Is the concept of phylogenetic constraint of heuristic value only

Considering all the possible temporal shifts in the boundaries of the set PPFx within the set FPF discussed in the previous section, from expansion (Fig. 7.17) to contraction (Fig. 7.18) to shifting (Figs. 7.19 and 7.20) and to fragmentation (Fig. 7.21), one might conclude that anything is possible, given enough evolutionary time. That is, that the concept of phylogenetic constraint is an interesting way of thinking about how the evolutionary process might occur, but that it is impossible to use the concept in the actual analysis of evolution and thus that the concept is of heuristic value only. This question regularly appears in the works of evolutionary biologists, from the simple statement that the constraint concept is 'central to current perceptions of the evolutionary process, but operationally, it is difficult to apply' (Schwenk, 1994, p. 251) to the rhetorical question: 'phylogenetic constraint in evolutionary theory: has it any explanatory power?' (McKitrick, 1993, p. 307). On the other hand, other researchers have extended the concept of phylogenetic constraint to modelling ecological phenomena, and not just morphological (Cattin et al., 2004).

Theoretical morphospace techniques offer the powerful possibility of actually using the concept of phylogenetic constraint as an analytical tool. The discipline of theoretical morphology is in its infancy (McGhee, 1999, 2001a), yet already we have a very possible example of phylogenetic constraint in operation: the inability of the nautilid cephalopods to invade the empty morphospace previously occupied by the ammonoid cephalopods (Fig. 7.16). The morphologies present within the empty region of morphospace clearly belong to the set of functional possible form, FPF, a fact the ammonoids clearly demonstrated in their long evolutionary history. These currently nonexistent forms would function just as well today as they did for the ammonoids in the Palaeozoic, why then do they remain nonexistent? Why have not the related swimming cephalopods, the nautilids, evolved these functional forms in the 65 million years following the ammonoids demise? It is difficult to escape the conclusion that phylogenetic constraint is in operation, and that natural selection is unable to overcome this constraint. McKitrick (1993, p. 307) defines phylogenetic constraint to be 'any result or component of the phylogenetic history of a lineage that prevents an anticipated course of evolution in that lineage'. In Figure 7.16 the technique of theoretical morphospace analysis has actually demonstrated an anticipated course of evolution for a lineage that has been prevented. Under the expectations of the theory of natural selection, the functional forms present in the empty region of morphospace should have been discovered by the nautilids in the past 65 million years. Yet they have not.

There remains one final constraint that might produce empty regions within functional possible morphospace, that might produce the nonexistence of forms that nevertheless clearly belong to the set intersection PPFx n FPF (Fig. 7.5). This is the constraint of development, the final determinant of the actual subset of existent form for lineage x (Fig. 7.6). We shall consider the possible analysis of this challenging intrinsic constraint in the next chapter.

The heuristic power of building theoretical morphospaces rests on the capability of generating hypothetical morphologies out of real processes, thus surpassing the usual analytical observation of natural occurrences. At some level, any experimental manipulation involving gain and loss of gene function is a strategy that parallels morphospace building. In both cases, natural occurrences are violated, and new forms appear that have to be explained with normal biological processes. The gain in insight is enormous: looking at the logic of theoretical occurrences can single out the logic of real occurrences.

Rasskin-Gutman and IzpisUa-Belmonte (2004, p. 411)

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