Species as lineages

An alternative way of conceiving of species is to see them as segments of historical lineages. A historical lineage is, according to Bradley E. Wilson, “a sequence of reproducing entities, individuated in terms of its components”.⁷⁸² Kevin de Queiroz describes it as “a single line of direct ancestry and descent”.⁷⁸³ A species is then a properly chosen segment of a historical lineage. The explanation to why it must be a segment and not a whole historic lineage is that species otherwise would be absurdly large. In fact, all organisms descend from the same RNA-molecule, which means that if we talked about whole historic lineages as species, all organisms would belong to the same species.⁷⁸⁴ Not any segment can count as a species, however. There is no consensus about how the segments should be cut, but for de Queiroz it is important that a segment that counts as a species has a demarcation line created by some critical event.⁷⁸⁵

782 Wilson, Bradley E. 1995 p.339

783 de Queiroz 1999 p.50

784 When I use the term ‘lineage’ I will therefore use it as short for ‘segment of historical lineage’.

785 de Queiroz 1999 p.53

The lineage idea is often seen as part of the idea that species are individuals but it has also been proposed as an independent ontology. Both de Queiroz and Bradley E. Wilson believe that the idea of species as segments of historical lineages (or for short – lineages) can be seen as an argument that species are individuals.⁷⁸⁶ Wilson also believes, however, that by only seeing species as lineages we can avoid the problems that follow with the notion of species as individuals.⁷⁸⁷

This seems to be true about the most important problem we found in connection with the idea of species as individuals. By seeing species as lineages but not as individuals we do not have to claim that all organisms of a species are organised and interact in the same way as parts of an individual organism, organisation, country, etc. The only thing we have to maintain regarding the organisms belonging to the same lineage is that they share the same descendant or are descendants of each other.

Another benefit of the term ‘historical lineage’ over the term ‘individual’ is that the former is less prone to misuse and misunderstanding. The paradigm cases of moral objects are organisms. If we see species as individuals in a way that is very different from the way in which organisms are individuals without being very clear about the differences, we run the risk that species are also regarded as organisms in a morally relevant sense. We would therefore also risk that what looks like a victory for ecocentrism turns out to be a simple linguistic shift of meaning.

A major advantage of the lineage idea is, according to de Queiroz, that it fits with all species concepts. He claims that all species concepts actually assume that species are segments of lineages. Seeing species as lineages is, in his view, the one thing that all species concepts have in common.⁷⁸⁸ What distinguishes the different species concepts is, according to him, just how the segments are cut.⁷⁸⁹

During my investigation of species concepts and ontologies I have found that many of the authors on the subject do, at least occasionally, talk about species as lineages independently of what concept or ontology they defend.⁷⁹⁰ This seems to support de Queiroz’s claim. My investigation has, however, been very limited and there are others who have reached other conclusions. Thomas Reydon has consulted a number of overviews of species concepts, and concluded that only a minority of them see species as lineages or segments of lineages.⁷⁹¹

I do not believe that the question of whether most of the proponents of different species concepts talk about them as lineages is a conclusive argument in either direction, however. The really interesting question is whether species,

786 de Queiroz 1999 p.67, Wilson, Bradley E. 1995 p.339

787 Wilson, Bradley E. 1995 pp.339, 341, 353f, passim

788 de Queiroz 1999 pp.49, 60ff, 79. Horvath also claims that all taxa are lineages (Horvath 1997 p.228).

789 de Queiroz 1999 p.53. At one point, Dupré argues along the same lines (Dupré 1999 p.10).

790 See e.g. Donoghue 1985 p.179, Dupré 1999 p.10, Ereshefsky 1991 pp.86ff, Hull 1999 p.41, Mishler 1999 p.313, Horvath 1997 pp.226, 228, Rolston 1988 p.149, Rolston 1994 p.36, Rolston 1999 p.123, Sober 1993 pp.155f, Sokal 1973 p.363, Sterelny 1999 pp.120, 123

791 Reydon 2005 p.141

according to all the serious contenders among species concepts, can be seen as lineages.

That phylogenetic species can be seen as existing in the form of historical lineages is quite clear.⁷⁹² Seeing species as segments of historical lineages is in fact a perfect fit for the phylogenetic species concept.

According to de Queiroz, the biological species concept can also be fitted into this ontology because interbreeding populations can be seen as “time-limited” segments of historical lineages.⁷⁹³

This might work as long as we have reasonably strong criteria for what counts as an interbreeding population, and as long as we do not use too restrictive time limits. To talk about a historical lineage of one generation seems absurd.

The really troublesome case ought to be the phenetic species concept. It is not easy to see a species defined by common properties as a lineage the way lineages have been defined by de Queiroz and Bradley E. Wilson. Nothing guarantees that organisms with the same defining set of properties belong to the same segment of historical lineage.

On the other hand, belonging to the same lineage can, in itself, be seen as a property even though this probably is quite far from the kind of properties that the proponents of phenetics traditionally have had in mind. Even so, this is how de Queiroz chooses to argue. If we instead see properties other than belonging to the same lineage segment as just contingent properties instead of necessary criteria, and concentrate on the species as lineage segments, the species problem will, according to him, disappear.⁷⁹⁴

By maintaining that there is one property (being a segment of a certain historical lineage) that is unique for the species category and that defines all species as species, de Queiroz can maintain that species exist objectively as segments of lineages, but that different segments can be identified as species based on different contingent properties.⁷⁹⁵

As an effect of this, he also believes that the lineage concept can account for both monism and pluralism. It allows for pluralism in the details but has a monistic basis – the lineage. Thereby he argues that the conflict between monism and pluralism disappears.⁷⁹⁶

As I interpret him, all species exist in the form of lineages, but the lineages are cut off from other lineages in different ways. Some species – i.e. some lineage segments – are distinguished by their ecologic niche while other species are distinguished by intrinsic reproductive isolation, etc.

One writer who opposes the suggestion that lineages can be an ontological category, is Walter Bock. He claims that those who see species as lineages have conflated two distinct concepts, viz. representations and ontologies. According to

792 Wilson, Robert A. 1999:1 p.192

793 de Queiroz 1999 p.54

794 de Queiroz 1999 pp.75, 79

795 de Queiroz 1999 p.75

796 de Queiroz 1999 p.73

Bock, lineages are representations of a species history and do not really exist in nature.⁷⁹⁷

I believe it does make sense to see lineages as a representation of a species history. On the other hand, I do not see that it only has to be a representation. We usually talk about human family lines as something that actually exist even though some or most of the human individuals who make up the family line are dead. It is true that we also say that a family line is dead when all members of that line are dead, but that does not contradict the previous statement. In fact, if we reason in the same way about species as lineages, it will solve one of the problems that haunted the ideas of species as classes and species as natural kinds, viz. that classes and natural kinds cannot really be exterminated even if all their members disappear. In order to talk about extinction we need an ontology that allows species to go extinct. We saw that the notion of species as individuals allowed that, and it seems that species as lineages also allows for it, which clearly talks in favour of this idea from an ecocentric perspective.

The only ecocentrist who explicitly talks about species as lineages is Rolston, and he does it rather often.⁷⁹⁸ On the other hand, we have also seen that he talks about species both as natural kinds and as individuals.

Once again, it might be possible to conceive of species as both kinds and individuals, but doing so makes things more complicated for the ecocentrists.

Here we have seen that the idea of species as lineages is often used to support the idea of species as individuals, but that the best use of the idea that species are lineages might be an alternative to the idea of species as individuals. De Queiroz does not mention natural kinds but he claims that seeing species as lineages can be used to reconcile the idea of species as individuals with the idea of species as classes. His way of doing so takes the form of extracting lineages as the real ontology. However, doing this with classes leads to a problem. It implies that we see species as “the class or set of organisms that make up a particular population-level lineage segment”.⁷⁹⁹ In this way, de Queiroz admits, classes will be spatiotemporally restricted which is not normally how they are conceived of in the classes/individuals debate.⁸⁰⁰ If we try the same manoeuvre with natural kinds we get the same result, and considering the central role the question of spatiotemporal restrictedness plays in the debate between the ideas of species as natural kinds and species as individuals, this is probably a very serious problem.

Rolston only briefly states in one passage how he sees the relation between lineages and natural kinds. He explains that “species are the actual historical lineages through which the natural kinds travel through time”.⁸⁰¹ He does not expand on what he means by this but it looks like he means that species are actually historical lineages while natural kinds are something else that move along the lineages. He does not give us any hint what the natural kinds that move

797 Bock 2004 p.179

798 Rolston 1988 p.149, Rolston 1994 p.36, Rolston 1999 p.123

799 de Queiroz 1999 p.67

800 de Queiroz 1999 pp.67f

801 Rolston 1994 p.36

along the lineages consist of, however. Considering that he in other places, clearly states that species are natural kinds, the statement above is quite confusing. If he means that species are both lineages and natural kinds, we have to ask why and how do species move through species?

What would Rolston and his colleagues say about dropping the other ontologies and just seeing species as lineages? It would make things much easier, and we would avoid the quite severe problems we have found in the other ontologies. The only draw back would be that it is intuitively less obvious that a lineage can be a moral object than that an individual can. However, as we saw above, this intuition can turn out to be an illusion based on a shift of meaning since we have found that the only way of conceiving of species as individuals is to accept a very different notion of ‘individual’ than we use when we talk about organisms as individuals. It might therefore instead be an advantage for the ecocentrists to have a “clean” terminology that lets them prove their point without any shift of meaning. All in all, it seems that seeing species as segments of historical lineages is the best bet for the ecocentrists.