Author: Bryan P. White
Original Publication: 10/11/2012
Edited for grammar/syntax: 07/31/2019
According to the traditional theory of allopatric speciation, species diversity in marine species should be low due to high dispersal rates, which should disallow breaks in gene flow that lead to speciation. However, this is not the case for many species, which has led to investigators looking at whether or not physical barriers exist either in the past (geologically speaking) and whether or not larval dispersal is limited. However, sympatric speciation may also be possible. I disagree with the author's definition of sympatric speciation. They say, “…speciation in the face of gene flow ('sympatric speciation'),” although the distinction could be purely a difference of grammar/syntax. I would define sympatric speciation as speciation occurring in the face of potential gene flow. In other words, populations could be undergoing gene flow between each other (there's no geographical restriction), but they are not due to some other reason. I do agree that ecological specialization is a potential cause for these cases of sympatric speciation. However, the current role of ecological factors in speciation is uncertain, although much work has been done on studying the effects of dietary specialization on potential ecological specialization.
Here the authors study a genus of carnivorous marine snails, Drupa, in the family Muricidae. Their goals were to produce a well-resolved molecular phylogeny of 12 morphospecies of Drupa, estimate the age of those species divergences, and map diet and range characters onto that phylogeny. They will then compare the two competing theories of speciation (allopatric vs. sympatric) by identifying whether or not sister species differ by dietary differences and/or range differences, or by range differences alone.
COI, 12S, and 28S were sequenced. A three-gene concatenation was performed and was tested for incongruence in gene trees (none was found). Initial trees were constructed using MrBayes, and ultrametric trees were constructed using BEAST to obtain divergence times and for input into the SPLITS package. Species were delimited using the SPLITS package in R, which “optimizes the likelihood of genetic clusters, where branching rates between species follow a Yule model, but rates within species follow a neutral coalescent model” (Pons et al. 2006; Fontaneto et al. 2007). They also examined uncorrected p-distances.
Nine significant species entities were found by the GMYC analysis, and the average pairwise distance among those GMYC entities was 12.5 to 31.8%, except for one species entity pair having only 3.5% distance. Average within entity distance was <1.5%, and they were estimated to diverge 25.5 mya. Allopatric speciation was the dominant form of speciation here, except for one species pair that may have undergone sympatric speciation, but rather than dietary differences this may be related to microhabitat differences.
I liked the introduction and rigorous methods taken to produce a good phylogeny and date-time for these species, as well as the use of the GMYC method of species delimitation, but felt the attempt at answering the question of ecological speciation fell short. Why were so many species difficult to identify their dietary patterns? Why start a study knowing that they would be unable to obtain this data? It seems like the authors thought they would be able to obtain the dietary information, but failed, and so offered little to resolve the question of allopatric vs. sympatric speciation. It seems like overall this paper has stronger evidence for species delimitations rather than to answer the question of allopatric vs. sympatric speciation.
Claremont, M., Reid, D.G. and Williams, S.T., 2012. Speciation and dietary specialization in Drupa, a genus of predatory marine snails (Gastropoda: Muricidae). Zoologica Scripta, 41(2), pp.137-149. doi: https://doi.org/10.1111/j.1463-6409.2011.00512.x