If you go to college (I studied Zoology) you learn a lot of exciting new words and phrases – things like parapatric speciation – very useful for impressing your friends at the bar (or maybe not!). Anyway, it’s a long time since I was at university and I must confess I had forgotten what parapatric meant! Researching the distribution of this little butterfly, the Bath White, I was reminded that parapatry is when the distributional ranges of two species, or two sub-populations, do not overlap, but instead are immediately adjacent to each other. Contiguous is the right word for it. As I understand it, there is no obvious geographical barrier, but something keeps the two species apart. Parapatry* is thought to be a rather rare phenomenon in nature.
Until about 30 years ago, the Bath White was thought to be a single species with a vast distribution: from the Canary Islands in the west, all the way across the Palaearctic region, to Japan in the east (John, 2013). It came as something of a surprise apparently, when a study by two Swiss biologists, Hansjürg Geiger and Adolf Scholl (published in 1982), revealed ‘two genetically well differentiated and geographically separated taxa’. Pontia daplidice occurs mainly in southwestern Europe and north Africa, including the Canary Islands (Gran Canaria), Morocco, Spain and France, but with some outlier populations in Cyprus, Lebanon and Israel. Pontia edusa on the other hand, is found mainly in eastern Europe, starting in Switzerland, Italy, Germany and Corsica across to the Balkans, Greece, southern Turkey, and so on (Geiger & Scholl, 1982; Porter et al., 1997). Subsequent DNA barcode studies, demonstrated large genetic distances between butterflies from these two different regions (Hausmann et al., 2011).
The DNA barcoding – which incidentally requires one or two legs from a poor adult butterfly to extract the DNA – suggested ‘an interruption of gene flow‘ between the two species or semi-species, which might suggest that there is little or no interbreeding between these two distinct taxa (but see below!). It is worth remembering however, that these studies are just based on genetic markers, which may, or may not be fully representative of the total genome (almost certainly aren’t). Nevertheless, they provided a clear indication of genetic diversity within a putative species and are widely used, although they do not, as I understand it, provide categorical evidence of separate species. The taxonomy is as yet unresolved, which is why some authors refer to P. edusa as a semi-species, taxonomically located within a so-called super-species called P. daplidice (Kurze et al., 2006; In German). Other authors refer to the Pontia daplidice species complex, with two sibling, semi-species.
The outcome of this research was to call Bath whites in the western region of Europe, Ponta daplidice (Linnaeus, 1758); and those in the eastern region, Pontia edusa (Fabricius, 1777) (Wagener, 1988). Modern butterfly guides (like Tolman and Lewington, 2008) illustrate the distribution of these two semi-species (usually called the Bath White and the Eastern Bath White). Older field guides (like Higgins and Riley, 1970) just had the one large distribution. The separation line, shown in all modern guides, runs north-south through western Europe, from the border of Holland and Germany on the North Sea, down through Switzerland and into northern Italy. Since this species is very mobile, and easily ably to colonise new habitats as they become available, it interesting to understand how the barrier between them is maintained.
Initial studies suggested that although they can mate with each other, and there is no ‘precopulatory barrier’ to stop them doing so, there are some ‘post-mating barriers’ which prevent some of the offspring from developing. At least for individuals that occur in populations that are far away from each other. Interestingly, pairings of male edusa with female daplidice were much more successful that pairings of male daplidice with female edusa. Showing that it really does matter which way round you swap partners!
As we have seen, the two taxa have a zone running down western Europe where their distributions abut each other. A hybrid zone was found in NW Italy where the two taxa came together. In 1990 and 1991, Adam Porter and other researchers based in the USA and Switzerland, carried out a survey, sampling 28 different sites in northern Italy (north of Genoa). The 167 km transect ran from east to west across the hybrid zone, from one area to the other, and samples were taken for electrophoretic analysis; the same technique as that which originally revealed the distinct taxa (Geiger & Scholl, 1982; Geiger et al., 1988). The study found evidence for strong selection against hybrids in a narrow zone at the centre of transect, which suggests to me that offspring of pairings between the two taxa are less likely to survive. However, the barrier was found to be relatively weak, with ‘amply opportunity for neutral gene exchange across their hybrid zone’ (Porter et al., 1997). The final conclusions of these workers was that these taxa are ‘best thought of as well-differentiated subspecies‘.
More recent records from Cyprus showed that Pontia daplidice was present on the island, as well as being in the nearby Levant (Israel and Lebanon) (John et al., 2013). These findings suggest to me that the east/west separation of these two taxa may be too simplistic and a much more nuanced distributional picture may eventually emerge. There appears to be an outlier of Pontia edusa in Brittany, for example.
So what should naturalists and field biologists make of these results? These two semi-species, or subspecies, are morphological indistinguishable. In other words, they cannot be separated in terms of the colours and patterns of their wings, or even their genitalia (according to John et al., 2013). So, in some ways it is a complete irrelevance that there are differences in allozyme and mitochondial DNA markers. On the other hand, these molecular techniques are revealing a number of so-called cryptic species, which appear almost identical but are genetically quite distinct. Even in very well-studied groups like European butterflies, there appear to be quite a few hidden, or undetected species; known to them of course, but not to us! Such cryptic biodiversity has implications for conservation of course, because you might be dealing with not one, but two or more species, which look, to all intents and purposes, the same! It is also I think, a salient reminder that nature is fluid and ever-changing, and what we see today has not always been the case, and what appears constant and unified (a species) is often a more nebulous concept than we like to admit.
The Bath White is extremely scarce migrant in the British Isles, and a long distance migrant to other northern European counties including Finland. They are strong migrants and were seen migrating through a high pass in the Pyrenees at 7,500 feet (Lack, 1951). The distribution map on p. 52 in Tolman and Lewington (2009), gives the impression that the occasional migrants found in the UK originate from the eastern region and are therefore P. edusa, but whether anyone has actually analysed them, I don’t know (expect not).
The Internet is a mine of information, and a great source of easy-to-obtain literature, photographs and reports, but it is worth constantly bearing in mind, that it is not always accurate. There is no Great God of the Internet checking every entry and making the necessary alterations to ensure absolute veracity! This story is a case in point. There are all sorts of sites, some local to a region, others just blogs like this one, offering slightly different versions of this story.
The published field guides I have seen generally refer to Pontia daplidice as a species complex, and the individual ‘sibling species‘ as semi-species. John et al. (2013) however, point to a degree of inconsistency, with some guides or atlases referring to Pontia edusa as a distinct semi-species, some as a subspecies of Pontia daplidice, and others as a P. daplidice/edusa species complex.
Internet sites are even less consistent. Wikipedia for example, appears to be quite informative about Pontia edusa but rather vague about Pontia daplidice, with no mention of the split. The best advice to students is, where possible, go back to the original sources. For some more photos and good descriptions of these species, on some excellent websites, see here and here.
An afterword on communicating science: We live in a sort of virtual Wikiworld, where all sorts of versions of the truth exist, but as an optimist, I like to think that over time we all start to converge on an accepted version. Until such time that is, as a Kuhnian revolution, or paradigm shift occurs, and we all come to accept a new version of the accepted facts! It is I think, incumbent on scientists in this modern age, to try to explain their studies – most of which are after all funded by tax payers – in as an intelligible and comprehensible a way as possible. Many scientific studies are so couched in jargon, that it is nearly impossible for the non-specialist to decipher them. A lot of good scientists do however, spend time writing books, blogs and articles for the intelligent layman, but there is still much to be done to improve the overall state of affairs.
*N.B sympatric species occur within the same or overlapping geographical areas; whilst allopatric species occur in separate non-overlapping geographical areas.
Geiger, H., Descimon, H. & Scholl, A. (1988). Evidence for speciation within nominal Pontia daplidice (Linnaeus, 1758) in southern Europe (Lepidoptera: Pieridae). Nota lepidopterologica 11: 7–20.
Geiger, H., & Scholl, A. (1982). Pontia daplidice (Lepidoptera, Pieridae) in Südeuropa – eine Gruppe von zwei Arten. Mitteilungen der Schweizerischen Entomologischen Gesellschaft 55 : 107-114.
Hausmann, A., Haszprunar, G., Segerer, A. H., Speidel, W., Behounek, G., & Hebert, P. D. (2011). Now DNA-barcoded: the butterflies and larger moths of Germany. Spixiana, 34(1), 47-58.
Higgins, L. G., & Riley, N. D. (1970). A field guide to the butterflies of Britain and Europe. A field guide to the butterflies of Britain and Europe.
John, E., Wiemers, M., Makris, C. & Russell, P. (2013). The Pontia daplidice (Linnaeus, 1758)/Pontia edusa (Fabricius, 1777) complex (Lepidoptera: Pieridae): confirmation of the presence of Pontia daplidice in Cyprus, and of Cleome iberica DC. as a new host-plant for this species in the Levant. Entomologist’s Gazette, 64, 69-78.
Kurze, B. J., Nuß, M., & Westphalen, M. (2006). Vorkommen und Lebensweise des Resedaweißlings (Pontia daplidice (Linnaeus, 1758)) in Sachsen (Lepidoptera: Pieridae). Sächsische Entomologische Zeitschrift, 1, 89-100.
Lack, E. (1951). Migration of insects and birds through a Pyrenean pass. The Journal of Animal Ecology, 63-67.
Porter, A. H., Wenger, R., Geiger, H., Scholl, A., & Shapiro, A. M. (1997). THE PONTIA DAPLIDICE‐EDUSA HYBRID ZONE IN NORTHWESTERN ITALY. Evolution, 51(5), 1561-1573.
Tolman, T., & Lewington, R. (2009). Collins Butterfly Guide. The most complete guide to the Butterflies of Britain and Europe: 384 pp.
Wagener, P. S. (1988). What are the valid names for the two genetically different taxa currently included within Pontia daplidice (Linnaeus, 1758)?(Lepidoptera Pieridae). Nota lepidopterologica, 11(1), 21-38.