VICARIANCE AND DISPERSAL ACROSS BAJA CALIFORNIA IN DISJUNCT MARINE FISH POPULATIONS
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Population disjunctions, as a first step toward complete allopatry, present an interesting situation to study incipient speciation. The geological formation of the Baja California Peninsula currently divides 19 species of fish into disjunct populations that are found on its Pacific Coast and in the northern part of the Gulf of California (also called the Sea of Cortez), but are absent from the Cape (Cabo San Lucas) region. We studied the genetic makeup of disjunct populations for 12 of these 19 fish species. Phylogeographic patterns for the 12 species can be separated into two major classes: a first group (eight species) showed reciprocal monophyly and high genetic divergence between disjunct populations. A second group (four species) displayed what appeared to be panmictic populations. Population structure between Pacific Coast populations, across the Punta Eugenia biogeographic boundary, was also evaluated. While dispersal potential (inferred by pelagic larval duration) was a poor predictor of population structure between Gulf of California and Pacific populations, we found that population genetic subdivision along the Pacific Coast at Punta Eugenia was always positively correlated with differentiation between Pacific and Gulf of California populations. Vicariant events, ongoing gene flow, and ecological characteristics played essential roles in shaping the population structures observed in this study.Keywords:
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Disjunct distributions among wet-zone taxa of the Indian subcontinent have intrigued biologists for decades. Most authors have invoked variations of either the dispersal or the vicariance model to explain disjunct distribution. However, some have noted that incorrect taxonomy, due to convergence in morphological characters, can erroneously suggest disjunct distribution. An appropriate approach to test these models (vicariance, dispersal and convergence) is to use molecular phylogenetic methods. A survey of recent molecular phylogenetic studies on Indian systems with disjunct distribution suggests that convergence may be quite common. Therefore, I propose that the first step in studying disjunct distributions is to determine if the observed pattern is real (true disjunct) and not due to convergence, i.e. an artifact of incorrect taxonomy (false disjunct).
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Aim To illustrate the use of natural historical data to evaluate vicariance and dispersal as hypotheses competing to explain disjunct populations. Location Nine disjunct areas on the margin of the Tyrrhenian basin of the Mediterranean Sea. Methods First describe how each hypothesized mechanism might explain the observed morphological variation in the model species complex, Genista ephedroides (Fabaceae); then confront the hypotheses with natural historical data including geology, oxygen isotopes, palynology, macro‐, micro‐ and nano‐fossils, and sea level changes, and with the ecological tolerances of the model species complex. Results Dispersal seems the more credible explanation. Main conclusion Patterns of morphological (or other) variation among related disjunct taxa can fit both vicariance and dispersal hypotheses. However they can possibly be distinguished by considering natural historical data.
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Abstract The present paper reviews advances in the study of two major intercontinental disjunct biogeographic patterns: (i) between Eurasian and western North American deserts with the Mediterranean climate (the Madrean–Tethyan disjunctions); and (ii) between the temperate regions of North and South America (the amphitropical disjunctions). Both disjunct patterns have multiple times of origin. The amphitropical disjunctions have largely resulted from long‐distance dispersal, primarily from the Miocene to the Holocene, with available data indicating that most lineages dispersed from North to South America. Results of recent studies on the Mediterranean disjuncts between the deserts of Eurasia and western North America support the multiple modes of origin and are mostly consistent with hypotheses of long‐distance dispersal and the North Atlantic migration. Axelrod's Madrean–Tethyan hypothesis, which implies vicariance between the two regions in the early Tertiary, has been favored by a few studies. The Beringian migration corridor for semiarid taxa is also supported in some cases.
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Allopatry and allopatric speciation can arise through two different mechanisms: vicariance or colonization through dispersal. Distinguishing between these different allopatric mechanisms is difficult and one of the major challenges in biogeographical research. Here, we address whether allopatric isolation in an endemic island lizard is the result of vicariance or dispersal. We estimated the amount and direction of gene flow during the divergence of isolated islet populations and subspecies of the endemic Skyros wall lizard Podarcis gaigeae, a phenotypically variable species that inhabits a major island and small islets in the Greek archipelago. We applied isolation-with-migration models to estimate population divergence times, population sizes and gene flow between islet-mainland population pairs. Divergence times were significantly correlated with independently estimated geological divergence times. This correlation strongly supports a vicariance scenario where islet populations have sequentially become isolated from the major island. We did not find evidence for significant gene flow within P. g. gaigeae. However, gene-flow estimates from the islet to the mainland populations were positively affected by islet area and negatively by distance between the islet and mainland. We also found evidence for gene flow from one subspecies (P. g. weigandi) into another (P. g. gaigeae), but not in the other direction. Ongoing gene flow between the subspecies suggests that even in this geographically allopatric scenario with the sea posing a strong barrier to dispersal, divergence with some gene flow is still feasible.
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More than 50 species of tropical shore fishes occur on both sides of the Pacific, despite the 5000 km separating the islands of the central Pacific from the offshore islands of the eastern Pacific. We examined 12 such trans-Pacific species, using populations from Hawaii and the eastern Pacific, with starch gel electrophoresis, in order to determine whether the allopatric populations are characterized by significant genetic differences. Values of genetic distance (D), computed for an average of 35 presumptive gene loci, ranged from <0.01-0.06. These values are markedly lower than those reported for amphi-American populations separated by the Isthmus of Panama for about three million years. For Priacanthus cruentatus which is circumtropical, comparison of the Pacific samples gave a D of <0.01, but the Atlantic vs Pacific comparison gave a D of 0.14. The results are compatible with a long-range dispersal hypothesis, with either recent dispersal or continuing gene flow. They are not consistent with a vicariant hypothesis which would regard the trans-Pacific species as relicts of a former world-wide, Tethyan distribution.
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Abstract The Baja California region provides a natural setting for studying the early mechanisms of allopatric speciation in marine systems. Disjunct fish populations from several species that occur in the northern Gulf of California and northern Pacific coast of Baja California, but are absent from its southern shores, were previously shown to be genetically isolated, making them excellent candidates for studying allopatry. In addition, one of these species, the sargo Anisotremus davidsonii , has two pairs of congeneric Panamic trans‐isthmian geminate species that allow for internal molecular clock calibration. Phylogeographic and demographic approaches based on mitochondrial (cytochrome b ) and nuclear (S7 ribosomal protein) sequences showed that A . davidsonii entered the gulf from the south, and later colonized the Pacific coast, approximately 0.6–0.16 million years ago. Pacific coast colonization may have used a route either around the southern cape of Baja California or across the peninsula through a natural seaway. However, while several seaways have been described from different geological times, none matches the dates of population disjunction, yet much geological work remains to be done in that area. At the present time, there is no evidence for dispersal around the southern tip of the Baja California Peninsula. Signatures of incipient allopatric speciation were observed, such as the reciprocal monophyly of disjunct populations for the mitochondrial marker. However, other characteristics were lacking, such as a strong difference in divergence and coalescence times. Taken together, these results suggest that disjunct populations of A . davidsonii may be consistent with the earliest stages of allopatric speciation.
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Journal Article Randomness in Allopatric Speciation Get access Sydney Anderson, Sydney Anderson Department of Mammalogy, The American Museum of Natural HistoryCentral Park West at 79th Street, New York, New York 10024 Search for other works by this author on: Oxford Academic PubMed Google Scholar Mary K. Evensen Mary K. Evensen Department of Mammalogy, The American Museum of Natural HistoryCentral Park West at 79th Street, New York, New York 10024 Search for other works by this author on: Oxford Academic PubMed Google Scholar Systematic Biology, Volume 27, Issue 4, December 1978, Pages 421–430, https://doi.org/10.1093/sysbio/27.4.421 Published: 01 December 1978 Article history Received: 01 July 1977 Revision received: 01 August 1978 Published: 01 December 1978
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