Cepaea Snails Evolution: Divergence and Ecological Adaptations in C. nemoralis and C. hortensis

The study of Cepaea snails evolution provides valuable insights into how environmental pressures drive divergence in closely related species such as Cepaea nemoralis and Cepaea hortensis.

Red Cepaea Hortensis

Divergence and Molecular Phylogeny

Recent molecular phylogenetic studies have reshaped our understanding of Cepaea evolution. A comprehensive analysis by Neiber & Hausdorf (2015) confirmed that C. nemoralis (the brown-lipped or grove snail) and C. hortensis (the white-lipped snail) are each other’s closest relatives and the only true members of Cepaea. Other snails once classified in Cepaea (like C. vindobonensis and C. sylvatica) were reassigned to different genera. This means the two lipped snails form a distinct lineage within the Western Helicinae clade of land snails. Their mitochondrial and nuclear DNA sequences are highly divergent from each other, to the point that aligning some genes is challenging. Such deep genetic divergence suggests a long independent evolutionary history for the two species. While precise molecular clock estimates for the C. nemoralis–C. hortensis split are not universally agreed, the data indicate an ancient separation. For context, the broader helicid snail family shows splits dating back to the late Eocene (~34–40 million years ago). Cepaea likely diverged from its eastern relatives by that time, and the C. nemoralis vs. C. hortensis speciation would have occurred well after those Eocene events. Indeed, paleontological evidence shows both species present as distinct entities by the Pliocene in Britain, implying their divergence took place several million years ago (likely in the mid-to-late Miocene or earlier). In summary, molecular clock and phylogenetic evidence point to a significant, ancient split between C. nemoralis and C. hortensis, consistent with their status as long-established sister species.

Juvenile Cepaea Nemoralis

Genetic Structure, Hybridization, and Species Boundaries

Despite their close relatedness, C. nemoralis and C. hortensis maintain clear genetic and reproductive boundaries. Classic genetic surveys (e.g. by Ochman, Jones & Selander 1987) of 231 populations across Europe found that each species has its own allele frequency clines and patterns of variation. Both snails display similar overall levels of genetic polymorphism, but their geographical genetic structures differ: C. hortensis shows a smooth north–south gradient in allele frequencies from Britain to Spain, whereas C. nemoralis exhibits clines that reverse direction between the continent and Britain. Notably, these patterns are largely independent in the two species – allelic changes in co-occurring populations do not track each other. This indicates they respond to environmental pressures separately, underscoring firm species boundaries. There is no evidence that their extensive geographic variation is merging or that one is in the process of budding off the other; in fact, Ochman et al. concluded that such differentiation is “not a precursor of speciation” but rather reflects long-standing separation.

Reproductive isolation between these snails is reinforced by both behavior and anatomy. In areas of sympatry they rarely, if ever, hybridize in the wild. Mating is assortative, aided by differences in their love dart morphology and reproductive structures. The love dart of C. nemoralis has a cross-section with simple blades, whereas C. hortensis’ dart has bifurcated blades; additionally, C. nemoralis typically has 3 or fewer mucous gland branches in its dart apparatus versus 4 or more in C. hortensis. These anatomical distinctions likely prevent successful mating between species in nature, even though the two look very similar externally. Interestingly, when interspecific crosses have been done in laboratory conditions, the genetic control of shell color and banding behaves almost identically in the two species – dominance relationships are preserved in hybrids. This suggests that the underlying supergene for shell polymorphism is structured similarly in both snails. However, such crosses are artificial; in the wild, C. nemoralis and C. hortensis remain “sibling species” that coexist without blending. Their separate gene pools and consistent anatomical differences define a clear species boundary, honed by millions of years of divergence.

Fossil Evidence and Historical Distribution

The fossil and subfossil record, though fragmentary for land snails, provides key insights into the history of Cepaea. Shells attributable to C. nemoralis and C. hortensis have been found in Pliocene deposits in Britain, indicating that these snails were part of European faunas over 2–5 million years ago. Throughout the Pleistocene and into the Holocene, Cepaea shells appear in various sedimentary contexts. Notably, C. nemoralis has an unbroken Holocene fossil record in Ireland spanning at least ~8000 years BP. Subfossil shells from Neolithic layers (e.g. Newlands Cross, Co. Dublin, ~7600 BP) show that C. nemoralis survived glacial cycles and was present in Ireland soon after the last Ice Age. Intriguingly, the dominance of a unique mitochondrial lineage (lineage C) in Irish grovesnails, which today is otherwise only found in the Eastern Pyrenees, suggests a human-mediated jump dispersal. Grindon & Davison (2013) argued that Mesolithic peoples might have inadvertently transported snails from Iberia/France to Ireland, creating a disjunct “Lusitanian” distribution. This is supported by the genetic distinctiveness of Irish C. nemoralis and the absence of that lineage in intervening regions, as well as the fact that these snails were a known food source in prehistoric Pyrenean communities.

Fossil timing also hints at colonization sequences in the British Isles. It appears C. nemoralis reached Britain slightly before Ireland during postglacial times. One study notes that Britain may have been colonized perhaps a thousand years earlier, with Ireland’s populations becoming established soon after, possibly via mixings of British and Iberian stock. Once established, both species have persisted and expanded. C. hortensis fossils are rarer in the record (perhaps due to its more northerly distribution), but it is the only Cepaea found in post-glacial Iceland, suggesting it colonized that island (likely with Viking-age human help) in the late Holocene. Aside from their native range in Europe, humans have introduced Cepaea snails to North America in the 19th century, where they are now naturalized in parts of the USA and Canada . In summary, fossil and archaeological evidence traces Cepaea snails from the Pliocene to the present, illuminating their historical biogeography: long-term natives of Europe that have occasionally hitched rides with humans to new locales.

Ecological Interactions and Habitat Adaptation

Throughout their evolution, C. nemoralis and C. hortensis have been shaped by ecological interactions and habitat preferences. These two snails often live side by side in similar habitats – typically woodlands, hedgerows, grasslands, and gardens with ample moisture and calcium for shell-building. Yet, subtle differences in ecology do exist. C. hortensis tends to be more cold-tolerant, allowing it to range further north (it extends into northern Scotland, Scandinavia, and is the sole Cepaea in Iceland). C. nemoralis prefers somewhat milder conditions and can grow slightly larger; it often dominates in lowland western Europe and has been more successful in human-altered landscapes. Where their ranges overlap, ecological studies have looked for competitive interactions. Laboratory experiments show that they do compete for resources – for example, when kept together they may interfere with each other’s feeding and reproduction (Cameron & Carter 1979, 1987). There is evidence of diet overlap, although C. hortensis might have a greater taste for decaying plant material, whereas C. nemoralis readily consumes fresh green vegetation.

In natural settings, however, competition does not appear to lead to exclusion. A classic field study titled “Competition, invasion, but no niche displacement” (Functional Ecology 1987) found that even when one species invaded a site already occupied by the other, both could maintain breeding populations without one outcompeting the other. In sympatric colonies, each snail species shows independent fluctuations in abundance and morph frequencies, driven more by microhabitat conditions and climate than by direct interspecific effects. In essence, coexistence is common: one species might be more abundant on an open sunny bank and the other a few meters away in shady undergrowth, but both can thrive in the same general area. This suggests that their niches are broadly overlapping, and any competitive advantage is minor or context-dependent. Predation pressures are also similar – both Cepaea are preyed on by song thrushes and other birds, which break the shells to eat the snail. The polymorphism in shell colour is thought to be maintained partly by visual predation: interestingly, thrushes do not distinguish between the two species, treating a yellow-banded C. nemoralis the same as a yellow-banded C. hortensis. Thus, both snails experience parallel natural selection forces (e.g., frequency-dependent predation on common morphs, and thermal selection on shell color) in their environments. Over evolutionary time, these ecological interactions – competition, predation, climate adaptation – have shaped shell polymorphism and behavior in Cepaea, but without disturbing the fundamental coexistence of the two species. Each has adapted to a broad range of habitats, demonstrating flexibility and resilience that have no doubt contributed to their long-term survival since the Pliocene.

Cepaea Hortensis

Cepaea as Model Organisms in Evolutionary Research

Both C. nemoralis and C. hortensis are famous in evolutionary biology and ecology, serving as model organisms for over a century. Their striking shell colour and banding polymorphism has been a textbook example of balanced polymorphism and natural selection. Early geneticists like Cain & Sheppard (1950s–1960s) used Cepaea to study Mendelian inheritance in natural populations, laying groundwork for ecological genetics. In modern times, Cepaea nemoralis has been reinvigorated as a model by new techniques. It has even featured in citizen-science projects (e.g. the “Evolution Megalab”) engaging the public to map shell colour frequencies across Europe . These snails’ accessibility and variable traits make them ideal for such large-scale evolutionary studies. As Neiber & Hausdorf noted, Cepaea remains crucial for understanding ecogenetics. Recent genomic research has further solidified this status. In 2021, a draft genome sequence of C. nemoralis was published (the first genome for any helicid snail), revealing an approximately 3.5-gigabase genome with over 43,000 predicted genes. Having the genome opens the door to identifying the loci underlying shell colour and banding. Indeed, studies in Molecular Ecology have used RAD-sequencing to map markers flanking the supergene that controls the shell polymorphism. The supergene’s architecture is of great interest because it produces multiple morphs maintained by selection.

Researchers also use Cepaea to explore broader evolutionary questions. For example, a 2022 study in Journal of Evolutionary Biology examined the phylogeography of C. nemoralis, finding deep genetic structure and evidence of long-distance post-glacial migrations. The present-day Irish populations were shown to be an admixture of Pyrenean lineage snails with others from a separate refugial source, highlighting how historical events (like human trade or natural dispersal) can shape genetic diversity. Another very recent study (Davison et al. 2024) investigated mitochondrial genome evolution in Cepaea. By sequencing C. nemoralis, C. hortensis, and 20 other snail species, they discovered extremely high mtDNA divergence and heteroplasmy in Cepaea, shedding light on the evolution of mutation rates in land snails . Meanwhile, C. hortensis has been somewhat less studied (earning the nickname “the poor relation” in polymorphism research) , but it is gaining attention as scientists recognize the value of comparative studies between the two sister species. In Journal of Molluscan Studies, researchers have begun to document C. hortensis polymorphism and its ecological correlates, ensuring it is no longer overshadowed by C. nemoralis .

In conclusion, the grove snail and the white-lipped snail stand as important model organisms at the intersection of evolution, genetics, and ecology. Their evolutionary history – from an ancient divergence, through survival of ice ages (recorded in fossils), to contemporary studies on genetics and climate adaptation – is exceptionally well documented. Ongoing research in top journals (e.g. Molecular Ecology, Evolution, Journal of Molluscan Studies) continues to refine our understanding of Cepaea speciation, population structure, and ecological adaptation. This rich body of work, with contributions spanning from classical field studies to modern genomics, makes Cepaea nemoralis and Cepaea hortensis key species for understanding how evolution operates on natural variation over both deep time and contemporary timescales.

Sources:

• Neiber, M.T. & Hausdorf, B. (2015). Mol. Phylogenet. Evol. 93: 143–149.

• Ochman, H. et al. (1987). Heredity 58: 127–138.

• Cameron, R.A.D. & Carter, M.A. (1987). Functional Ecology 1(2): 91–97.

• Grindon, A.J. & Davison, A. (2013). PLoS ONE 8(6): e65792.

• Dowle, E.J. et al. (2022). J. Evol. Biol. 35(8): 1110–1124.

• Saenko, S.V. et al. (2021). G3: Genes Genomes Genet. 11(2): jkaa071.

• Davison, A. et al. (2024). BMC Biol. 22: 3 (High mtDNA heteroplasmy in Cepaea). 

• Silvertown, J. et al. (2011). J. Molluscan Stud. 77(3): 241–249.   (Polymorphism in C. hortensis)

• Kerney, M.P. et al. (1983). Atlas of the Land and Freshwater Molluscs of Britain and Ireland. (For fossil and distribution context).