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Mallet J.. Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation. Phil Trans Roy Soc B Biol Sci 363: 2971-2986

Galton Laboratory, University College London, 4 Stephenson Way, London NW1 2HE, UK.
Philosophical Transactions of The Royal Society B Biological Sciences (Impact Factor: 6.31). 07/2008; 363(1506):2971-86. DOI: 10.1098/rstb.2008.0081
Source: PubMed

ABSTRACT Species are generally viewed by evolutionists as 'real' distinct entities in nature, making speciation appear difficult. Charles Darwin had originally promoted a very different uniformitarian view that biological species were continuous with 'varieties' below the level of species and became distinguishable from them only when divergent natural selection led to gaps in the distribution of morphology. This Darwinian view on species came under immediate attack, and the consensus among evolutionary biologists today appears to side more with the ideas of Ernst Mayr and Theodosius Dobzhansky, who argued 70 years ago that Darwin was wrong about species. Here, I show how recent genetic studies of supposedly well-behaved animals, such as insects and vertebrates, including our own species, have supported the existence of the Darwinian continuum between varieties and species. Below the level of species, there are well-defined ecological races, while above the level of species, hybridization still occurs, and may often lead to introgression and, sometimes, hybrid speciation. This continuum is evident, not only across vast geographical regions, but also locally in sympatry. The existence of this continuum provides good evidence for gradual evolution of species from ecological races and biotypes, to hybridizing species and, ultimately, to species that no longer cross. Continuity between varieties and species not only provides an excellent argument against creationism, but also gives insight into the process of speciation. The lack of a hiatus between species and ecological races suggests that speciation may occur, perhaps frequently, in sympatry, and the abundant intermediate stages suggest that it is happening all around us. Speciation is easy!

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    • "While many biologists still favor the biological species concept, which relies on complete reproductive isolation to delineate species, several authors highlight the problems that this approach might have with regard to understanding the underlying processes that actually drive speciation (Abbott et al. 2008; Mallet 2008; Abbott et al. 2013). Complete isolation is merely the end-point of the divergence process, but effective barriers to gene flow due to factors like ecology, geography, and phenology can arise well before full isolation is achieved (Butlin et al. 2008; Lexer and Widmer 2008; Nosil and Feder 2012). "
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    ABSTRACT: An increasing number of studies of hybridization in recent years have revealed that complete reproductive isolation between species is frequently not finalized in more or less closely related organisms. Most of these species do, however, seem to retain their phenotypical characteristics despite the implication of gene flow, highlighting the remaining gap in our knowledge of how much of an organism's genome is permeable to gene flow, and which factors promote or prevent hybridization. We used AFLP markers to investigate the genetic composition of three populations involving two interfertile Rhododendron species: two sympatric populations, of which only one contained hybrids, and a further hybrid-dominated population. No fixed differences between the species were found, and only 5.8% of the markers showed some degree of species differentiation. Additionally, 45.5% of highly species-differentiating markers experienced significant transmission distortion in the hybrids, which was most pronounced in F1 hybrids, suggesting that factors conveying incompatibilities are still segregating within the species. Furthermore, the two hybrid populations showed stark contrasting composition of hybrids; one was an asymmetrically backcrossing hybrid swarm, while in the other, backcrosses were absent, thus preventing gene flow.
    Ecology and Evolution 07/2015; DOI:10.1002/ece3.1570 · 1.66 Impact Factor
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    • "Traditional taxonomy based purely on phenotypic characters has been faced with the accumulation of molecular data, and mainly with the growing notion that speciation is a continuous process (de Queiroz, 2007; de Queiroz et al., 1998; Edwards et al., 2005; Mallet, 2008). One of the most visible consequences of that is the lack of a consensual species definition, which is mirrored by the emergence of different species concepts and delimitation methods (de Queiroz, 2007; Edwards et al., 2005; Yang and Rannala, 2010). "
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    ABSTRACT: The genus Xiphorhynchus is a species rich avian group widely distributed in Neotropical forests of Central and South America. Although recent molecular studies have improved our understanding of the spatial patterns of genetic diversity in some species of this genus, most are still poorly known, including their taxonomy. Here, we address the historical diversification and phylogenetic relationships of the X. guttatus / susurrans complex, using data from two mitochondrial (cyt b and ND2) and one nuclear (β-fibint7) genes. Phylogenetic relationships were inferred with both gene trees and a Bayesian-based species tree under a coalescent framework (∗BEAST). With exception of the nuclear β-fibint7 gene that produced an unresolved tree, both mtDNA and the species tree showed a similar topology and were congruent in recovering five main clades with high statistical support. These clades, however, are not fully concordant with traditional delimitation of some X. guttatus subspecies, since X. g. polystictus, X. g. guttatus, and X. connectens are not supported as distinct clades. Interestingly, these three taxa are more closely related to the mostly trans-Andean X. susurrans than the other southern and western Amazonian subspecies of X. guttatus, which constitutes a paraphyletic species. Timing estimates based on the species tree indicated that diversification in X. guttatus occurred between the end of the Pliocene and early Pleistocene, likely associated with the formation of the modern Amazon River and its main southern tributaries (Xingu, Tocantins, and Madeira), in addition to climate-induced changes in the distribution of rainforest biomes. Our study supports with an enlarged dataset a previous proposal for recognizing at least three species level taxa in the X. guttatus / susurrans complex: X. susurrans, X. guttatus, and X. guttatoides. Copyright © 2015 Elsevier Inc. All rights reserved.
    Molecular Phylogenetics and Evolution 02/2015; 85. DOI:10.1016/j.ympev.2015.02.004 · 4.02 Impact Factor
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    • "In comparative ecology, the proper naming of species is essential. Historically, ecological studies have assigned a particular name to a particular entity based on the Darwinian species concept, which uses morphological characters to separate clusters of individuals into species (Darwin 1859; Mallet 2008). While studying ecological patterns at large scales, ecologists are often unable to identify all individuals encountered in the field to species. "
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    ABSTRACT: While studying ecological patterns at large scales, ecologists are often unable to identify all collections, forcing them to either omit these unidentified records entirely, without knowing the effect of this, or pursue very costly and time-consuming efforts for identifying them. These "indets" may be of critical importance, but as yet, their impact on the reliability of ecological analyses is poorly known. We investigated the consequence of omitting the unidentified records and provide an explanation for the results. We used three large-scale independent datasets, (Guyana/ Suriname, French Guiana, Ecuador) each consisting of records having been identified to a valid species name (identified morpho-species - IMS) and a number of unidentified records (unidentified morpho-species - UMS). A subset was created for each dataset containing only the IMS, which was compared with the complete dataset containing all morpho-species (AMS: = IMS + UMS) for the following analyses: species diversity (Fisher's alpha), similarity of species composition, Mantel test and ordination (NMDS). In addition, we also simulated an even larger number of unidentified records for all three datasets and analyzed the agreement between similarities again with these simulated datasets. For all analyses, results were extremely similar when using the complete datasets or the truncated subsets. IMS predicted ≥91% of the variation in AMS in all tests/analyses. Even when simulating a larger fraction of UMS, IMS predicted the results for AMS rather well. Using only IMS also out-performed using higher taxon data (genus-level identification) for similarity analyses. Finding a high congruence for all analyses when using IMS rather than AMS suggests that patterns of similarity and composition are very robust. In other words, having a large number of unidentified species in a dataset may not affect our conclusions as much as is often thought.
    Ecology and Evolution 12/2014; 4(24). DOI:10.1002/ece3.1246 · 1.66 Impact Factor
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