The Concept Of Clade Selection

I turn now to clade selection, discussed by authors including Williams (1992), Sterelny (1996a), Vermeij(1996), and Nunney (1999). A clade is a monophyletic group of species, that is, a group comprising an ancestral species, all of its descendent species, and nothing else. Clades are thus located further up the (genealogical) hierarchy than species.7 Clade selection is often presented as a natural generalization of species selection. Thus, for example, Williams (1992) writes: 'there is no reason why species selection should be recognized as a special process different from any other kind of clade selection. . . selection can take place among clades of higher than the species level' (p. 125). Similarly, Nunney (1999) says that species selection 'can be subsumed under the more general category of clade selection' (p. 247).

This idea may seem plausible, particularly if one endorses the view that all monophyletic groups, not just species, are 'individuals' in the Ghiselin/Hull sense. But in fact it faces a critical problem, for the notion of clade fitness turns out to be incoherent.

The fitness of a species is normally defined as the number of offspring species it leaves.8 This notion makes sense because species reproduce,

7 According to standard cladistic usage, followed here, a single species does not count as a clade and cannot be or fail to be monophyletic. Monophyly is a property of collections of species, not single species (Hennig 1966). However, some theorists have tried to apply the concept of monophyly to single species, usually by arguing that a species is a monophyletic group of populations (cf. Mishler and Brandon 1987). This goes hand-in-hand with a broader use of 'clade', according to which single species do count as clades.

8 Some authors define species fitness slightly differently, as the difference between speciation rate and extinction rate, by analogy with the Malthusian parameter (e.g. Vrba 1984). A similar definition of organismic fitness is used by Michod (1999). The argument below—that there is no coherent notion of clade fitness comparable to the notion of species fitness—applies whichever definition of species fitness we prefer.

that is, beget other species. But do clades reproduce too? Proponents of clade selection believe that they do. Williams (1992) explicitly describes cladogenesis as reproduction for clades (p. 52). Similarly, van Valen (1988) argues that supra-specific taxa can beget other supra-specific taxa, hence be subject to selection; he talks about the probability that 'one family gives rise to another' (p. 59).9 Sterelny (1996a) apparently concurs. He argues that clades have adaptations, and he insists that adaptations must be heritable characters, so they can be honed by cumulative selection. He rules out some alleged clade adaptations on the grounds that the characters in question are unlikely to be heritable; Vermeij (1996) makes a similar argument. Heritable means transmitted from parents to offspring, so Sterelny and Vermeij presumably think that clades can beget other clades.

However, there is a complication. For clades are by definition monophyletic, and as a matter of logic, monophyletic clades cannot stand in ancestor-descendent relations (cf. Nelson and Platnick 1984; Eldredge 1985, 2003). Ataxon which contains all the descendents of its members as proper parts cannot be ancestral to any other such taxon. To see this point, consider the cladogram in Figure 7.1. If we ask what the ancestor of the highlighted clade A is, then the answer can only be a species, not another clade. Clade A is of course a part of the larger clade B, but it is not the offspring of B. For offspring must have an independent existence from their parents, and be able to outlive them.

Figure 7.1. Cladogram showing two nested clades

9 Though van Valen does not actually use the term 'clade selection', for he does not accept cladism.

But clade A cannot outlive clade B. The only way a monophyletic clade can cease to exist is if all of its constituent species go extinct, which implies that all the sub-clades which are parts of it must cease to exist too. If clade B ceases to exist, then clade A must do so too. So A is not the offspring of B; it is simply a part of it.10

It follows that Williams's idea that cladogenesis constitutes 'reproduction for clades' is incorrect. Reproduction means one entity giving rise to another entity of the same type, but clades cannot do this. In cladogenesis, the entity that splits is a species lineage. Presuming it splits into two (which is most usual), and given the standard cladistic convention that a species automatically goes extinct when it splits, the result is a new clade containing three species—the original one (now extinct), and two new ones. But the new clade is not the offspring of any of the clades to which the original species belonged, but rather a part of them. Williams's claim that there is no reason to focus on species selection rather than clade selection is therefore wrong. There is such a reason: species give rise to offspring, hence form ancestor—descendent lineages, but clades do not. Monophyletic clades are not the sorts of thing to which fitness can be meaningfully ascribed.

How might defenders of clade selection respond? One response would be to concede that clades do not reproduce, but argue that differential extinction of clades might still occur. This is true enough. However, selection on entities that do not reproduce their kind is not very interesting, and will not lead to adaptations. All sorts of entities are subject to selection in this weak sense. A collection of atoms may have different probabilities of radioactive decay, a collection of buildings may have different probabilities of being demolished, and so on. Natural selection is only an interesting idea when applied to entities that reproduce. Moreover, clade selection in this weak sense is not a more general version of species selection. It is precisely because species do reproduce that species selection is a potentially interesting evolutionary mechanism.

Note that this is not to say that interesting cases of natural selection must involve fecundity rather than viability selection, which is certainly untrue. Rather, the point is that natural selection, whether it operates

10 In Okasha (2003a), this argument is expressed by saying that the parent—offspring relation must be capable of becoming the ancestor—descendant relation, i.e. if two entities are related as parent and offspring, it must be possible for them to become related as ancestor and descendant, at a later point in time. This means that death ofthe parental entity must not necessarily entail death of the offspring.

on differences in survival or fecundity, is only interesting when applied to entities that do in fact reproduce. Differential survival of organisms and species is interesting, because organisms and species reproduce their kind. Differential survival of clades is not, because clades do not.

A second possible response is to concede that clade fitness in the sense of number of offspring clades does not make sense, but to replace it with another notion. Why not let a clade's fitness refer to the number of sub-clades it comes to have as parts? And by heritability, we could mean resemblance between a larger clade and its sub-clades, rather than its offspring clades. Clade selection in this sense could help explain differences in 'bushiness' between clades. Fitter clades are the ones whose traits lead them to grow bushier than others.

One might object that redefining clade fitness this way means that clade selection ceases to be a genuinely Darwinian process. We do not normally appeal to differences in 'branch fitness' to explain why some branches of an oak tree contain more twigs than others, but structurally this is parallel. But in any case, there is a deeper objection to the proposed way of salvaging clade selection.

Consider the clades marked A and B in Figure 7.2, each containing two extant species. If clade A is fitter than clade B, according to the suggested redefinition, this means that A will come to contain more sub-clades as parts. But cladogenesis only occurs when species lineages split, so this means that the species in A must leave more offspring species than those in B. Therefore clade selection in the suggested sense is redundant—species selection can do all the work. The fact that clade A grows bushier than clade B is explained by the fact that the species in A are fitter than those in B. Defining clade fitness as number of sub-clades and then invoking a process of clade selection is pointless. For clade

Figure 7.2. Cladogram showing two sister clades

selection in this sense explains nothing that is not already explained by species selection.

A third (related) response also argues for a redefinition of clade fitness. Why not define the fitness of a clade as the average fitness of its constituent species? In effect, this is to suggest that we should treat clade selection as an MLS1 process, in which the focal units are not the clades themselves but rather their constituent species. The relation between clades and species would thus be analogous to the relation between groups and individuals in most models of group selection. Clearly this would circumvent the problem that clades cannot reproduce their kind; for in MLS1 it is the particles not the collectives that must stand in parent—offspring relations.

Though logically coherent, it is hard to see what the point of treating clade selection this way would be. The point of a group selection model which defines group fitness as average individual fitness is to model situations where the fitness of an individual depends on the composition of its group. If there are no group-level effects on individual fitness, there is no need for a group selection model of this sort—individual selection is the only force at work. In the clade case, there are unlikely to be clade-level effects on species fitness, in the way there are often group-level effects on individual fitness. Why should the fitness of any species depend on which other species are in its clade? Such a dependence is of course possible. For example, if a given species goes extinct, this could increase the fitness of a sister species with which it competes for resources. But it is unlikely to be a common phenomenon.

Therefore, treating clade selection as an MLS1 process, with species as the focal units, is conceptually coherent but unlikely to have useful empirical application. If average species fitness is greater in clade A than clade B, then clade A will become bushier than B. But almost certainly, this falls within the purview of species selection—the clade-level trend is a by-product. Nothing is gained by defining clade fitness as average species fitness and then attributing the difference in bushiness to clade selection. In the absence of systematic clade-level effects on species fitness, this is artificially to multiply levels of selection for no reason.

The foregoing arguments suggest that the concept of clade selection is at worst incoherent and at best simply collapses into species selection. I suggest that evolutionists should therefore abandon the concept.11

11 Though see Haber and Hamilton (forthcoming) for an attempt to salvage clade selection from the objections presented here.

Finally, let us consider a different way in which clades might be relevant to species selection. It is sometimes suggested that species selection can only take place among the members of a monophyletic clade; Vrba (1989) makes this part of her definition of species selection. Note that this has nothing to do with treating clades themselves as units of selection; rather, the suggestion is that the clade is the entity whose composition gets changed by species selection, that is, the analogue of what in ordinary Darwinian selection is usually called the 'population'.

I think that this suggestion unnecessarily restricts the concept of species selection (cf. Grantham 1995). In general, the population of entities undergoing natural selection, at whatever level, is demarcated by the requirement that its members experience a common selective force (cf. Sober and Lewontin 1982). This requirement was implicit in the abstract treatment of selection developed in previous chapters; for it is implied by the idea that character differences must cause fitness differences, within the population of entities undergoing selection. Nothing follows about the genealogical connections, if any, among the entities in the population.12

This point holds good for species selection too. It may be the case that the species in a monophyletic clade are more likely to be subject to common causal influences than those in a non-monophyletic group. For selective forces are often phylogenetically mediated, as Sterelny (1996a) notes. Just as conspecific organisms are often affected similarly by environmental changes, the same may be true of con-cladistic species. But this is an empirical issue. It does not justify making monophyly a condition of any set of species whose composition can be changed by species selection. Again, this point is illustrated by the hypothesis that species selection favoured sexual reproduction. As noted in Section 7.3, if this hypothesis is correct, then the set of species on which selection acted consists of all those species, sexual and asexual, whose fitness was affected by their capacity to evolve. There is no a priori reason to think that this set was monophyletic.

12 Grantham (1995) argues that 'the term population has a genealogical component . . . and an ecological component' (p. 311), on the grounds that populations are generally assumed to contain conspecific organisms. This is admittedly how the term 'population' is often used. But the requirement of conspecificity is indefensible. Just as selection can act on a population of asexual organisms, so it can act on a population of sexual organisms which do not exchange genetic material with each other.

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  • Dennis Thomas
    How do you count the clades?
    8 years ago

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