L’espèce incertaine et les taxons flous

Advances in knowledge about evolution and speciation often raise the question of the “right” definition of species. 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 had already emphasised the importance of “doubtful species” in his argument about variation. We illustrate this property of living organisms to resist classification with the example of fuzzy taxa. These can be summed up in two main types, the syngameon 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 represents a major aspect of biodiversity issues.