Rispondi a: LA "CADUTA" DI DARWIN…

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Conclusion

An experience-based analysis of the causal powers of various explanatory hypotheses suggests purposive or intelligent design as a causally adequate– and perhaps the most causally adequate– explanation for the origin of the complex specified information required to build the Cambrian animals and the novel forms they represent. For this reason, recent scientific interest in the design hypothesis is unlikely to abate as biologists continue to wrestle with the problem of the origination of biological form and the higher taxa.

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End Notes

1 Specifically, Gilbert et al. (1996) argued that changes in morphogenetic fields might produce large-scale changes in the developmental programs and, ultimately, body plans of organisms. Yet they offered no evidence that such fields– if indeed they exist– can be altered to produce advantageous variations in body plan, though this is a necessary condition of any successful causal theory of macroevolution.

2 If one takes the fossil record at face value and assumes that the Cambrian explosion took place within a relatively narrow 5-10 million year window, explaining the origin of the information necessary to produce new proteins, for example, becomes more acute in part because mutation rates would not have been sufficient to generate the number of changes in the genome necessary to build the new proteins for more complex Cambrian animals (Ohno 1996:8475-8478). This review will argue that, even if one allows several hundred million years for the origin of the metazoan, significant probabilistic and other difficulties remain with the neo-Darwinian explanation of the origin of form and information.

3 As Crick put it, “information means here the precise determination of sequence, either of bases in the nucleic acid or on amino acid residues in the protein” (Crick 1958:144, 153).

4 To solve this problem Ohno himself proposes the existence of a hypothetical ancestral form that possessed virtually all the genetic information necessary to produce the new body plans of the Cambrian animals. He asserts that this ancestor and its “pananimalian genome” might have arisen several hundred million years before the Cambrian explosion. On this view, each of the different Cambrian animals would have possessed virtually identical genomes, albeit with considerable latent and unexpressed capacity in the case of each individual form (Ohno 1996:8475-8478). While this proposal might help explain the origin of the Cambrian animal forms by reference to preexisting genetic information, it does not solve, but instead merely displaces, the problem of the origin of the genetic information necessary to produce these new forms.

5 Some have suggested that mutations in “master regulator” Hox genes might provide the raw material for body plan morphogenesis. Yet there are two problems with this proposal. First, Hox gene expression begins only after the foundation of the body plan has been established in early embryogenesis. (Davidson 2001:66). Second, Hox genes are highly conserved across many disparate phyla and so cannot account for the morphological differences that exist between the phyla (Valentine 2004:88).

6 Notable differences in the developmental pathways of similar organisms have been observed. For example, congeneric species of sea urchins (from genus Heliocidaris) exhibit striking differences in their developmental pathways (Raff 1999:110-121). Thus, it might be argued that such differences show that early developmental programs can in fact be mutated to produce new forms. Nevertheless, there are two problems with this claim. First, there is no direct evidence that existing differences in sea urchin development arose by mutation. Second, the observed differences in the developmental programs of different species of sea urchins do not result in new body plans, but instead in highly conserved structures. Despite differences in developmental patterns, the endpoints are the same. Thus, even if it can be assumed that mutations produced the differences in developmental pathways, it must be acknowledged that such changes did not result in novel form.

7 Of course, many post-translation processes of modification also play a role in producing a functional protein. Such processes make it impossible to predict a protein's final sequencing from its corresponding gene sequence alone (Sarkar 1996:199-202).

8 Erwin (2004:21), although friendly to the possibility of species selection, argues that Gould provides little evidence for its existence. “The difficulty” writes Erwin of species selection, “…is that we must rely on Gould's arguments for theoretical plausibility and sufficient relative frequency. Rarely is a mass of data presented to justify and support Gould's conclusion.” Indeed, Gould (2002) himself admitted that species selection remains largely a hypothetical construct: “I freely admit that well-documented cases of species selection do not permeate the literature” (p. 710).

9”I do not deny either the wonder, or the powerful importance, of organized adaptive complexity. I recognize that we know no mechanism for the origin of such organismal features other than conventional natural selection at the organismic level– for the sheer intricacy and elaboration of good biomechanical design surely precludes either random production, or incidental origin as a side consequence of active processes at other levels” (Gould 2002:710). “Thus, we do not challenge the efficacy or the cardinal importance of organismal selection. As previously discussed, I fully agree with Dawkins (1986) and others that one cannot invoke a higher-level force like species selection to explain 'things that organisms do'– in particular, the stunning panoply of organismic adaptations that has always motivated our sense of wonder about the natural world, and that Darwin (1859) described, in one of his most famous lines (3), as 'that perfection of structure and coadaptation which most justly excites our admiration'“ (Gould 2002:886).

10 Theories in the historical sciences typically make claims about what happened in the past, or what happened in the past to cause particular events to occur (Meyer 1991:57-72). For this reason, historical scientific theories are rarely tested by making predictions about what will occur under controlled laboratory conditions (Cleland 2001:987, 2002:474-496). Instead, such theories are usually tested by comparing their explanatory power against that of their competitors with respect to already known facts. Even in the case in which historical theories make claims about past causes they usually do so on the basis of preexisting knowledge of cause and effect relationships. Nevertheless, prediction may play a limited role in testing historical scientific theories since such theories may have implications as to what kind of evidence is likely to emerge in the future. For example, neo-Darwinism affirms that new functional sections of the genome arise by trial and error process of mutation and subsequent selection. For this reason, historically many neo-Darwinists expected or predicted that the large non-coding regions of the genome– so-called “junk DNA”– would lack function altogether (Orgel & Crick 1980). On this line of thinking, the nonfunctional sections of the genome represent nature's failed experiments that remain in the genome as a kind of artifact of the past activity of the mutation and selection process. Advocates of the design hypotheses on the other hand, would have predicted that non-coding regions of the genome might well reveal hidden functions, not only because design theorists do not think that new genetic information arises by a trial and error process of mutation and selection, but also because designed systems are often functionally polyvalent. Even so, as new studies reveal more about the functions performed by the non-coding regions of the genome (Gibbs 2003), the design hypothesis can no longer be said to make this claim in the form of a specifically future-oriented prediction. Instead, the design hypothesis might be said to gain confirmation or support from its ability to explain this now known evidence, albeit after the fact. Of course, neo Darwinists might also amend their original prediction using various auxiliary hypotheses to explain away the presence of newly discovered functions in the non-coding regions of DNA. In both cases, considerations of ex post facto explanatory power reemerge as central to assessing and testing competing historical theories.