Descent (Filiation)

Abstract

Darwing’s writings about filiation have not always been understood, especially by the French readership. The French translation of the term 'descent is part of the problem. The tree that Darwin published in 1859 is a theoretical genealogical framework but, here again, it has not always been understood as such. We show that the classificatory agenda of Darwin included a constraint of monophyly. Darwin clearly recommended what to do with classifications, but was mistranslated, and did not provide the methods to achieve the agenda. Those methods will arrive a century later, through the phylogenetic systematics proposed by Willi Hennig. The basic reasoning of the method is summarized. We explain why the notion of organization plan is an anti-phylogenetic concept, and why homology is not a circular concept. The difference between genealogy and phylogeny is important to recall here, as we do not confuse them anymore. We distinguish three trees: the theoretical genealogical tree at the ontological level, the phylogenetic tree at the epistemic level, and the metaphor of 'tree of life used in the step of summarizing knowledge, often to the public. We explain why, in the phylogenetic realm, ancestor-descendants relationships between two concrete organisms (either extant or fossil) are flawed. Modern phylogenetics is called precisely when concrete genealogy is not accessible anymore. This could appear paradoxical, but it is not. Concrete ancestors are individually unknowable at the empirical level, abstract ancestors are theoretically required at the ontological level, and this is why abstract ancestors can be partially reconstructed by a phylogeny at the epistemic level. We claim, opposing to some recent writings, that in the realm of phylogenetics, concrete ancestors are unknowable. If we do know them individually, phylogenetics is not useful anymore and genealogy comes back at the epistemic level. Then we discuss the conditions and meanings of exportation of trees in other scientific fields than biological systematics.