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. London, UK: John Murray, Feder, J. Genetic differentiation between sympatric host races of Rhagoletis pomonella. Nature , 61—64 Funk, D. Isolating a role for natural selection in speciation: Host adaptation and sexual isolation in Neochlamisus bebbianae leaf beetles.
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Mutation order — A major stochastic process in evolution. Mayr, E. Systematics and the Origin of Species. The Evolutionary Synthesis. Niemiller, M. Recent divergence with gene flow in Tennessee cave salamanders Plethodontidae: Gyrinophilus inferred from gene genealogies. Molecular Ecology 17 , — Nosil, P. Host-plant adaptation drives the parallel evolution of reproductive isolation.
Nature , — Ecological explanations for incomplete speciation. Divergent selection and heterogeneous genomic divergence. Molecular Ecology 18 , — Panhuis, T. Sexual selection and speciation.
Ramsey, J. Pathways, mechanisms and rates of polyploid formation in flowering plants. Annual Review of Ecology, Evolution, and Systematics 29 , — Ritchie, M. Annual Review of Ecology, Evolution, and Systematics 38 , 79— Rundle, H. Ecological speciation. Ecology Letters 8 , — Schluter, D. Ecology and the origin of species. Evidence for ecological speciation and its alternative. Science , — Seehausen, O. Speciation through sensory drive in cichlid fish. Turner, T.
Genomic islands of speciation in Anopheles gambiae. PLoS Biology 3 , e On the origin of species by natural and sexual selection. Via, S. Sympatric speciation in animals: The ugly duckling grows up. The Hardy-Weinberg Principle. Evolution Introduction. Life History Evolution. Mutations Are the Raw Materials of Evolution. Speciation: The Origin of New Species. Avian Egg Coloration and Visual Ecology. The Ecology of Avian Brood Parasitism.
The Maintenance of Species Diversity. Neutral Theory of Species Diversity. Population Genomics. Semelparity and Iteroparity. Geographic Mosaics of Coevolution. Comparative Genomics. Cybertaxonomy and Ecology. Ecological Opportunity: Trigger of Adaptive Radiation. Evidence for Meat-Eating by Early Humans. Resource Partitioning and Why It Matters. The Evolution of Aging. Citation: Safran, R. Nature Education Knowledge 3 10 How do new species form?
Like most areas of Evolutionary Biology, research related to the formation of new species - 'speciation ' - is rich in historical and current debate. Here, we review both early and modern views on speciation, starting with Darwin and finishing with current genomics-era insights. Aa Aa Aa. Darwin's "Mystery of Mysteries". The Modern Synthesis. Barriers to reproduction. The role of geography in speciation. Biologists have long been fascinated with — and sought to explain — the origin and maintenance of biological diversity within and among species.
Natural selection is generally recognized as a central mechanism of evolutionary change within species. Thus, natural selection plays a major role in generating the array of phenotypic and genetic diversity observed in nature.
But to what extent is selection also responsible for the formation of new species i. To what extent do phenotypic and species diversity arise via the same processes, as proposed by Darwin? Figure 4. Ecological speciation in host-plant associated populations of Timema cristinae walking-stick insects individual populations feed on either the Ceanothus spinosus host plant or on Adenostoma fasciculatum.
The role of sexual selection in speciation. A view that is becoming increasingly popular is that sexual selection, or selection related to variation in reproductive success, plays a role in speciation Panhuis et al. This model suggests that differential patterns of trait variation related to reproductive success within populations contribute to the reproductive isolation among populations.
A compelling example is related to the explosive radiation of cichlid fishes in the African Rift Lakes, where populations with overlapping distributions are diverging as a function of the differential preference of male color in mate selection Seehausen et al. Some models of speciation do not include a role for selection of any sort, but rather invoke a key role for chance events.
Current views: Mutation-order vs. A lack of strong examples for speciation by genetic drift, yet evidence for ecologically-similar species pairs Price , has led to the development of a powerful alternative mechanism to ecological speciation. In essence, different populations find different genetic solutions to the same selective problem.
In turn, the different genetic solutions i. Whereas different alleles are favored between two populations under ecological speciation, the same alleles would be favored in both populations under mutation-order speciation i. Divergence occurs anyway because, by chance, the populations do not acquire the same mutations or fix them in the same order. Divergence is therefore stochastic but the process involves selection, and thus is distinct from genetic drift.
Selection can be ecologically based under mutation-order speciation, but ecology does not favor divergence as such, and an association between ecological divergence and reproductive isolation is not expected. How might mutation-order speciation arise? Sexual selection might cause mutation-order speciation if reproductive isolation evolves by the fixation of alternative advantageous mutations — for example those which increase individual attractiveness — in different populations living in similar ecological environments.
For a summary of these models, see Table 1. References and Recommended Reading Butlin, R. Sympatric, parapatric or allopatric: The most important way to classify speciation? Coyne, J. Sunderland, MA: Sinauer Associates, Animal Species and Evolution. Price, T. Speciation in Birds.
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