutionary status. Research paper.
- Three-spined sticklebacks — Gasterosteus aculeatus
Marine sticklebacks repeatedly colonised freshwater lakes after the last Ice Age. Different populations subsequently evolved changes in body shape, armour, feeding structures and behaviour suited to their particular lakes. Many marine and freshwater populations can still interbreed, but hybrids between the most strongly divergent forms may possess poorly matched combinations of traits and reduced fitness. Different populations therefore occupy different positions along the speciation continuum. Research paper.
- Rough periwinkles — Littorina saxatilis
Populations of this marine snail have repeatedly evolved into “Wave” and “Crab” ecotypes. Wave-exposed shores favour small snails with relatively thin shells and large apertures, whereas sheltered shores containing predatory crabs favour larger, thicker and more resistant shells. The ecotypes meet and exchange genes, but natural selection continually recreates their differences. Swedish populations show weak reproductive barriers, while comparable Spanish populations are much closer to reproductive isolation—all within the same recognised species. Research paper.
- Apple maggot flies — Rhagoletis pomonella
The ancestral population bred on native hawthorn, but some flies switched to introduced apple trees. Because apples ripen earlier than hawthorns, apple-associated flies emerge and mate earlier. The flies also prefer the smell of their respective host fruit and usually mate on or near it. Apple and hawthorn flies therefore form genetically differentiated host races living in the same geographical area. Gene flow continues, however, so reproductive isolation remains incomplete. Research paper.
- European house mice — Mus musculus musculus and M. m. domesticus
These two subspecies evolved separately before expanding across Europe and meeting in a narrow hybrid zone. They differ genetically and behaviourally, show preferences for mates of their own subspecies, and some hybrid males have reduced fertility. Nevertheless, hybridisation and gene flow continue wherever their ranges meet. They consequently remain subspecies rather than completely isolated species. Research paper.
- Walking-stick insects — Timema cristinae
Populations living on Ceanothus and Adenostoma shrubs have evolved different colour patterns that camouflage them against their respective host plants. They also differ in host preference and are more likely to mate with insects from the same ecotype. Hybrids and migrants are less effectively camouflaged, but interbreeding still occurs. Natural selection therefore promotes divergence while continuing gene flow prevents complete separation. Research paper.
These examples show why there is no universal point at which one population instantaneously becomes a new species. Divergence may increase until the populations become independently evolving species, remain stable with limited gene flow, or disappear if environmental change brings the populations back together. Speciation is a process with several possible outcomes, not a single predetermined event.
Publication of the paper in the Journal of the Marine Biological Association of the United Kingdom was accompanied by a news release from the University of Exeter:
British Risso’s dolphins belong to single population
Risso’s dolphins around the British Isles are part of a single population – genetically distinct from those elsewhere in the Atlantic, new research shows.
Scientists studied the DNA of dolphins that stranded in England, Scotland and Wales from 1992 to 2016.
The results showed that that Risso’s dolphins found around the British Isles are their own genetic entity, suggesting they don’t mix much with other Risso’s dolphins from further away and are genetically distinct from the closest populations in the Azores, the Mediterranean, and possibly the wider north Atlantic.
The results support the notion of Risso’s dolphins in British waters having been isolated from other populations for some time. However, we did not sample every nearby part of the Atlantic, so it’s not yet clear where the boundaries of this unit lie.
Nicola Hodgins, co-lead author
Centre for Ecology and Conservation
School of Biosciences
University of Exeter
Exeter, UK.
While the study found no significant genetic differences between the animals around different parts of the British Isles, there may still be evidence of “sub-structuring” – meaning that the dolphins may still live in or return to specific areas around the British Isles, in addition to remaining in specific groups over their lifetime.
For example, recent analysis of Risso’s dolphins in Scottish waters found less than 3% matches between individual dolphins photo-identified on the west and northern coasts, implying that Risso’s dolphins exhibit a significant level of preference for particular areas. This is particularly important because Risso’s dolphin numbers may be decreasing in some regions.
To explore these boundaries and find out whether Risso’s dolphins in British waters are truly unique to this area, more genetic samples are needed, with comparisons from nearby regions such as Norway, the Celtic Sea, the south‑west English Channel and the Bay of Biscay.
Using more advanced DNA techniques could further add resolution and give a fine-grained picture of connectedness between groups of Risso’s dolphins around the British Isles – key information needed to accurately assess their conservation status.
It could be argued that the population should be recognised as a “sub-population” by the IUCN – as has been done for the Mediterranean sub-population, which is classified as Endangered.
Future research combining information on behaviour, appearance, movements, vocalisations and genetics will help researchers better understand how Risso’s dolphins around the UK are connected, how they use different habitats, and how best to protect them.
Good conservation requires habitat-specific management. While Risso’s dolphins are currently managed as a single UK population, local populations can face different threats and use habitats in different ways. Effective protection must therefore focus on safeguarding important habitats and addressing pressures at a local scale, where conservation measures can have the greatest impact.
Nicola Hodgins.
The research team included MARA, Nova Atlantis, Environmental Investigation Agency, Whale and Dolphin Conservation, Zoological Society of London, and the universities of Exeter, Glasgow, Zurich, Western Australia and Taiwan.
Samples used in this study were collected by the UK Government funded stranding programmes, the Scottish Marine Animal Strandings Scheme (SMASS) and the Cetacean Strandings Investigation Programme (CSIP).
Publication:
What the researchers have found is not a newly formed species but something equally instructive: a population caught at one point along an evolutionary continuum. Risso’s dolphins around the British Isles remain genetically connected to one another, yet their low diversity and distinctive mitochondrial lineage preserve evidence of colonisation, isolation and restricted gene flow with populations farther afield.
Whether that divergence will ever produce a separate species cannot be predicted. Continued isolation could allow genetic differences to accumulate until reproductive barriers develop; renewed contact could restore gene flow and erase some of those differences. Evolution has no predetermined destination. Populations diverge, reconnect, merge or become permanently separated according to changing environments and historical circumstances.
This is precisely why creationist appeals to immutable “kinds” are biologically meaningless. There is no detectable boundary at which permissible variation supposedly ends, no genetic mechanism preventing differences from accumulating, and no objective test by which creationists can identify the limits of a “kind”. Every stage that evolutionary biology predicts—from local variation and population structure to partial reproductive isolation and completed speciation—is found in nature.
The British Risso’s dolphins are also a reminder that evolutionary significance does not depend upon whether taxonomists award a population a new species name. Their distinctive ancestry, low genetic diversity, specialised habitat use and possible behavioural subdivisions make them a population worth protecting in their own right. No magical creation event is needed to explain them—only migration, inheritance, isolation, selection and genetic drift operating, as they demonstrably do, generation after generation.
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