what is ‘natural’ in natural selection?

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What is ‘Natural’ in Natural Selection?Abhijeet S BardapurkarSchool of Educational StudiesAmbedkar University, Delhi(Integrated Institute of Technol-ogy Campus)Sector-9, DwarkaNew Delhi 110 077, India.Email: [email protected],[email protected]

To understand Darwin’s concept of natural selection, wehave to contrast it with his characterization of artificialselection, and then ask: what is natural in natural selection?While we do this, we develop two distinctions: one between‘change by transformative action’ and ‘change by selection’,and another between ‘artificial selection’ and ‘naturalselection’. The first distinction helps us understand evolutionby selection and the second natural selection.

We cannot make a significant claim to comprehend science,unless we understand how (at least some of) the scientific theoriesexplain what they propose to explain. The focus of this article isonDarwin’s theory of natural selection: how this theory, developedby Charles Darwin and Alfred Russel Wallace in the mid-nineteenthcentury, explains organic evolutionary adaptivechange.Darwin’s explanation is unique in various ways and our aim hereis to understand the structure of this explanation.

Darwin explains organic evolution by characterizing it as adaptivechange by selection. To understand Darwin’s explanation is tounderstand how selection leads to evolutionary adaptive change.Change by selection is best understood in contrast with, what Iterm as, change by transformative action. The selection/transformation contrast is central to the pedagogy of Darwin’stheory; to develop an understanding of natural selection, studentshave to understand the meaning of ‘selection’ and its biologicalconsequences1. This inevitably brings us to another contrast thatis equally central to our present concerns: that between artificialselection and the natural selection. The selection analogy is thecornerstone of Darwin’s theory. It defines the natural in naturalselection.

1. Transformation Versus Selection

Organisms change in thehands of breeders. Breeders start breeding

KeywordsUnderstanding causal explana-tion, organic evolutionary change,natural selection, selection anal-ogy, Charles Darwin.

1 For a review of students’ un-derstanding of organic evolutionsee, for example:http://dx.doi.org/10.1007/s12052-008-0048-5

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2 Strictly, one may rightly insistthat adaptive variation – not merevariation – is a precondition toselection. This will become clearas we progress further in thisarticle. But, note that a neutralvariation may turn out to be adap-tive should the conditions inwhich a particular populationthrives change.

one variety and may end with another. This is an instance ofevolutionary change. Darwin’s theory is about such evolutionarychanges from one variety to another, and he explained it bydeveloping the idea of ‘selection’, better called as the ‘selectionanalogy’.

Darwin’s breeders do not act on the individuals to transformthem. They select. They eye individuals with traits that aredifferent – even slightly different – from the rest. They look forindividual variants, and select some of these for breeding.Variation is common; the breeder selects a variant of her interestfrom the existing pool. Evolution of a novel variety in the handsof breeders is caused by the selective preservation and collectionof available variants in each generation. This is evolutionarychange by cumulative selection.

In contrast, in what I refer to as change by transformative action,one is not just preserving but changing what is available. Let ustake a couple of mundane examples to illustrate this distinction.These examples are from the physical (not biological) world andare meant to illustrate just the selection/transformation contrast.If instead of acting (say by hitting with a hammer) on a stone totransform it into a pile of sand, you begin by selecting stones ofsmaller and smaller size, as and when they become available.And, eventually, if you are lucky to have the stones of the desiredsize, you end up having a pile of sand particles. This pile of sand,we say, is caused by selection. Instead of creating or making orproducing a pile of sand particles from a stone by transformativeaction, you kept on selecting and accumulating the stones, of sizeclosest to the desired size of sand particles, from the availablestones. To take another example, one could either produce waterby combining hydrogen and oxygen, or could have a water bodyby accumulating small water droplets.

Variation then is a precondition to selection2. Without variation –among the individuals of a population – selection is impossible toimagine. Evolutionary change by selection is possible only whenthe individual change exists: only when siblings vary from each

Biological changecould be at a

particular individuallevel, and it could be

at the level ofpopulations. If we

underline thedistinctive nature ofchanges at different

levels of biologicalorganisation, we canunderstand how they

are related in the longhistory of life.

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other and from their parents in each generation, and when at leastsome of the variant traits are transmitted to the followinggenerations3. If biological reproduction were to be clonal, whereindividuals of a specific population are identical to each other,then that population normally will not evolve. It could evolveonlyby transformationof the individuals4 – onlyby transformativeaction – for example, through a spontaneous change in thehereditary material.

2. What is Artificial in Artificial Selection?

When do the numerous (supposed or experienced) changes in theliving and non-living world qualify as ‘natural’ changes? Tounderstand ‘natural’ is to contrast it with both ‘supernatural’ and‘artificial’5. A change is a natural changewhen its cause originatesneither in the act of supernatural power nor in the act of humanbeings6. Of course, this definition is anthropocentric, because we– human beings – are attempting to understand the causalityaround us, and from this perspective, what is artificial is causedby us, mostly for us. Note that the existence and inheritance ofvariation among individuals is not an artifact of human deeds –whatever may be its cause, it is not caused by a breeder with theview of modifying the breed in a certain manner. Both variationand inheritanceare naturally availableto the breeder, not artificiallyproduced for the purpose.What makes artificial selectionartificialis not the variation and its inheritance, but the human act ofselection. In each generation, the breeder selects for breedingthose individuals who vary from others in being more beautiful ormore useful to the human eye. Without this selector, there will beno selection of the variation useful to human beings.

What is the consequence of this selection? The selected variations,when are hereditary, “accumulate” (Darwin, 1859; for examplesee p.32) in a certain direction decided by the breeder. Over thegenerations, the number of particular variants as well as themagnitude of this variation increases. For example, the tail lengthas well as the number of long-tailed pigeons would increase as aresult of selection of long-tailed pigeons.

3 In the post-Darwin classicalunderstanding, hereditary varia-tion is synonymous with geneticvariation, and genetic variationis the cause of phenotypic varia-t ions. Phenotypic variationmeans variation in the physical,physiological and behavioraltraits. As discussed in the fol-lowing sections in the text, se-lected adaptive traits will accu-mulate in the population onlywhen they are inherited.

4 Richard Lewontin would call it“transformational evolution” dis-tinguishing it from “variationalevolution”.

5 Worldviews differ in ways theydefine the domains of the ‘super-natural’ and the ‘artificial’. Mypresent purpose is not to debatethe ‘correctness’ of one view overanother but to underline the peda-gogical significance of artificial/natural/supernatural contrast,and give it a minimalist render-ing that is conducive to under-stand Darwin’s theory of naturalselection.

6 Historically, the meaning of‘natural’ emerged in contrast tothat of ‘supernatural’. See, forexample: J C Greene, The deathof Adam: Evolution and its im-pact on western thought, p.57,78 and pp.54–55, State Univer-sity Press, Iowa, 1959.

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When we compare a variety that has changed due to selection,with the one with which the breeder had begun, we notice that thevariety after continued selection, compared to its ancestralpopulation, is more in tunewith “wants or fancies” (Darwin, p.38,1859) of human beings – the newer variety is adapted to ourconditions of life and beauty. It would appear as if this adaptivechange is produced or madeby thebreeder’s enbloc transformativeaction on the ancestral population. But, as we know, hereadaptive change is a consequence; and consequence not of anytransformative action, but of cumulative selection: selection ofslightly useful, and hence adaptive, variation available in thepopulation. And, as we noted earlier, the cumulative selection iscaused by selective breeding: the adaptive variation is preservedin each generation by the selector, and from one generation to thenext, by the natural mechanism of inheritance. But, at this point,we have to take a long view of the causal explanation we areexploring. Accumulation of a particular variation is caused by itspreservation, but why a particular variation is selected in the firstplace? It is selected because it suits the human needs and fancies.Usefulness or profitability of a variation for the breeder-selectoris the cause of its preservation by selection. Make a note of thecausal role of the adaptive-profitability of a variation –understanding of this causal thread is crucial to comprehendDarwin’s selection analogy.

We began with the characterization of the adaptive evolutionarychange as change by selection, and then went on to understand itin ‘artificial selection’. The following sectionwill discuss adaptiveevolutionary change by natural selection. What is the nature ofthis change, and how is it distinct from the change in man’sselection? What is natural in natural selection?

3. What is Natural in Natural Selection?

Darwin characterized organic evolutionary change as adaptivecumulative change by selection, and applied the same ingeniousidea of selection across the domesticated and wild varieties. Inboth artificial as well as natural selection, the same question is

The changing worldperplexes us. To

understand change isto explain it: to explain

how the change inquestion is caused.

Human world could bea guide to understandchange in the natural

world, particularlywhen the change in

question is caused bythe human agency.

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asked: how are individual variants, so common in both domestica-ted andwild varieties, selected and preserved through generations?The causal agency is clearly demarcated in the case of artificialselection: “nature gives successive variations; man adds them upin certain directions useful to him” (Darwin, 1859; p.30). What isthe case in natural selection? How is the individual variationselected and preserved naturally – without the agency of man?

When an individual varies from others in the population, andwhen this variation is useful to the individual in its own survivalor reproduction, then the variation is naturally selected andpreserved: in the sense that the variation is repeatedly reproducedin the following generations, with more and more variantindividuals in the population. Selection and preservation of avariation is a natural consequence of its usefulness to the variant.See the analogy here: be it artificial selection or be it naturalselection, the preservation of variation is the consequence of itsusefulness. If the variation is useful to the human beings, theselector-breeder causes its preservation; if the variation is usefulto the variant individual itself, this usefulness-to-the-self causesits preservation. And in both cases, the inheritance of the variationcauses its accumulation. Here are a couple of (hypothetical)situations to illustrate natural selection.

Imagine a bacterial population growing in the environment wheretwo types of sugars – SA and SB, are available. Initially theenvironment hadno fungal species in the vicinity of the population.Later a fungal species producing antibiotic FA is recentlyintroduced. Now, if a few bacteria vary from others in beingresistant to the FA antibiotic, these will be naturally selected andpreserved. This variation has always beenpresent in thepopulation,but was not useful enough to cause the selection until the fungalspecies arrives on the scene. Take another identical population inan identical situation, but without any antibiotic producing source.Let us suppose, a fewbacterial cells undergoa hereditary variationthat enables effective uptake and catabolism of sugar SB. Thisvariation would be profitable to them, while the rest of bacterialpopulation depends entirely on sugar SA for survival. Thevariants

Selection andpreservation of avariation is a naturalconsequence of itsusefulness to thevariant.

In developing histheory, Darwin drewthe analogy betweenthe human world andthe natural world. But,Darwin’s explanationis naturalistic. Itinvokes no humancause. For Darwin,evolution is a naturalconsequence of theaccumulation ofadaptive variation inthe population.

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will benaturally selected: compared to the others in the population,they will naturally outgrow in numbers7. Note that, in the secondinstance, unlike the first, there is no change in the externalconditions, and still there is natural selection.

Profitability or usefulness of a variation for the variant individualcannot be conceptualized in a vacuum – a variation is useful inrelation to the organic and physical conditions in which thevariant organism is living. But, students of Darwin will be unableto circumvent a major misconception, if they fail to appreciatethat natural selection is consequent on the variation’s usefulnessfor the variant, not on the static or changing environmentalconditions. For example, Darwinwrites, “[A]nyvariation, howeverslight and from whatever cause proceeding, if it be in any degreeprofitable to an individual of any species, in its infinitely complexrelations to other organic beings and to external nature, will tendto the preservation of that individual, and will generally beinherited by its offspring.” (Darwin, p.61, 1859). Darwin “owes”(ibid.) this process of selection to the “struggle for life”, but oneshould not overplay the causal role of the struggle – and of theenvironmental changes – to the extent of equating it with naturalselection. To do this is to nurture a naïve interpretation of naturalselectionwhere the selector in artificial selection is simply replacedwith the “environment” or the “struggle”, without giving the duecausal role to the natural accumulation of useful variation in thepopulation. Variation is to be viewed in the context of itscomplex relationship with other variants and their physicalenvironment; the worth of a variant is dependent on this context;and the “struggle” adds to the causal value of the variation in anindependent sense. The struggle in itself does not cause thevariation. Variation occurs independently and is selected naturallynot merely because organisms possessing them are struggling tosurvive, but because the variation proves to be useful in thevariant’s struggle to survive and reproduce. This is analogouswith the artificial selection: here too, the selection does dependon how “educated” are the selector’s eyes; nevertheless, theselector selects not because he is well trained in identifying

7 Of course, had sugar SB beenabsent or had there been nocompetition for sugar SA, thevariation that enables the effec-tive utilization of Sugar SB wouldnot be profitable.

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variants, but because the variation is of use to him. The cause ofselection lies in the usefulness of variations: in their being “usefulto man” or “useful in someway to each being” (Darwin, 1859; pp.80–81). “[U]nless profitable variations do occur, natural selectioncan do nothing … … natural selection can act only through andfor the good of each being … preserving and adding up all that isgood” (ibid., p.82, p.84).

4. Selection and Adaptation

Naturally selected individual variation is useful, and in this senseis adaptive. What Darwin’s theory does is to explain how existingadaptive individual variation is augmented in the population,over numerous generations, in the prevalent conditions of life8.What is common between an (evolutionary) adaptation and itsancestral individual variation is that both of them have beenbeneficial. The question of the immediate origin of variation –what is the cause of variation in the development of an individual– is irrelevant for its selection, provided the variation is stable andis transmitted to the next generation. Note: what matters inselection is not the cause of variation but its effect, its usefulness,its adaptive advantage. When an individual variation is useful, itactually contributes in its ownselectionbyvirtue of this usefulness.And then, over the generations, it has historical accounts thatexplain its existence. Not one but two complementary historicalaccounts exist: an individual-developmental history accountingfor its origin, and an evolutionary history accounting for itspreservation, consequent accumulation and augmentation in thepopulation. Darwin’s theory of natural selection is an explanatorynarrative of the journey from a least distinct but slightly adaptivestage of difference to another more distinct and greatly adaptivestage of difference. In Darwin own words:

I look at individual differences, though of small interest to thesystematist, as of high importance for us, as being the first steptowards such slight varieties asarebarely thoughtworth recordingin works on natural history. And I look at varieties which are inany degree more distinct and permanent, as steps leading to more

8 It is important to note that “ad-aptation” is commonly assumedto be a consequence of naturalselection, and is also employedto denote the process of selec-tion. And also, every beneficialtrait does not necessarily inheritthe causal history of selectionand just being beneficial to thevariant is not sufficient to be anadaptation. For a comprehen-sive discussion of this issue see,for example: R N Brandon, Con-cepts and methods in evolution-ary biology, pp.3–45, CambridgeUniversity Press, Cambridge,1996.

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strongly marked and more permanent varieties; and at theselatter, as leading to sub-species, and to species. Thepassage fromone stage of difference to another and higher stage may be, insome cases, due merely to the long-continued action of differentphysical conditions in two different regions; but I have not muchfaith in this view; and I attribute the passage of a variety, from astate in which it differs very slightly from its parent to one inwhich it differs more, to the action (sic) of natural selection inaccumulating... differences of structure in certain definitedirections (ibid., p.51–52; my emphasis).

In Darwin’s account, organisms are “modified through variation,and the [existing] modifications are accumulated by naturalselection for the good of the being” (ibid., pp.85–6). Studentsshould appreciate that the origin of individual variation is beyondthe domain of Darwin’s theory: it is the theory of evolution ofvariation, not of the origin of variation; it is the theory ofconsequences, of effects, not the theory of causes. The theorycould distance itself from the question of the origin because: one,it explains the evolution of existing individual variation; and two,it explains the evolution by selection. Darwin could develop anaturalistic9 explanation of organic evolutionary change becausethe (causal) structure of his theory is such that: one, the origin ofchange – the cause of individual variation – is irrelevant for histheory; and two, the selector is not necessary for the selection. Toconclude: Selection is a natural effect of inherited self-advantage,and to understand the organic evolutionary change by selectionstudents should be able to “sumup in their minds slight differencesaccumulatedduringmanysuccessive generations.” (Darwin, 1859;p.29).

9 See the margin note 5 and therelated text.

Acknowledgements

Earlier versions of parts of thiswork were presented in a semi-nar at the Centre for EcologicalStudies, Indian Institute of Sci-ence, Bangalore. I have benefit-ted from the enriching discus-s ion with RaghavendraGadagkar and the audience ofthe seminar. I am also gratefulto Arvind Kumar, Beena Choksi,G Nagarjuna, Jon Hodge, PeterLipton, and VidyanandNanjundiah for their kind com-ments and discussions at vari-ous stages of this article.

Suggested Reading

Literature on Darwin’s theory is enormous, but reading (at least the first four chapters of) thefirst edition of Darwin’s following book is vital for understanding the idea of natural selection:Charles Darwin, On the origin of species: A facsimile of the first edition, Harvard UniversityPress, Cambridge MA, 1859/1964. This book and all other works of Darwin are now availableat http://darwin-online.org.uk. The students and teachers may also like to read some of theDarwin correspondence, now easily accessible at http://www.darwinproject.ac.uk/

what is ‘natural’ in natural selection?

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