Do rock-dwelling land snail species avoid their congeners?

What is it about?

It is long known that adaptation is the engine of evolution, at the same time non-adaptive factors may also have some role in this process. Some malacologists claim that obligate rock-dwelling land snails are among the best examples for animal groups where non-adaptive factors play a key role in the early phases of species development. In order to confirm this hypothesis, we studied co-occurrences within the Balkan land snail genus Montenegrina. Based on thousands of land snail distribution records from all over the Balkan Peninsula (SE Europe), we tested how frequently Montenegrina congeners (different species of the same genus) occur together compared to the value presuming random distribution. If species development is driven not principally by adaptation (but mostly by random factors like founder effect, genetic drift, etc.), the newly diverged sister species do not necessarily have different ecological niches at the beginning of this process. In this case, until their niches are identical or largely overlap, due to competitive exclusion, they are not able to co-occur. In contrast, if this is mainly an adaptive process, we would expect niche partitioning already at an early phase of species divergence and consequently, we would not expect any competitive exclusion among closely related species. We used probabilistic null-models to simulate taxon distributions under the null hypothesis of no taxon interactions and calculated co-occurrences. Independence of taxon occurrences was tested by comparing observed co-occurrences to simulated values. We found significantly fewer than expected co-occurrences among Montenegrina species and intrageneric clades. In our interpretation, this indicates that species divergence preceded niche partitioning and suggests a primary role of non-adaptive processes in the speciation of rock-dwelling gastropods.

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Why is it important?

This study has a double importance. As malacologists, we feel important to have an evidence — even if an indirect one — for the role and relative significance of non-adaptive mechanisms in rock-dwelling land snails' evolution. At the same time, the method itself might be also important. We introduced a taxon-specific metric that characterizes the occurrence probability at a given location. This probability was calculated as a distance-weighted mean of the taxon’s presence and absence records at all sites. The importance of the method is that it can account for the effects of distributional constraints in range-wide datasets, and as such it is suitable for testing ecological, biogeographical, or evolutionary hypotheses where interactions of partly allopatric taxa are in question.

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