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When monophyly is not enough: exclusivity as the key to defining a phylogenetic species concept

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Abstract

A natural starting place for developing a phylogenetic species concept is to examine monophyletic groups of organisms. Proponents of “the” Phylogenetic Species Concept fall into one of two camps. The first camp denies that species even could be monophyletic and groups organisms using character traits. The second groups organisms using common ancestry and requires that species must be monophyletic. I argue that neither view is entirely correct. While monophyletic groups of organisms exist, they should not be equated with species. Instead, species must meet the more restrictive criterion of being genealogically exclusive groups where the members are more closely related to each other than to anything outside the group. I carefully spell out different versions of what this might mean and arrive at a working definition of exclusivity that forms groups that can function within phylogenetic theory. I conclude by arguing that while a phylogenetic species concept must use exclusivity as a grouping criterion, a variety of ranking criteria are consistent with the requirement that species can be placed on phylogenetic trees.

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Notes

  1. By a ‘diverging’ phylogeny I mean that branches split but never join. Equivalently, each node (other than the root) has precisely one parent. It is tempting to think that a diverging phylogeny means that the horizontal distance between tips represents phenotypic divergence as well but this is not correct. For example, instances of convergent evolution take place on a diverging tree.

  2. Baum and Shaw (1995) anticipate much of the above discussion and conclude that taxa should be exclusive groups, but deny that exclusivity can be understood in MRCA terms. They do this because they wish to define species as basal taxa and some exclusive groups would be too small to be species. However, here, I do not assume that species are the smallest exclusive groups; rather, I am searching for a general phylogenetic grouping criterion and a general understanding of exclusive groups of organisms.

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Acknowledgments

Thanks to Matt Barker, David Baum, Marc Ereshefsky, Casey Helgeson, Brent Mishler, Greg Novack, Elliott Sober, Kim Sterelny, and John Wilkins, all of whom provided helpful comments on earlier drafts of this paper.

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Correspondence to Joel D. Velasco.

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Velasco, J.D. When monophyly is not enough: exclusivity as the key to defining a phylogenetic species concept. Biol Philos 24, 473–486 (2009). https://doi.org/10.1007/s10539-009-9151-4

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  • DOI: https://doi.org/10.1007/s10539-009-9151-4

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