Abstract
As two lineages diverge from one another, mitochondrial DNA should evolve fixed differences more rapidly than nuclear DNA due to its smaller effective population size and faster mutation rate. As a consequence, molecular systematists have focused on the criteria of reciprocal monophyly in mitochondrial DNA for delimiting species boundaries. However, mitochondrial gene trees do not necessarily reflect the evolutionary history of the taxa in question, and even mitochondrial loci are not expected to be reciprocally monophyletic when the speciation event happened very recently. The goal of this study was to examine mitochondrial paraphyly within the Orchard Oriole complex, which is composed of Icterus spurius (Orchard Oriole) and Icterus fuertesi (Fuertes' Oriole). We increased the geographic sampling, added four nuclear loci, and used a range of population genetic and coalescent methods to examine the divergence between the taxa. With increased taxon sampling, we found evidence of clear structure between the taxa for mitochondrial DNA. However, nuclear loci showed little evidence of population structure, indicating a very recent divergence between I. spurius and I. fuertesi. Another goal was to examine the genetic variation within each taxon to look for evidence of a past founder event within the I. fuertesi lineage. Based on the high amounts of genetic variation for all nuclear loci, we found no evidence of such an event – thus, we found no support for the possible founding of I. fuertesi through a change in migratory behavior, followed by peripheral isolates speciation. Our results demonstrate that these two taxa are in the earliest stages of speciation, at a point when they have fixed differences in plumage color that are not reflected in monophyly of the mitochondrial or nuclear DNA markers in this study. This very recent divergence makes them ideal for continued studies of species boundaries and the earliest stages of speciation.
Highlights
For the last 25 years, neutral molecular markers, especially mitochondrial DNA, have been used to examine closely related species – elucidating their evolutionary histories and looking for evidence of gene flow
Our study focuses on a pair of taxa that represent a well-documented case of mitochondrial paraphyly: the Orchard Oriole complex
The lack of genetic variation within I. fuertesi reported by Baker et al (2003) could have resulted from differences in sampling effort – there were roughly twice as many I. spurius individuals as I. fuertesi included in their study. We addressed these questions regarding the mitochondrial paraphyly between these taxa, and the potential founder event involving I. fuertesi by sequencing multiple loci and through an increase in sample size, adding nine I. fuertesi samples
Summary
For the last 25 years, neutral molecular markers, especially mitochondrial DNA (mtDNA), have been used to examine closely related species – elucidating their evolutionary histories and looking for evidence of gene flow. Studies of closely related taxa often address species delimitation by looking for reciprocal monophyly in mitochondrial genes (Moritz 1994; Zink and McKitrick 1995; Funk and Omland 2003; Hebert et al 2004; Zink and Barrowclough 2008). Mitochondrial DNA has two key advantages that make it useful in this regard: (1) mtDNA has a rapid mutation rate, allowing for increased chances of accumulating variation; (2) mtDNA has faster sorting (genetic drift) due to its maternal inheritance and generally lower effective population sizes (ESSs; Avise 1994; Zink and Barrowclough 2008). Individual gene trees of closely related taxa can have unpredictable amounts of variation due to stochasticity in mutation, genetic drift,
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