Addressing cumulative selection.

In his unfavorable review (“God as genetic engineer,” Books et al. , 8 June, p. [1427][1]) of my book, The Edge of Evolution ([1][2]), Sean Carroll writes that “Behe's chief error is minimizing the power of natural selection to act cumulatively,” and implies that I fail to discuss “pyrimethamine resistance in malarial parasites … —a notable omission given Behe's extensive discussion of malarial drug resistance.” The insinuation is that I included only what fit my purposes. Yet I explicitly discuss multiple mutations of pyrimethamine resistance: “Although the first mutation (at position 108 of the protein, as it happens) grants some resistance to the drug, the malaria is still vulnerable to larger doses. Adding more mutations (at positions 51, 59, and a few others) can increase the level of resistance” [([1][2]), p. 79]. In the same section, I also discuss the development of insecticide resistance in mosquitoes by “tiny, incremental steps—amino acid by amino acid—leading from one biological level to another.” Furthermore, in other sections, I describe a cumulative Darwinian route to antifreeze proteins and extensively address hemoglobin C-Harlem to illustrate the crucial difference between beneficial intermediate mutations and deleterious intermediate ones. 1. 1.[↵][3]1. M. J. Behe , The Edge of Evolution: The Search for the Limits of Darwinism (Free Press, New York, 2007). # Response {#article-title-2} Behe did indeed discuss pyrimethamine resistance on pages 75 and 76 of his book ([1][4]). My criticism is that Behe omitted the clear evidence for the cumulative selection of multiple changes in the drug target protein in nature and that he invoked an altogether different and unsupported explanation in an attempt to bolster his main premise. In his Letter, Behe has misrepresented the thrust of the actual text of his book. With respect to the latter, the passage he quotes in his Letter about how “[a]dding more mutations … can increase the level of resistance” is immediately followed in his book by the disclaimer that “[h]owever, as usual there's a hitch. Some of those extra mutations (but not the first one) seem to interfere with the normal work of the protein” (p. 75). Behe is clearly seeking to convey the message that there is some impediment to Darwinian evolution via multiple intermediates, both in this specific case and in general (hence the phrase “as usual”). However, this is not the case. Careful inspection of the data in the reference I cited ([2][5]) reveals that, in fact, certain mutations (e.g., Cys59→Arg) increase specific parameters of the enzyme's performance. Structural studies suggest that this mutation, found at very high frequency in drug-resistant parasites in nature, improves enzyme binding to substrates in the context of otherwise adverse mutations ([3][6]). Furthermore, pyrimethamineresistant dihydrofolate reductase enzymes actually have activities equal to or better than the wild-type enzyme ( 4 ). Behe also neglects to note the fact that such triple and quadruple mutant enzymes have been found in isolates from India, Southeast Asia, Eastern Africa, and South America, including areas where pyrimethamine use has been limited. The latter suggests that mutant parasites may be as fit as wild-type parasites. Instead of enlightening his readers with an explanation of how sequential mutation and cumulative selection has operated in this example, Behe changes the direction of the discussion back to the main (and erroneous) argument of his book—the necessity for two or more simultaneous mutations for changes in function. He speculates that “two further, simultaneous mutations seem to be necessary” for the evolution of pyrimethamine resistance, despite the fact that the authors I cited ([2][5]) explicitly demonstrated two different pathways to triple and quadruple mutants via stepwise processes. Behe does not cite this work and he obfuscates the clear but inconvenient message in this body of data. If, as Behe now seems to imply in his Letter, he is a greater proponent of cumulative selection than I gave him credit for, why would he, with so many available examples, characterize it as “rare”? It is because cumulative selection is fully capable of producing what he claims Darwinian evolution cannot do. The minimization of cumulative selection and the complete disregard of a massive literature surrounding protein interactions are crucial to Behe's entirely unfounded conclusion that “complex interactive machinery … can't be put together gradually” (p. 81) and must therefore be designed. 1. 1.[↵][7]1. M. J. Behe , The Edge of Evolution: The Search for the Limits of Darwinism (Free Press, New York, 2007). 2. 2.[↵][8]1. W. Sirawaraporn 2. et al. , Proc. Natl. Acad. Sci. U.S.A. 94, 1124 (1997). [OpenUrl][9][Abstract/FREE Full Text][10] 3. 3.[↵][11]1. J. Yuvaniyama 2. et al. , Nat. Struct. Biol. 10, 357 (2003). [OpenUrl][12][CrossRef][13][PubMed][14][Web of Science][15] [1]: /lookup/doi/10.1126/science.1145104 [2]: #ref-1 [3]: #xref-ref-1-1 "View reference 1. in text" [4]: #ref-2 [5]: #ref-3 [6]: #ref-4 [7]: #xref-ref-2-1 "View reference 1. in text" [8]: #xref-ref-3-1 "View reference 2. in text" [9]: {openurl}?query=rft.jtitle%253DPNAS%26rft.stitle%253DProc.%2BNatl.%2BAcad.%2BSci.%2BUSA%26rft.aulast%253DSirawaraporn%26rft.auinit1%253DW.%26rft.volume%253D94%26rft.issue%253D4%26rft.spage%253D1124%26rft.epage%253D1129%26rft.atitle%253DAntifolate-resistant%2Bmutants%2Bof%2BPlasmodium%2Bfalciparum%2Bdihydrofolate%2Breductase%26rft_id%253Dinfo%253Adoi%252F10.1073%252Fpnas.94.4.1124%26rft_id%253Dinfo%253Apmid%252F9037017%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [10]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6NDoicG5hcyI7czo1OiJyZXNpZCI7czo5OiI5NC80LzExMjQiO3M6NDoiYXRvbSI7czoyNDoiL3NjaS8zMTgvNTg0OC8xOTYuMS5hdG9tIjt9czo4OiJmcmFnbWVudCI7czowOiIiO30= [11]: #xref-ref-4-1 "View reference 3. in text" [12]: {openurl}?query=rft.jtitle%253DNature%2Bstructural%2Bbiology%26rft.stitle%253DNat%2BStruct%2BBiol%26rft.aulast%253DYuvaniyama%26rft.auinit1%253DJ.%26rft.volume%253D10%26rft.issue%253D5%26rft.spage%253D357%26rft.epage%253D365%26rft.atitle%253DInsights%2Binto%2Bantifolate%2Bresistance%2Bfrom%2Bmalarial%2BDHFR-TS%2Bstructures.%26rft_id%253Dinfo%253Adoi%252F10.1038%252Fnsb921%26rft_id%253Dinfo%253Apmid%252F12704428%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [13]: /lookup/external-ref?access_num=10.1038/nsb921&link_type=DOI [14]: /lookup/external-ref?access_num=12704428&link_type=MED&atom=%2Fsci%2F318%2F5848%2F196.1.atom [15]: /lookup/external-ref?access_num=000182536300011&link_type=ISI