An Extended Synthesis for Evolutionary Biology

Publish in

Articles & News Stories


Please download to get full document.

View again

of 11
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Massimo Pigliucci, evolutionary theory; Darwinism; Modern Synthesis; Extended Synthesis; philosophy of science; natural selection; agency; efficacy; scope
  THE YEAR IN EVOLUTIONARY BIOLOGY 2009 An Extended Synthesis forEvolutionary Biology Massimo Pigliucci Departments of Ecology and Evolution and of Philosophy, Stony Brook University,Stony Brook, New York, USA Evolutionary theory is undergoing an intense period of discussion and reevaluation.This,contrarytothemisleadingclaimsofcreationistsandotherpseudoscientists,isnoharbingerofacrisisbutrathertheopposite:thefieldisexpandingdramaticallyintermsof both empirical discoveries and new ideas. In this essay I briefly trace the conceptualhistoryofevolutionarytheoryfromDarwinismtoneo-Darwinism,andfromtheModernSynthesis to what I refer to as the Extended Synthesis, a more inclusive conceptualframeworkcontainingamongothersevo–devo,anexpandedtheoryofheredity,elementsof complexity theory, ideas about evolvability, and a reevaluation of levels of selection.I argue that evolutionary biology has never seen a paradigm shift, in the philosophicalsense of the term, except when it moved from natural theology to empirical science inthe middle of the 19th century. The Extended Synthesis, accordingly, is an expansion of the Modern Synthesis of the 1930s and 1940s, and one that—like its predecessor—willprobably take decades to complete.  Key words:  evolutionary theory; Darwinism; Modern Synthesis; Extended Synthesis;philosophy of science; natural selection; agency; efficacy; scope The scientific theory of evolution is just over150 years old. In 2009 scientists all over theworld are celebrating one and a half cen-turies since the publication of   The Origin of  Species   (Darwin 1859), which followed closelythepresentationofajointpaperbyDarwinandWallace (1858) at the Linnean Society of London.Rathersuddenly,theconceptofevolu-tionmovedfirmlyawayfrombeingaquasimys-tical notion, and biology left Paley’s (1802) nat-ural theology forever behind to enter the realmofrespectablescience,justlikephysicshaddonetwo centuries before.Of course, what we consider “evolutionarytheory”todayisnotDarwin’soriginalwork,notany more than today’s physics is “Newtonian”in nature. Instead, we currently work underthe conceptual framework that was built overthe course of several decades at the beginning   Address for correspondence: Massimo Pigliucci, Departments of Ecol-ogyandEvolutionandofPhilosophy,StonyBrookUniversity,StonyBrook,NY 11794.  of the 20th century and is well known as theModernSynthesis(MS;Mayr&Provine1980).Thegoalofthisessayistosummarizewhyearly21st century biology is poised to take the nextstep in evolutionary theory to produce whatis being referred to as the Extended Synthesis(ES) (Pigliucci & Muller 2010).To appreciate this perspective of where thefield is going, we need to understand where itcame from and why it developed the way it did.These are tasks where historians and philoso-phers of science complement the practicing evolutionist, with the three disciplines combin-ingeffortstoidentifyabroaderfieldof“theory”than is customary in everyday science. We shallsee that both the srcinal Darwinism and theMS went through at least two distinct incar-nations and that each iteration of evolutionarytheory addressed problems left unresolved bythe previous one. This will then set the stagefortheidentificationofissuesnottackledbytheMS itself, which will lead us to sketch an ES. At the end of my discussion I will argue that The Year in Evolutionary Biology 2009: Ann. N.Y. Acad. Sci. 1168: 218–228 (2009).doi: 10.1111/j.1749-6632.2009.04578.x   c  2009 New York Academy of Sciences. 218  Pigliucci  :  An Extended Synthesis for Evolutionary Biology   219 evolutionary biology has not undergone aparadigm shift since Darwin and Wallace’swork; instead, its progress can be best under-stood in terms of Gould’s 2002 model of a con-tinuously expanding conceptual tree.  A Brief Conceptual History ofEvolutionary Theory  Theconceptofevolutionaschangeovertimehad, of course, been around before Darwinand Wallace’s seminal report, for instance, inthe writings of Darwin’s grandfather, Erasmus.Darwin and Wallace, however, provided threefundamental advances: the clear presentationof two cardinal concepts for the new theory,as well as a wealth of evidence from a vari-ety of fields to support the inference that thetheory was in fact correct. The cardinal con-cepts were (1) common descent of all organ-isms from one (or a few) srcinal ancestors and(2) natural selection as the process responsiblefor the apparent “fit” between organisms andtheir environment. The evidence came frommany fields, with biogeography, comparativeanatomy, behavioral ecology, and paleontologyamong them.The new theory was immediately controver-sial, and not just for its perceived theologicalimplications. Darwin devoted time and energytoconvincemajorphilosophersofhistime,suchasWilliamWhewell,thathistheoryconstitutedgood science. Whewell was engaged in a dis-pute with John Stuart Mill on the best way toimprove on Francis Bacon’s notion of induc-tion, which was considered the centerpiece of the scientific method. Mill’s position was closerto Bacon’s, that new observations representeda starting point for generating novel hypothe-ses. Whewell, however, thought that hypothe-ses had to guide scientific discovery and thecollection of new observations. In this context,Darwin’s work in  The Origin of Species   was ini-tially dismissed as an egregious example of de-duction, and therefore bad science. This wasobviously not the case: Darwin’s method ac-tually followed closely what Whewell referredto as “consilience,” a convergence of differ-ent sources of evidence toward the same infer-ence.Indeed,Darwinhimselfwrotetoafriend:“How odd it is that anyone should not see thatallobservationmustbefororagainstsomeviewif it is to be of any service!” (Darwin & Seward1903). Today it might seem odd that a scientistwas at pains to justify his research to a pair of philosophers, and yet evolutionary biology stillpartially retains the unfortunate reputation of beinga“softscience”(Pigliucci2002),inpartasa consequence of lingering philosophical con-fusion about the nature of historical versus ex-perimental research (Cleland 2002).What the original Darwinism was reallymissing was not a solid philosophical founda-tion but rather a theory of heredity. Darwinfamously struggled with it, alternatively flirt-ing with both Lamarckism and his own theoryof blending inheritance. Mendel published hisseminal paper on the genetics of peas in 1865,soonafter TheOriginofSpecies   andwhileDarwinwasstillactive,buthisworkremainedunknownuntil the turn of the century (the perils of pub-lishing in obscure journals while pursuing anadministrative career). In the meantime, how-ever,Lamarckismwasbeingsecurelyexpungedfrom evolutionary theory, mostly through theefforts of Wallace and those of August Weiss-man, who proposed the theory of separationof germ and somatic lines (which would soonprove to be inapplicable to many organisms,beginning with plants). It is this “Darwinismsans Lamarckism” that should be historicallyreferred to as neo-Darwinism (not, as ErnstMayr repeatedly pointed out, to be confusedwith the later MS).By the turn of the century neo-Darwinismstill did not have a theory of heredity, andthe concept of natural selection itself was in-creasingly questioned in favor of alternativessuch as orthogenesis and mutationism (Bowler1983). Indeed, it was the latter notion, togetherwith the emergence of the new discipline of “Mendelism,” that caused a crisis in the fieldandseemedtospellthedemiseofDarwin’sview  220  Annals of the New York Academy of Sciences  ofevolution.Inanutshell,itdidnotappearpos-sibletoreconcileDarwin’sinsistenceoncontin-uous variation and gradual evolution with thenew data on discrete inheritance of Mendeliancharacters or with the equally discrete effect of mutations that began to be isolated in the lab-oratory. It was no coincidence that ThomasHunt Morgan, one of the early Drosophilageneticists, initially accepted the idea of evo-lution but rejected the mechanism of naturalselection.The resolution of this impasse began withtheclassicworkofFisher(1918),Wright(1932),and Haldane (1932), who laid down the foun-dations of the field of population-statistical ge-netics.Theseauthorselegantlyshowedthatthe joint expressions of several discrete Mendeliangenes can cumulatively produce the effect of a continuous, Gaussian distribution of pheno-types. Not only was there no contradiction be-tween Darwinism and Mendelism, but in factthe latter provided the much sought-after the-ory of heredity that had eluded Darwin and hiscontemporaries.This interdisciplinary reconciliation, how-ever, was only the beginning of the MS, whichunderwent a second, expansive phase during the 1930s and 1940s. The additional work took the shape of an impressive series of booksby Dobzhansky (1937), Huxley (1942), Mayr(1942), Simpson (1944), and Stebbins (1950).The resulting body of theory truly deserves theappellation of “synthesis” in that it wove to-gether not only neo-Darwinism and popula-tion genetics but also zoology, botany, paleon-tology, and natural history. Entire new fields of research, such as that focusing on speciationmechanisms, opened up, and old explanations,such as orthogenesis to account for macroevo-lutionarytrends,wereshowntobeunnecessaryas gradual Darwinism was extended to paleon-tological time scales.The MS is still the accepted version of evo-lutionary theory, summarized in the following manner in one of the leading textbooks in thefield (Futuyma 2006): “The major tenets of theevolutionary synthesis were that populationscontain genetic variation that arises by randommutation and recombination; that populationsevolve by changes in gene frequency broughtabout by random genetic drift, gene flow, andespecially natural selection; that most adaptivegenetic variants have individually slight phe-notypic effects so that phenotypic changes aregradual; that diversification comes about byspeciation, which normally entails the gradualevolution of reproductive isolation among pop-ulations; and that these processes, continuedfor sufficiently long, give rise to changes of suchgreat magnitude as to warrant the designationof higher taxonomic levels.”  An Extended Synthesis The question that has been raised by an in-creasingnumberofevolutionarybiologistsoverthepastdecadeorsoiswhether,morethanhalf a century after the consolidation of the MS, anupdatetotheconceptualstructureofevolution-ary biology is needed (Rollo 1995; Schlichting & Pigliucci 1998; Carroll 2000; Gould 2002;Muller 2007; Pigliucci 2007). The fundamen-tal reason for an ES was put forth early onby philosopher Karl Popper (Platnick & Rosen1987)whenhewrotethat“[theMS]isstrictlyatheoryofgenes,yetthephenomenonthathastobeexplainedinevolutionisthatofthetransmu-tation of form.” Actually, evolutionary theoryneedstoexplainboththeevolutionofgenesandtheevolutionofforms,andtheMS,particularlythrough its population genetics backbone, doesa good job at the former. The latter, however,has remained largely peripheral until the ad- vent of “evo–devo” in the mid-1990s, and evensoonlythesurfacehasarguablybeenscratchedso far (Robert 2004).More specifically, we can begin to articulatethe need for an ES by posing a series of ques-tions for which the MS provides partial an-swers or no answer at all. For instance: Whatcausal roles does development play in evolu-tion? Is evolutionary change always gradual(andwhatdowemeanby“gradual”)?Isnatural  Pigliucci  :  An Extended Synthesis for Evolutionary Biology   221 selection the only organizing principle produc-ing biological complexity? Does natural selec-tionproduceevolutionatotherhierarchicallev-els in addition to the organism, identified byDarwin, and the gene, added after the consol-idation of the MS? Is there, once and for all,a discontinuity of some sort between so-calledmicro- and macroevolution (and again, can weagree on the meaning of these terms)? Is thequestion of inheritance completely settled orare there additional mechanisms besides thestandard genetic one? How do novel pheno-types arise, and do they represent a distinctclass of phenotypic change? How do ecologyandevolutionarybiologymesh;thatis,caneco-logical and evolutionary theories be related? A satisfactory exploration of these questionsis obviously beyond the scope of this essay, butin the following I sketch the outline of someapproaches that are being pursued (Table 1).This outline emerged largely from a workshopon the structure and future of evolutionary the-oryattheKonradLorenzInstituteinAltenberg (Vienna, Austria) during the summer of 2008,the full proceedings of which will be publishednext year (Pigliucci & Muller 2010). Of course,those present at Altenberg were but a sampleof the theoretical biologists, empiricists, andphilosophers of science that are actively in- volved in attempts to articulate an ES. As a first issue, biological theory still mustcome to grips with the question of contingency versus necessity (Beatty 2006). Darwin him-self highlighted the opportunism exhibited bynatural selection in shaping biological forms,for instance, in the example of the flowersof an orchid species that are twisted 360 ◦ topresent a particular petal to the pollinator, be-cause they derive from ancestors whose flowerparts were already rotated by 180 ◦ . JacquesMonod (1971) and Franc¸ois Jacob (1977) fa-mouslywroteabouttheroleofchanceinevolu-tion, and of course Gould (2002) made it a cen-terpiece of his evolutionary analyses. To someextent the issue is now becoming experimen-tally tractable (Travisano  et al.  1995), but it isstillmarredbyconceptuallydifficultissuescon-cerning historicity (Cleland 2002) and whetherit is possible, even in principle, to separate con-tingent from deterministic processes in evolu-tion (Pigliucci & Kaplan 2006). Another issue is posed by the necessity fora significant renovation of the mathematicaltheory that accompanies the MS. One exam-ple concerns the models and especially themetaphor of the adaptive landscape that haveguided population genetics theory virtually un-questioned throughout the 20th century. Theidea was introduced by Wright (1932) to modelthe problem of the relative role of contingency versus necessity (where contingency is repre-sented by the effects of genetic drift). Wright’smetaphorgeneratedamajorlineofmostlythe-oretical research hinging upon questions suchas how populations effect a “peak shift” whentheyare stuckin asuboptimal area oftheadap-tive landscape. Modern computing power anda serious reanalysis of the whole idea of “land-scape” suggests that, in the (ubiquitous) case of  very high dimensionality, talk of “peaks,” “val-leys,” and—  a fortiori   —peak shifts, simply losesmeaning (Gavrilets 2004; Pigliucci & Kaplan2006). It remains to be seen whether this is aninstance of population genetics theory reach-ing its limits in terms of the degree of biologicalcomplexity it can handle (Dupr´e 1993) or if anew but substantial reformulation of its meth-ods and conceptual domain will move thingsforward again. A second example of expansion of the math-ematical theory underpinning evolutionary bi-ology is the idea of   multilevel selection theory . Thishas been around for decades in the form of kin and group selection, but recently enoughtheoretical (and, to a much lesser extent, em-pirical) work has been done to put multilevelselection theory on the main stage of evo-lutionary research (Okasha 2006; Wilson &Wilson 2007). It is now clear that several levelsof the biological hierarchy are, at least theoreti-cally, legitimate targets of selection, from genesto individuals, from groups of kin to popula-tions to species (but, interestingly, and contraryto a relatively widespread opinion, not clades
Related Search

Previous Document


Next Document


We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks