The «correct» definition of species is regularly called into question by advances in knowledge about evolution and speciation. This question is far from being resolved because speciation is at work in current biodiversity and the taxa of specific rank are therefore not certain species, but rather hypotheses of species that need to be tested. Darwin was already emphasising the importance of «doubtful species» in his argument about variation. We
Let us illustrate this property of living organisms to resist classification with the example of fuzzy taxa. These can be summarised in two main types, the syngamon and incipient species. The frequency of gene flow between taxa of specific rank, and the ecological and evolutionary role of introgression, show that uncertain species are a major aspect of biodiversity issues.
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