Mallet J.. Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation. Philos Trans R Soc Lond 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: 7.06). 07/2008; 363(1506):2971-86. DOI: 10.1098/rstb.2008.0081
Source: PubMed


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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Available from: James Mallet, Oct 04, 2015
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    • ", might be required to reduce gene flow between niche-adapted genotypes before clusters of selectively neutral genome-wide diversity may develop. Mosaic sympatry essentially means that niches are distributed patchily, without being completely allopatric (Mallet 2008). This situation might readily describe many microbial environments, such as soil, oceans, and animal hosts, in which resources are distributed in small-scale patches, but patches may be short-lived and colonizing populations may mix frequently because of the need to recolonize new patches (Polz et al. 2006). "
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    ABSTRACT: What are species? How do they arise? These questions are not easy to answer and have been particularly controversial in microbiology. Yet, for those microbiologists studying environmental questions or dealing with clinical issues, the ability to name and recognize species, widely considered the fundamental units of ecology, can be practically useful. On a more fundamental level, the speciation problem, the focus here, is more mechanistic and conceptual. What is the origin of microbial species, and what evolutionary and ecological mechanisms keep them separate once they begin to diverge? To what extent are these mechanisms universal across diverse types of microbes, and more broadly across the entire the tree of life? Here, we propose that microbial speciation must be viewed in light of gene flow, which defines units of genetic similarity, and of natural selection, which defines units of phenotype and ecological function. We discuss to what extent ecological and genetic units overlap to form cohesive populations in the wild, based on recent evolutionary modeling and population genomics studies. These studies suggest a continuous "speciation spectrum," which microbial populations traverse in different ways depending on their balance of gene flow and natural selection.
    Cold Spring Harbor perspectives in biology 09/2015; DOI:10.1101/cshperspect.a018143 · 8.68 Impact Factor
    • "For instance, it is often challenging to distinguish between primary divergence in sympatry vs. a secondary contact following differentiation in allopatry because neutral markers often used for demographic inference may converge to the same equilibrium under both scenarios (Endler, 1977; Barton & Hewitt, 1985; Bierne et al., 2013). In addition, population divergence after primary or secondary contact does not always lead to complete reproductive isolation (sensu Mayr, 1947) and to the formation of a new species (Mallet, 2008; Hendry, 2009; Elias et al., 2012; Nosil, 2012). As a result, it is important to combine experimental approaches analysing reproductive barriers with inferences of gene flow in replicated population pairs to measure the level of reproductive isolation between putative species (e.g. "
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    ABSTRACT: Ecologically based divergent selection is a factor that could drive reproductive isolation even in the presence of gene flow. Population pairs arrayed along a continuum of divergence provide a good opportunity to address this issue. Here we used a combination of mating trials, experimental crosses and population genetics analyses to investigate the evolution of reproductive isolation between two closely related species of lampreys with distinct life histories. We used microsatellite markers to genotype over 1000 individuals of the migratory parasitic river lamprey (Lampetra fluviatilis) and freshwater-resident nonparasitic brook lamprey (L. planeri) distributed in 10 sympatric and parapatric population pairs in France. Mating trials, parentage analyses and artificial fertilizations demonstrated a low level of reproductive isolation between species even though size assortative mating may contribute to isolation. Most parapatric population pairs were strongly differentiated due to the joint effects of geographic distance and barriers to migration. In contrast, we found variable levels of gene flow between sympatric populations ranging from panmixia to moderate differentiation, which indicates a gradient of divergence with some population pairs that may correspond to alternative morphs or ecotypes of a single species and others that remain partially isolated. Ecologically-based divergent selection may explain these variable levels of divergence among sympatric population pairs but incomplete genome swamping following secondary contact could have also played a role. Overall, this study illustrates how highly differentiated phenotypes can be maintained despite high levels of gene flow that limit the progress toward speciation. This article is protected by copyright. All rights reserved.
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    • "Importantly the lineage perspective helps us reconsider the process of evolution over long time periods. For instance, because evolutionary lineages are continuous in time, those lineages represented by taxonomic units such as species can be subdivided into further lineages that reflect classification of subspecies, varieties or metapopulations that encompass intraspecific variation (Mallet, 2008a; Dynesius & Jansson, 2014). "
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    ABSTRACT: Using the framework of evolutionary lineages to separate the process of evolution and classification of species, we observe that ‘anagenesis’ and ‘cladogenesis’ are unnecessary terms. The terms have changed significantly in meaning over time, and current usage is inconsistent and vague across many different disciplines. The most popular definition of cladogenesis is the splitting of evolutionary lineages (cessation of gene flow), whereas anagenesis is evolutionary change between splits. Cladogenesis (and lineage-splitting) is also regularly made synonymous with speciation. This definition is misleading as lineage-splitting is prolific during evolution and because palaeontological studies provide no direct estimate of gene flow. The terms also fail to incorporate speciation without being arbitrary or relative, and the focus upon lineage-splitting ignores the importance of divergence, hybridization, extinction and informative value (i.e. what is helpful to describe as a taxon) for species classification. We conclude and demonstrate that evolution and species diversity can be considered with greater clarity using simpler, more transparent terms than anagenesis and cladogenesis. Describing evolution and taxonomic classification can be straightforward, and there is no need to ‘make words mean so many different things’.
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