Abstract
We examine how molecular challenges to adaptationism, as recently put into philosophical focus by Sarkar (2015), fare against different types of adaptationist theses that have been distinguished in the philosophical debate on adaptationism. Our aim is to defend a weak form of empirical adaptationism according to which a majority of phenotypic traits at the organismal level are fixed by natural selection. Sarkar (2015) indicates a possible puzzle for this view by claiming that the same arguments that challenge adaptationism at the level of the genome can potentially apply at higher levels. We argue that many of the disputes about the importance of neutrality of molecular evolution as a challenge to the adaptationist thesis stem from unprecise use of terms such as phenotypic traits or complex phenotypes. As an important step toward a solution of this problem, we propose a strategy that tracks how changes at the molecular level can cause phenotypic effects at the organismal level, and identifies as important those phenotypic changes where a new function at the organismal level was introduced. We argue that for such cases, it is justified to conclude that selection was responsible for the fixation of a new trait. While this does not yet vindicate empirical adaptationism until further empirical research is done, it does provide us with the strategy of testing it. It shows how the puzzle can be solved by pointing to cases where new and important functions at the organismal level arise and where it can be clearly shown that selection was strong enough to counteract the effects of drift.
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Notes
- 1.
Already Darwin in the later editions of the Origin of species discussed the relation between natural selection and other evolutionary factors.
- 2.
When discussing phenotypic evolution, Kimura focuses on fossil records, so phenotypic traits in this case refer to morphological characters. In addition, it can be noted that Kimura does not think that the neutral theory denies the role of natural selection in determining the course of adaptive evolution (Kimura 1989).
- 3.
For an informal but relevant debate on this topic, where the issues of clear definition of phenotypic traits are an important part of the discussion, see this blog: http://sandwalk.blogspot.hr/2011/02/dawkins-darwin-drift-and-neutral-theory.html.
- 4.
For a survey and criticisms of attempts to offer adaptationist stories that would account for the architecture of the human genome and eukaryotic genomes in general see Sarkar (2015).
- 5.
However, it should be noted that there are many other mechanisms underlying the origins of new genes other than gene duplication. Novel genes can arise from messenger RNAs of ancestral genes, protein-coding genes metamorphosed into new RNA genes, and genomic parasites co-opted as new genes and new protein. Moreover, RNA genes can be composed from previously non-functional sequences (Kaessmann 2010). Here we take gene duplication as a good example because it is a well-studied source of new genes and potentially new functions at the level of organismal phenotype.
- 6.
We acknowledge the fact that in many cases where positive selection fixes traits at the molecular level, there are methods for detecting the act of selection, such as showing that nonsynonymous nucleotide substitutions exceed synonymous nucleotide substitutions during the early stages after duplication. However, we take it that this is not enough to conclusively establish adaptationism at the molecular level, and this is not the adaptationist thesis that we are concerned with. We limit adaptationism to the claim about the evolution of phenotypic traits. In addition, in order to reach the conclusion about the act of selection, further information about environmental conditions and the usefulness of the new evolved function should be taken into consideration.
- 7.
Sarkar (2015) addresses this issue and acknowledges that the lack of this information makes his argument “qualitative” instead of quantitative, but still not merely verbal as adaptationist “just so stories.” We think that our strategy is a good starting point in avoiding the accusation of offering merely verbal accounts of evolution of phenotypic traits. This strategy is still not backed by enough quantitative data, but we take it that it represents a substantial step toward testing the adaptationist hypothesis regarding organismal phenotypic traits. In our case, this consists in tracing how the mechanisms at the molecular level cause organismal phenotypic changes.
- 8.
There have also been proposals to divide the adaptationist views in more detail. For instance, Lewens (2009) distinguishes seven types of adaptationism (while acknowledging the main three types, which he then subdivides into distinct subtypes).
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Acknowledgments
For helpful discussions of earlier drafts of the paper, we would like to thank Elliott Sober, Thomas Reydon, Marko Jurjako, Pierdaniele Giaretta, and audiences at the conferences “The 19th Evolutionary Biology Meeting in Marseille,” “Model Selection: Ockham’s Razor and Related Issues,” Padova, and “Philosophy, Society, and the Sciences,” Rijeka, which all were held in 2015. Many and special thanks also to Pierre Pontarotti. Work on this paper was partly supported by a grant from the European Social Fund for the project “Building a Support System for Young Researchers.”
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Šustar, P., Brzović, Z. (2016). Molecular Challenges to Adaptationism. In: Pontarotti, P. (eds) Evolutionary Biology. Springer, Cham. https://doi.org/10.1007/978-3-319-41324-2_16
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