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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    • "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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    • "Whether and which ancient Homo taxa (including Pleistocene H. sapiens ) can be considered monophyletic, genomically coherent species will have to await sequences of numerous genomes from across the range of their distribution(s). However, this description of what keeps species distinct also implies that the ability to coexist geographically (sympatry) without the fusion of lineages is a result of being separate species (Mallet 2008). It is less clear whether this was the case. "
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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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