Species from Darwin onward.

Controversy regarding the species problem has been going on for many decades and no consensus has ever been reached about what a "species" really is and how best to define the concept. De Queiroz (1998) introduced a distinction between two aspects of this problem: on the one hand, the definition proper, and on the other, the criteria allowing biologists to recognize species in practice. This distinction is a first step on the way toward a solution of the problem. In the present paper, we show that de Queiroz's distinction is made possible by the radical theoretical change introduced by Darwin. We emphasize that the species problem did not appear in the 20th century, but long before, and that Darwin addresses it indirectly in the Origin of Species. It might seem paradoxical to refer to Darwin's views about species, because they are usually considered as unclear. However, we propose that an analysis of these views in the context of Darwin's own theory of evolution might reveal how a definition of the concept of species is made possible by being anchored to the very theory of evolution. To this aim, we present a plausible reconstruction of Darwin's implicit conception of species and show how this conception fits with the debates on species that took place in the 18th and 19th centuries. We then turn to today's biology and show what changes Darwin's implicit conception of species has brought about relative to the species concept and species delimitation.