Abstract
Here I advance two related evolutionary propositions. (1) Natural selection is most often considered to require competition between reproducing “individuals”, sometimes quite broadly conceived, as in cases of clonal, species or multispecies-community selection. But differential survival of non-competing and non-reproducing individuals will also result in increasing frequencies of survival-promoting “adaptations” among survivors, and thus is also a kind of natural selection. (2) Darwinists have challenged the view that the Earth’s biosphere is an evolved global homeostatic system. Since there is only one biosphere, reproductive competition cannot have been involved in selection for such survival-promoting adaptations, they claim. But natural selection through survival could reconcile Gaia with evolutionary theory.
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Notes
Both Van Valen (1989) and Dawkins, in The Selfish Gene (1976), describe the differential survival of more durable non-reproducing minerals or (prebiotic) molecules as a kind of natural selection, the latter noting that “Darwin’s ‘survival of the fittest’ is really a special case of a more general law of survival of the stable” (Dawkins 1976, p. 12). A difference here is that survivors as in Fig. 3 can become intrinsically more persistent with time (see also footnote 2).
Okasha (2006) and Godfrey-Smith seem to accept that the process I illustrate in Fig. 3 is a kind of (or at least akin to) Darwinian natural selection, but consider it “unimportant” or “uninteresting”. The former (Okasha 2006 p. 214), in arguing that clade selection, except for the limit case of species selection, is conceptually incoherent because clades do not reproduce, notes that defenders of clade selection might counter that “differential extinction of clades might still occur”. He writes: “This is true enough. However, selection on entities that do not reproduce their kind is not very interesting and will not lead to adaptations. All sorts of entities are subject to selection in this weak sense. A collection of atoms may have different probabilities of decay, a collection of buildings may have different probabilities of being demolished, and so on. Natural selection in only an interesting idea when applied to entities that reproduce.” Of course physicists and architects might well find the differential survival of atoms or buildings quite interesting, motivating further inquiry. More relevant to my case is that differential persistence of biological communities is likely to entail adaptations that are themselves the product of lower-level selection by reproductive competition (such as LGT, see footnote 3). Persistent communities are expected to be evolving communities, changing fundamentally over time. Those surviving longest are likely to have become progressively more persistent intrinsically (as denoted by thicker lines in Fig. 3). At least for atoms, this would not be true, while for buildings conscious intervention by managers would be required. Godfrey-Smith is more dismissive, writing that “it is possible to bend a partially Darwinian description around change in collections of things lacking reproduction, but this is a very artificial extension of the theory.” (Godfrey-Smith 2009, p. 40.) Indeed it seems artificial when persistence is recast as reproduction, each instant or era begetting the next. But at the core of Godfrey-Smith’s dismissiveness seems to be the concern that without the possibility of reproductive multiplication, “the only way there can be fitness differences is for the population to get smaller”. Thus the “evolutionary possibilities are very limited; selection cannot play a role in origin explanations, for example … When ‘reproduction’ does not include the possibility of multiplication, the result is at most a low-powered Darwinian process.” (Godfrey-Smith 2009, p. 104). Lack of power does not seem a charge that can be leveled against the basic predictive claim made here, that more persistent non-reproducing but evolving entities are more likely over time to have accumulated processes or elements that favor their persistence. We can surely consider such processes or elements as, or analogous to, “adaptations”, although indeed selection at levels lower than that of a unitary biosphere may be essential for “origin explanations”.
Rather more specific objections concerning feasibility might also be raised. One might be that any major change to a complex system is more likely to be maladaptive than adaptive, while successive fixation of minor adaptive changes would requires populations much larger than the limit here (n = 1). This argument misses the point, I think. The survival-promoting adaptations I imagine would most often be fixed by selection via reproductive competition at lower levels and, like rare fortuitous base changes in DNA, only coincidentally contributory to higher-level adaptation—through either reproductive competition or, in the case of Gaia-like entities, survival. A prime example would be lateral gene transfer (LGT), rife among prokaryotes and almost certainly the consequence of selection for promiscuity at the genomic level among the “selfish” agents of transfer (viruses and plasmids). Because of LGT, local prokaryotic populations quickly adjust to even drastic environmental change, and on a global scale, nutrient cycles are maintained over evolutionary time by an ever-shifting cast of microbial characters reading from a shared and enduring script (the reservoir of exchangeable genes [Falkowski et al. 2008]). So LGT (or its equivalent in alternative biologies) would comprise a Gaian adaptation, expected to be prevalent amongst longer-persisting non-reproducing biospheres, though its origin and maintenance in any single such biosphere would have lower-level reproductive causes. Similarly, biological diversity arises as a consequence of lower level evolutionary and ecological forces, but to the extent that it promotes biospheric longevity is expected to be preferentially characteristic (selected for) among more persistent biospheres. Another concern about feasibility might highlight the distinction drawn by Dawkins, in The Extended Phenotype (1982), between reproduction and growth (see also footnote 2). Evolution of complex multicellular organisms entails evolution of ever more complex and coordinated developmental sequences through an accumulative succession of reproductive events, “each one in the series being a slight improvement on its predecessor” (Dawkins 1982, p. 258). Gaia can only grow, as might a tumor in somatic tissue, and would have limited ability to evolve new functions or coordinate them over time. But again, my claim is not that Gaia exhibits anything like the integrated complexity of multicellular organism, only that she is expected to exhibit more stability-conferring subsystems than either chance or lower-level reproductive selection alone could account for because, as a persistent entity, she is more likely (and if sufficiently persistent, actually certain) to have accumulated persistence-promoting adaptations. Bouchard’s (2008) description of the increasing survivability of an aspen grove is a good metaphorical match to the process I imagine. With some plausibility one might also venture that Earth’s biosphere has survived several near-universal extinctions and that its re-establishment after each of these [Saheny and Benton 2007], represents a sort of reproductive event in which high-survival lineages individually or as collectives enjoy advantage.)
This is indeed surely the most contestable of the claims made here. A further thought experiment might make it more palatable. Suppose we trace in our imagination the history of accumulated adaptive changes over a succession of replication events leading back from a single contemporary bacterial cell to its direct ancestor, that many adaptations ago. Assume this cell has always been part of a population of reproductively competing (but never recombining) conspecifics. This populations’ evolution would be described as a succession of events of “periodic selection”, in each of which the clone of descendants of a cell in which an advantageous mutation occurred replaced all other cells in the population (Atwood et al. 1951). But we would know nothing about the cells that have been replaced, only about the succession of unique winners and their step-by-step acquisition of adaptive mutations. Our adaptive narrative thus does not depend on the existence of these countless losers in previous generations, though its plausibility and whether or not we describe the process as ‘selection’ do seem to. If there were ever only a single succession of cells, the probability of stringing so many favorable mutations together would indeed be absurdly small. But this improbability (so long as it is not zero) is trumped (rendered a certainty) by the fact that today’s multiply adapted cell does exist. It is at this point that the model advanced here might seem “anthropic” in character, and thus outside the scope of ‘selection’, “low-powered Darwinian processes” or possibly of scientific reasoning. But, I’d suggest, there is negotiable boundary between anthropic and selectionist ways of thinking, in the region of which selection for persistence (especially for unitary entities) can be found. In cosmology, boundary negotiations are already under way (Smolin 2007).
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Acknowledgments
I thank Rich Campbell, Letitia Meynell, Andrew Fenton and Eleftherios Zouros for comments on an earlier version of this manuscript, and Kim Sterelny and an anonymous reviewer for comments on the original submission.
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Ford Doolittle, W. Natural selection through survival alone, and the possibility of Gaia. Biol Philos 29, 415–423 (2014). https://doi.org/10.1007/s10539-013-9384-0
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DOI: https://doi.org/10.1007/s10539-013-9384-0