Tuesday, 12 September 2017

Snake Evolution in Progress

Barred Grass Snake, Natrix helvitica

Photo: Wolfgang Böhme
Hybridization patterns in two contact zones of grass snakes reveal a new Central European snake species | Scientific Reports

Arguably the most attractive British snake of the three native species, and certainly the largest, is the grass snake.

Now it seems, according to a paper published in Scientific Reports a few days ago, it is the Western representative of what is an example of speciation in progress. It also illustrates the conflict between genetics and taxonomy.

Based on an analysis of two mitochondrial markers and 13 microsatellite loci the team concluded that the western or barred grass snake, found in Britain, France, Western Germany, Switzerland and Italy is a distinct species (Natrix helvetica) rather than a subspecies (Natrix natrix helvetica) as current taxonomy classifies them. They arrived at this conclusion by examining grass snakes in two 'contact zones' where different genetic lineages overlap.

Distribution of mitochondrial lineages of 1,580 grass snakes used in this study. Total sample size of each clade shown in the legend. Eight allochthonous grass snakes with haplotypes of Italian lineages caught in southern Great Britain and Hesse, Germany, not shown. Map was created using ARCGIS 10.2 (http://www.esri.com/arcgis) and ADOBE ILLUSTRATOR CS6 (http://www.adobe.com/products/illustrator.html). Inset: Natrix natrix helvetica (Linz am Rhein, Germany).
Photo: Wolfgang Böhme.
Recent studies found major conflicts between traditional taxonomy and genetic differentiation of grass snakes and identified previously unknown secondary contact zones. Until now, little is known about gene flow across these contact zones. Using two mitochondrial markers and 13 microsatellite loci, we examined two contact zones. One, largely corresponding to the Rhine region, involves the western subspecies Natrix natrix helvetica and the eastern subspecies N. n. natrix, whereas in the other, more easterly, contact zone two lineages meet that are currently identified with N. n. natrix and N. n. persa. This second contact zone runs across Central Europe to the southern Balkans. Our analyses reveal that the western contact zone is narrow, with parapatrically distributed mitochondrial lineages and limited, largely unidirectional nuclear gene flow. In contrast, the eastern contact zone is very wide, with massive nuclear admixture and broadly overlapping mitochondrial lineages. In combination with additional lines of evidence (morphology, phylogeny, divergence times), we conclude that these differences reflect different stages in the speciation process and that Natrix helvetica should be regarded as a distinct species. We suggest a nomenclatural framework for presently recognized grass snake taxa and highlight the need for reconciling the conflicts between genetics and taxonomy.

Eastern Grass Snake (Natrix natrix natrix)

Photo: Melita Vamberger
Contact zones such as these are important to biology in that they act as living laboratories where the incidence of, and viability of, the offspring of hybridisation can be observed. Where there is little or no hybridisation or where hybrids are non-viable this is strongly indicative that there are two different species. On the other hand, where hybridisation is common and produced viable offspring, then the best that can be said is that there are two subspecies, as was thought to be the case with all the Western Eurasian grass snakes.

The research team found that at the Western contact zone, the grass snakes had speciated to give N. helvetica but at the eastern contact zone, speciation was still in progress, with N. n. natrix and N. n. persa still hybridising freely to produce viable offspring.

As the Senckenberg Research Institute press release says:

The two contact zones examined in this study represent different stages in the speciation process: The eastern contact zone reveals a complete mixing of the involved genetic lineages over hundreds of kilometers. In the Rhine region, on the other hand, the hybrid zone is less than 50 km wide, and the admixture is very limited and unidirectional, primarily with Barred Grass Snakes cross-breeding with Eastern Grass Snakes, but rarely the other way around. “This indicates the presence of reproductive barriers,” explains Fritz. They arise during the speciation process to prevent mismatched pairings among different species. These reproductive barriers and the narrow hybrid zone show that the Barred Grass Snake constitutes a distinct species.

Again we have an example of something that creationists keep insisting can't be there. It would be good to think this sort of evidence would induce creationists to change their minds, but just another example of their dogma being at odds with reality is hardly likely to raise a creationist eye-brow. It will already have been dismissed on the absurd grounds that no evidence, no matter how convincing, is enough for them to admit to being wrong.

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