Abstract
We examine the use of the notion of natural selection in the philosophical debate on functions in biology. This debate has been largely shaped by the way in which different accounts assess various selective pressures in justifying claims about biological functions. Cummins (Functions: new essays in the philosophy of psychology and biology. Oxford University Press, Oxford, pp 157–172, 2002), one of the main proponents of the causal role account of biological functions, argues that a correctly understood neo-Darwinian notion of natural selection has nothing to do with functional talk in biology. In this paper, we counter Cummins’ account by showing that progress in the molecular approaches to evolutionary biology—specifically scientific data available in neo-functionalization research—offers valuable support to the etiological selectionist approach to functions in biological and biologically-related sciences. Finally, we use the presented data to build our own account of biological functions, which tries to avoid the wrong turns taken by both major strands in the biological function debate, namely causal role and etiological accounts. According to our account, the function of a certain gene or a protein in the biological system that contains it is a particular causal activity, or a group of causal activities whose manifestation is in a specific way determined by corresponding mechanisms of genetic expression. Also, we argue that in many important cases this particular expression of genetic activity was positively selected at a certain point in evolutionary history. Since we take selection as an important but not the only factor that grounds biological functions, we are committed to a weak etiological account.
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Notes
One of the main consequences of the latter point is highly implausible saltationism, which the defender of etiological account is forced to accept. We will not here venture further into any of Cummins’ other specific reasons for this objection. Rather, we will concentrate on the part of Cummins’ critique, which draws out a better proposal on how to accommodate natural selection into biological functional talk. For Cummins’ Darwinian orthodoxy on gradualism, see in particular his 2002 paper (pp. 165–166).
This is how Cummins describes the problem, however, to be more precise it seems that it is correct to say that in order for wings to be selected because they enable flight, there must be a subpopulation in which organisms have wings that enable flight, while organisms in the rest of the population do not have wings. This is because of the fact that, if they were to have wings, then we would not be able to say that wings were selected because they enable flight, but rather a type of wings were selected that enables flight. This might be a semantic issue, since if we define wings as organs that enable flight, it follows that the rest of the population with wing-like organs that do not enable flight cannot have wings.
For a more detailed update on the growth of evidence in molecular evolutionary biology that calls into question objection (1), see, e.g., Ohno (1970), Zhang (2003), Piatigorsky (2007). With regards to a broader scientific and philosophical context in which those data are embedded, cf. Sect. 3 in this paper.
Methods for detecting positive Darwinian selection include showing that nonsynonymous nucleotide substitutions (resulting in new amino acids) exceed synonymous nucleotide substitutions (no amino acid change) during the early stages after duplication (Hughes 1999). However, authors like Golding and Dean (1998) argue that in addition to this kind of evidence, information about the phenotypic level is needed. We address this issue in Sect. 4.
The endorsement of the weak etiological theory commits us to a form of adaptationism—explanatory adaptationism—according to which the apparent design of organisms and relations of adaptedness between organisms and their environment represents a crucial problem in biology that is explained by invoking selection (Godfrey-Smith 2001). It is important to stress that we accept explanatory adaptationism without endorsing empirical adaptationism, according to which most biologically significant features of organisms are shaped by natural selection (Sterenly and Griffiths 1999). We hold that natural selection has a special explanatory importance even if it does not occur often and even if the majority of traits in organisms are not adaptations.
In the case of the evolution of trichromacy an additional reason is often provided in favor of accepting the selection hypothesis: the same parallel amino acid replacements that generate red-sensitive opsins have ocurred independently in fish, reptiles and mammals which makes the hypothesis that selection is responsible for the fixation of trichromacy more likely then the hypothesis that all these independent events were the product of the random fixation of selectively neutral mutation (Golding and Dean 1998). This is because of the fact that if a new allele comprising duplicate genes is selectively neutral, the probability of its fixation is small, but if there is positive selection at work, then it is much more probable that it will be fixed in the population, especially if the same fixation ocurred independently at several times in evolutionary history (cf. Zhang 2003).
Perhaps Cummins could reply that his argument is sound since his philosophical targets are actually guilty of engaging in cheap tricks even though this is not the only strategy available for defending an etiological account of functions. However, it seems that this is not an answer available to Cummins since he claims that natural selection does not play a role in producing functions, so a more careful strategy without cheap tricks seems impossible regarding his account. We thank the reviewer for pointing out this possible strategy for Cummins.
It could be objected that we cannot simply claim that trivariance occurring at the photoreceptor level by itself results in trichromacy concerned with our ancestors’ feeding strategies. Firstly, by focusing on the photoreceptor level, we are not ignoring certain adjustments that should have taken place within the human eye in passing from the trivariance to the trichromacy in question (see e.g., MacLeod 2010). Secondly, the occurrence of the third cone class in the case of trivariance could have been supported by already existing higher-level mechanisms in dichromats. This is only to say that the described occurrence of trivariance in dichromats, which, then, results in the emergent function of trichromacy as illustrated by Fig. 3, fits within the basic explanatory scheme of positive Darwinian selection [for a clear statement of the basic scheme of natural selection, see (Skipper and Millstein 2005)].
As already pointed out on p. 16 this explanation is further strengthened by evidence that these changes in genetic traits brought about some important phenotypic changes.
Figure 3 is updated by the data presented in Sect. 3, in particular, within the part referring to the case of human color vision considered as an outcome of genetic redundancy, that is, as a case of neo-functionalization via gene duplication. For other cases in which natural selection maintains genetic redundancy in a population, see Thomas (1993, pp. 395–397).
As known, there are some important difficulties concerning Cummins’ notion of ’programmed manifestation’ in relation to his accounting for functional behavior of biological items. We will not explore its more general difficulties, as well as what would be an adequate framework for dealing with them. At this point in the paper, we will instead track the influence of positive Darwinian selective pressures in justifying biological functional claims. In particular, we will try to ascertain in what kind of situations, we may correctly consider causal activities of biological items as being selected for manifesting these activities. However, as we will summarize shortly, such kinds of situations do not exclude that, according to both theories of neo-functionalization, we may have legitimate functional ascriptions by appealing to merely neutral evolutionary considerations.
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Acknowledgments
We wish to thank Thomas Reydon and two anonymous referees for their careful reading of our manuscript and their insightful comments.
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Šustar, P., Brzović, Z. The function debate: between “cheap tricks” and evolutionary neutrality. Synthese 191, 2653–2671 (2014). https://doi.org/10.1007/s11229-014-0407-4
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DOI: https://doi.org/10.1007/s11229-014-0407-4