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The phylogenetic species concept

6. ECOCENTRISM

6.1. W HAT IS A SPECIES ?

6.1.4. The phylogenetic species concept

only real taxonomic level.582 Other taxonomic levels like sub-species, genera, etc.

can be seen as conventions, but reproductive isolation produces (according to the theory) objectively distinguishable groups.583 This is an advantage from an ecocentric perspective since it is important for them both that species have an objective existence and that species can be justifiably seen as relevantly different from the other taxonomic levels – which we, according to ecocentrism, do not have any duties to.

The most difficult problem presented to the ecocentrists by the biological species concept is essentially the same as with the phenetic species concept: How do they connect the general idea of species as interbreeding populations with their idea of species as moral objects with a non-reducible, independent and morally relevant interest in continued existence? How do gene flow and reproductive isolation generate this kind of interests? I suspect that finding such a connection will be quite a challenging task for the ecocentrists.

This, together with the other problems we have found, means that we have to conclude that the biological species concept might be at least as problematic as the phenetic species concept from an ecocentric point of view.

cohesive and integrated. The phylogenetic species concept cannot point out one level as more important than others.589 Why, for instance, do two brothers not make up a species? They share a common ancestor that they do not share with their cousins. We can also ask: why do all primates not count as a species? We clearly have a common ancestor some millions of years down the evolutionary tree and there is no other evolutionary linage between us. Indeed, it seems that the phylogenetic species concept could accept everything from individual organisms to the whole biosphere as species. It thus seems that the problem of drawing the line between species that we have seen in both the phenetic and the biological species concepts is also present in the phylogenetic species concept.

The only difference is that the problems in this case are along the vertical plane as well as the horizontal.

It is mainly the different ways of answering the question of where to draw the line that divides phylogenetics into different versions. One simple way of cutting the segments of the phylogenetic tree into species is to count all distinguishable monophyletic groups with invariable traits as one species.590 This is the case in some versions of the phylogenetic species concept but it has some drawbacks. One problem is that we will end up with a very large number of very small species.591 Another problem is that the species category will not be an evolutionary unit.592 The latter problem is particularly serious for the ecocentrists – both because it makes it less reasonable to talk about species as individuals, and because much of their reasoning is supposed to have an evolutionary basis.

There are many other suggestions, but to give a detailed account of them would take an investigation of its own. I will therefore confine myself to the example above as an illustration of how to cut the segments, and stress that there is no consensus among the advocates of phylogenetics regarding how to solve this problem.

None the less, the phylogenetic species concept has some advantages over the biological species concept. For instance, in many cases it has turned out to be more practical.593 For some systematists, strict genealogic groupings are important, which makes the phylogenetic species concept a better choice.594 The proponents of phylogenetics also like to point out that the biological species concept can accept non-monophyletic groups as species.595 This means that individuals on different branches of the evolutionary tree might form interbreeding populations even though there are other branches between them on the evolutionary tree that are not included in the species. This in turn means that an organism can be closer related in terms of common ancestors to some organisms outside the species than to some organisms within the species.

Donoghue considers this to be a very serious problem that shows the biological

589 Horvath 1997 p.226

590 Donoghue 1985 p.179, Dupré 1999 p.11

591 Dupré 1993 p.49, Dupré 1999 p.11

592 Sterelny 1999 p.136 note 3.

593 Harris & Froufe 2005 p.6

594 Donoghue 1985 p.177

595 Donoghue 1985 pp.175, 176f, Ereshefsky 1999 pp.292f

species concept to be arbitrary, misleading and counterintuitive.596 On the other hand, the advocates of the biological species concept can defend themselves by pointing out that this argument begs the question in that it is only a problem for those who already believe that phylogenetic descent is more important than belonging to a reproductively isolated population. It does not supply any argument to why phylogenetic descent should be more important than reproductive isolation.

There are other independent arguments for that, however. One thing that is seen as an advantage of sorting organisms by their evolutionary history is that it accounts for the importance of evolution as the process behind biodiversity.597 There are different opinions regarding the general appropriateness of the phylogenetic species concept when it comes to studying evolutionary processes, however. Grandcolas believes it is very well suited for that purpose,598 while Dupré claims that we may have to divide the organisms into groups that crosscut phylogenetic lineages in order to study their evolutionary roles.599

Another problem regarding the phylogenetic species concept is that actual classification is in practice made by DNA-analysis, but genetic similarity is not always a guarantee for evolutionary relatedness. Genes can, for example, transfer spontaneously between different species of micro-organisms (and possibly also between other species), which makes it difficult, in practice, to make the theoretically important distinction between genetic similarity and evolutionary relatedness.600

From an ecocentric viewpoint it is an advantage that the phylogenetic species concept fits very well with the ontological notion of species as individuals or lineages. This is also claimed to be the case with the biological species concept,601 but the phylogenetic concept is probably even more suited for giving species the ontological status as individuals. The notion of species as individuals is a favourite among ecocentrists and we will have a closer look at it later. The biggest problem with the phylogenetic species concept from an ecocentric perspective ought to be to figure out how the property of ‘being made up by organisms with a common ancestor’ can generate non-reducible and morally relevant interests. It does not look much easier to show how a common evolutionary history can generate such interests than how reproductive isolation or morphologic or genetic similarity may do so.

596 Donoghue 1985 pp.175ff

597 Mishler 1999 pp.309f

598 Grandcolas et al 1994 passim

599 Dupré 1999 pp.13f

600 Johansson, Birgitta 2003 p.10

601 Sober 1993 p.155