The biotic complexity of the Mexican Transition Zone and the evolution of Gonzalo Halffter's biogeographic thought

Abstract

Since the 19thcentury, 2 factors stood out during the studies of the complex species distribution in the central Mexican mountain ranges: the acknowledgment of biotic regions and the congruent distribution of widespread taxa. Such patterns were subsequently confirmed. However, towards the late 19thcentury and early 20thcentury, the acknowledgement of species diversity gradients prevailed, explained by the "Holarcticist approach", according to which the dominant species originated in the broad northern areas. The congruent species distribution was revisited in the second half of the 20thcentury, mainly by Léon Croizat. The complex Mexican biological diversity was now explained as the result of 2 or more convergent biotas. Afterwards, Gonzalo Halffter highlighted the fact that, within this apparently random mixture of species, several taxon assemblages with congruent geographical patterns exist, with their own evolutionary history, overlapping within the same geographical space, despite having different temporal and spatial origins. This work displays a synthesis of the development of his biogeographic ideas, from his initial dispersalist approach to his model of overlapping cenocrons, which contributed to the conception of the Mexican Transition Zone.