Abstract
BackgroundStudying species with disjunct distributions allows biogeographers to evaluate factors controlling species ranges, limits on gene flow, and allopatric speciation. Here, we use phylogeographic and population genetic studies of the barnacle Pollicipes elegans to discriminate between two primary hypotheses about the origin of disjunct distributions of extra-tropical populations: trans-tropical stepping-stone colonization versus an out-of-the tropics origin.ResultsNucleotide diversity peaked in the centre of the species’ range in samples from El Salvador and was lower in samples from higher latitudes at Mexico and Peru. Haplotypes from El Salvador samples also had a deeper coalescent, or an older time to a most recent common ancestor. A deep phylogeographical break exists between Mexico and all samples taken to the south (El Salvador and Peru). Isolation-with-migration analyses showed no significant gene flow between any of the three regions indicating that the difference in genetic differentiation among all three regions is explained primarily by differences in population separation times. Approximate Bayesian Computation model testing found strong support for an out-of-the tropics origin of extra-tropical populations in P. elegans.ConclusionsWe found little evidence consistent with a stepping-stone history of trans-tropical colonization, but instead found strong evidence for a tropical origin model for the largely disjunct distribution of P. elegans. Sea surface temperature and habitat suitability are likely mechanisms driving decline of populations in tropical regions, causing the disjunct distribution.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-015-0131-z) contains supplementary material, which is available to authorized users.
Highlights
Studying species with disjunct distributions allows biogeographers to evaluate factors controlling species ranges, limits on gene flow, and allopatric speciation
Sequence diversity statistics After alignment, visual inspection, and trimming, a single 590 bp partial mitochondrial cytochrome c oxidase subunit I gene (COI) sequence was obtained for all individuals
Nucleotide diversity was greatest in the central portion of the species range, where populations of P. elegans are currently most scarce; diversity peaked in SAL, with lower values in MEX and in PER (Table 1)
Summary
Studying species with disjunct distributions allows biogeographers to evaluate factors controlling species ranges, limits on gene flow, and allopatric speciation. Species with disjunct or geographically discontinuous ranges are important systems for understanding the factors controlling species’ distributions, population connectivity, and the process of allopatric speciation [1,2,3,4,5,6,7,8,9,10]. Marchant et al Frontiers in Zoology (2015) 12:39 tests based on the timing of population separation or speciation relative to known geological events rely fundamentally on estimates of mutation rates, which are often only known from related taxa and may depend on additional untestable assumptions. The potential pitfall of this alternative approach is that it cannot be used to reconstruct ancient biogeographic events [24] because the telltale population genetic signatures that they leave behind will be gradually erased by the evolutionary forces of mutation, gene flow, genetic drift, and natural selection [23, 27]
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