A major transition in Darwinism

(thecell) actinq’far thcgoiG 01the group (the organism) is. so to speak. turned uoside dawn and replaced by the concept that the autonnmy of the ,,n,t ,s con‘puredarwinism’. and use th<.term ‘suciai strained bytherules ofthegrnupl. in multidarwinism’inevitably with tbempiication cellular orgaoisms these rules include oi’red in toothandcla%.Thissuggeststo (I) the mhlbltion al replicaban of c‘tis by nowthat Ihe public image o, evolutunary gene-exfx~ssed tartars. present already theow is JS doamaticalty sim~tistic to&> io Vcllooxcolonies and other multias it was one hundred years ago cellular systems’-s: and (2) the installation Evolutionarv bioloaists should wel- of an ‘epigenetic memory‘ which maincome discuss&s directed towards two tains somatic ceil lines in the diiierenti. closely linked goals. firstly. 10 present ,u ated state: s Conflicts (latent or maoifest) the public (including creationists?) rl new between entities and system indicate that and better balanced view of evolutionary the transfer of autonomy, and thus the theory; dnd secondly, to suggest to bioirr- switch from one unit of selection to angists in general that. although the players uther. can never be absolute. Entities will have remained thesame. the focus ofevuinevitably be bwaking rules and regaioiutionary biology has changed in receot ing- temporarily at least -an autonomous years - the really important task having or semi-autonomous life. This is true for shifted irom explaining the ‘“ridin of spe- parasitic genes within the genume” for cies’ to the n,ude,,,n~ uf the processes by rranslormed cells in mutticeltutnr organwhich autonomous parts cooperate (or are isms. sod for worker bees in a bee cotcnyl’. coerced) to form systems of gruter orga. nizationai complexity. Complexity Leavmg aside the example of the oriII in the course of evoh?:on sxne eow gin of human societies, MSESdis!inguish ties (molecules. genes. hypercycles. cells. seven major transitions. ranging irom rep solitary individuals) have combined to ktating molecules lorming populations of form larger systems, structural and funcmolecules in compartments. to solnary tional camf,ienty m”st have Increased. individuals forming integrated soueties. However, :his does not imply that wokThe feature common to each of these tran- tionary design included a general drive sitions is that entities ‘capahie 01 iode. towards increasing complexity After all. pendent replication before the transition prokaryotes haw prohabiv re.nained at the same organlzationa, Ieve, 1,x morn can replicate only.% part of a larger whole after it’ It is the mechanisms behmd this than three billion years. and it is known traosfer of replicative autonomy from ‘e,,. that structural complexity has decreased tities’ to ‘wholes’ as well as the conflicts during the evolution of parasitic ibnes. between these two levels 01organizational Some authors. like McSbea” and even complexity that occupy the centrr stare MSES. remain sceptica, about the posst 01 today’s evolutionary theatre. A+Gnst biiity of objectively deiining increase in this background a number of cootrzwr complexity a.5an importmt feature of eusial issues of evolutionary biology appear karyotic evolution. To me it appears that in a new light, as the following examples such scepticism IS justiiied only ii a onrIllustrate. dimensional measure of complexity is ap oiied MSESbased their interoretatwn exUnilaof selection &sive~. on genome size. MfShea 011cell When entities combine to form a types and morphological features; both larger whole. their autonomy is part,a:,y authors~ametothecanclusion tha! these transferred to the newly created system. measuressuggest an increaseofcompiexIVhereas before the transition selectiw ityaiongsomeztretches of phylogeny but forces acted excius~veiy on entities (for not along others. However, if the back.. example on solitary ceils). it is the whole bone of biological evolution is considered (the multicellular organism in this case) focons~st olaserlesot that becomes the locus of selective sc- then :heevolution ofcompiexityaisom~~st tion after the transition. This statement have undergone major transitions. The woome size of mvcoolasms. the contains the essroce of the group selection controversy. although it loses its scli- smallest organisms. is abo& tdh base pairs righteous f,avaur if the notion at the unit (hpj. to bacteria. fun& &,c and lower

A mlajortransition in darwinis Wolfgang

Wieser

lines

Many of evidenceindicate that In recent years the IOCUB01evolutionary bir~lagyhas begunto ahilt tram exDtalninethe ‘orktn o‘swcler’ to the

coemad) to form ?iystemsof greater comtduxity.If such a major shift Is indeed rrcuning. then it shouldbe more widely promulgatedto counteraci the puMk image of evoiutionary theory, which ~pwam to be as do@na:kaiiy simpiistic taday 88 It was a century ago.

Smith and Eiirs Srathmary (USES)’ has been haiicd’ as one of the most important books on evolution since Darwin’s Origin ofSnec!es or. more modestlv. since t&i:, Ce:eml nreoryol~l”lu*lS;lectiun. By structuring a central theme that has emerged from 50 years of discovery and progress in biology, MSEShave provided what may turn out to be a platform from which ‘darwinism’ can he seen in a new light. To most people, evoiutianatytheory. in the guise ol darwinism. is still dominated by th,? ideas al competition (selertion acting on variability) and survival oi the ftttest.lt.; best known ptctorialsymbo, continues LDbe the phytogenetic tree. in the linht of the conceot of ‘transitions’. outlin-ed by MSFS. the’ popuiarlzation oi evoiuttona~ theory might enter a phase dominated by the ideas of cooperation and control (systemic constraints acting on autonomous entities). and of conflicts between units, of selecti m on different levets of argsnir.ltioo. SUC’Ia change in perspective would be des rabte for several reasons. oneofwhlch bGng that the news media of the westerir world continue to describe the present-day economy and economic philosophy stereotypically as TRIFI:,,o, 12. no 9 se,,,rmh,v ,wi

simple

‘majorfransitinns‘.

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PERSPECTIVES. 7

t Progress and ecological expansion AS MSES pLlint out. ‘IlIP “otion 01 pr* cress has a bad name among evolutionary biok,gists‘. Howevrr. if they had rellected upon Humpty Dumpty‘s dictum”, ‘When word it means just what choose i,tomean.. ‘.f.“‘7Scoutd havcadded that whether the name 01 prupress is bad or no, depends entirely on bow ihe word 1s defined. If ““P &fines ‘QrO&!ESS -
use II

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organisms to climb evolutionary ladders. then how can one account Ior the fact already lnentioned that prukaryoles do not seem to have progressed at a11far more than 3x 1~7~years? There are other ways. however. of definmg progress. For example, a specilic ecolugical niche or a spe cific biulugical lunction may be identified and tbe ouestion asked on which time sale (‘te~~po’) and along which pathway (‘mode’) dilferent line? of organisms progressed toward filling that niche or perfectinr ,“a, function. How did aquatic hving in the viscous regime of low Reynolds numbers manage to !“o”e into and gradually conquer the inertial regime of hlgh Reynolds numbers? Engineered by selection,variation and internal constramts. surely there has been prw gress along this ecologically delined pathrily,leadingtnlarge,lastswimmingsquids and fish, This statement is independent of the fact that in the same medium a multitude of organisms remained successfully in the viscous regime. and that some other urganisms moved into the inertial regime without exploiting to the full the potential veryfinteetidence however-criticiresths o, last traffic between distant places provalues reported lor the higher vertebrates. vided by this regime. but. at least from SDOK!~S to lower verteEach 01 the major transitions discussed by MSESrepresentsJnorganizationallevel, brates propartion~lity’between the numoccupied by a certain type of biological berolcelltyprs and theappearanceofanimaltsxaon thegwlogicaltimescaleseems system (4s. multicellular organisms. s* to be fa.irlywellestabllshed. &ties) that evolved and created phyla genetic relationships. Another type cdeve The inlormation available at present suggests that diversification of cellular lutiunary line, probably characterized by an outburst of conflicts between units of fbmctions proceeded at a faster geological rate in invertebratesand lowerveltebrates selection. connects the levels of increasing than in higher vertebrates (if it increased complexity.Thetransitionsfrom,let ussay. in the la&r at all). On the dther hand, in a the prokaryote to the cu’kayote, or from the unicellular to the multicellular level of well-known paper’“, A.C. Wilson showed organization. repraent aspects of bin&L that In v&b&s !he pace of morphocal evolution that cannot be understood by logical evolution accelerated with gealogi. cal tune, the acceleration constant being calculating degrees of genetic sinlilarfty or related to behavioural diversification and by establishing homologies. ‘Progress‘ in our understanding of these major events of relative brain size. Since brain size reflects the number of neural elements and conevolution required the introduction of elnections. it represents. by definltton. yet ements of game theory, population biology another measure al the complaxtty 01 orand developmental biology.

orga&s

di&sity of the organ& up xl a value of about 1Pbp. Beyond this value. in the major groups (11invertebrates. in chordates and flowering plants, the correlation breaks down In closely related species or genera. genome size may wry by more than two orders of magnir?:de. However, in fnrger taxa there is no furttrer increase of the owrog~ size of the genome. which wrks out at approximately 3x KPbp almost exactly the size cd the haploid human gaomr. This is the essence of the C-value paradnu”, which seems to imply that above a critical value genenome size is replaced by another measure of camplexity. lor example by the diversification 01 cellular function. According toVakntineet al.‘~.thenumberulcelltypes inmetazoan~ increases from 1 in placozow~s to about 203 in higher verteb!.ics McShwll -on 366

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PERSPECTIVES If one considers the mafur transitions to have been th? really important even!s in biological evnlutw Ibe llhylogenetlc ‘rceloses its promiwnt r&as a keysymblil “i darwmism fn Fig. 2.

I s”g$est an-

other kind of symbol: here. the prucess of evolutiun is represented by a wt 01 con. centric circles, each cbcle standing for a transition from a”to”“l”o”S entities to an inteeratrd wstem At each of rbese leveh organisms evolved and established re. latlonships that may be pictured in the form of convefit’anal branching patterns III additiuu. there areavenues (thick ;ines) connectingorgallilationaf ievefs. from the centre fjowest complexity) to lhe periphery Ollghest complexity). Conflicts between enttt!es and system are inwftable, constituting

just

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an cl.

ement of the evolutionary process as competitlon between entities. The model prusented also alknvs accommodation of the relationship between genetic and cultural evolution. i.e. gene-culture coe~ol~iion~~. The human individual is part 01 the gmetic seras 01 transitions, but at the same timeitalsooccupiesthecentre(I)oladiiferent set of transitions consisting ul so cial and cultural organizations olvariable size, strengthand quality(the nameschosen In Ftg. 2 serving only as examples to which others could be added). The two setsofconcentric circles arecl&ely bnked but are to some extent also iudependent of eachother. Conllicts between message from genome and braln are frequent and well-known ieaiures of human sociology. A set of concentric circles .w the key Visual symbol for the evolutionary pre cesscallsforth associationsdifferent from those evoked by a branching tree. The vertical dimen:ion of the tatter suggests totheobserveradistfnafon between’low’ and ‘high’, insinuating, lor example, the supremacyof thelndd;ermanic language and race” or the pathway to Omega and fullilment’8. In contrast, the equally arte Iactual set of concentric circles does not readily associate with the concept of llnear oro~~ress but. rather. with the idez of exp&i& with theXvir&ning*of a quality which so far have identified as systemic complexity. However. the driving force behlnd this expansion must besomething else, somerhtng on which selection can act, and this can only be ecological diversification. This is rbe quality that increases with each transition: specialization and differentiation allowing theexploitation of new sources of energy and materials; SW cial systems engireerfng dramatic expan sions of resource bases as compared to those used by solitary species? trophic tfnkriormfngtrophfcnehvarh;and,li~~ally, gene-culture cowolution setting into me tion the restructuring of ecological relationshlps ca a global scale.

I

Tbenatureof such anecnfoglcafvector is entirely different from that of the tek% fogicvectortowards perfectionvisurlized

byTetlharddeChardfn’x. It is afsoverylar removed irom the single skewed distribuLioncurve. which, arrordingtoS.J. GouWs 369

BOOK

REVIEWS

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~~_._~ Tropical forest: the quest for generality

~_~__

aim-t? and arwnd it chimges

Given that the withln certain environr.lental limits. I think largely passive and Sochaatw process 01 that our ecology would improve by asking questions about certain species d ISPersd leads to seedlings establishing III a --.._-_‘L, s’*es and not looking for presentlywide ranqc 01 subcanovv light cawhtions. it 1.dt certam is not w&sing that &ear; concluding that percewed tooad generalities that we know tree species arevely plastic In their light rewill have weak predictive power. The basic The Ecology of spunses, largely simtlar (certainly most are grawtb and reproduction Tropical Forest Tree Seedlings unkhelytu beniche differentiated). and prob are sha. zd everywhere, but rarely are the (Man and the ably corrvergent in their characteristics over conditions and the histories. Biosphere Series) evolutionary time. I we the same problem with the second edited bx Mi>. Slrarnr Patterns al seedling mor:ality withinand book. Urianselul. askwhether differences in ~tweens~iesareena~oustyvariableand species diversity in tropical ecosystems lead are J poor pred!ctlve guide to canopy cumto changes in ecosystem processes such as vositinn. Survival tot he sapling stage is cergrowth, nutrient cycling and water use: or tainly a precondition. but much else can deput more speclllcally, whether loss in birr Biodiversity and Ecosystem tem.in:~~L”h;pEc!csgron’tonaturity.~rly diversity afferls fox;: !unr:ionin%The idea Processes in Tropical Forests grotih IS morelikelyto besize-classdensi!yis that tree species can be placed intu ecoindepndnlt. butlaterSrnwthmoredensity physiological and structural groups so that edrred by C.H Orions. R Dine und J H Ctrshmon dependent. and nl course what is measured diversity within and between these groups currently (a-1Uyr typicaily) Is not a strong can be studied. This is not aa easy taskand Springer-Vertag, 1996. guldeto theconditionsof recruitment 01 the / this vohnneshows but the beginnings. There D~tl9R.W hbk (xi + 229 pages) (perhaps 50-2Wyr ago) since conISBN 3 540 592iS X are manygrrups butthese are non-exclusive dllions most likely will have changed and and are artdnged again along continua. For These voIun~es lrit w unsett!ed and not historical events staved a marked and vuncexamvle tree svecies able to cove with WY 1 much enlightened. The lack of current i tuatinr! role. There is also the arow%& re- lowdhusuhork soils mav al& be tolerant progress. I co&d. lies in an unsuccesslul aliz& that light requIreme& aad reofdrought.Not toounexpectedly, the conclusearch for generalities in community ecalof spec~cs lie on cnntinua, and the sin% Ior the moment are that there are few aw.trovical in this case. without the newsprevious classifications ol convenience are reliable and consistem pantropical trends ary and correct thwre!ical lramework Since giving way to a new appreciation of growlh wilh changing species richness and that at neither volume shows an appreciation of andsurvivalof juvenile trees. the level 01 broad groupings (such as shade slc&aSicity and sampling stattistics, unlorPerhaps we have come to expect too tolerant understorey trees) there is much ttmately they take us little further. redlmdnnrv. ~ much lromsyntheticoverviews. Manyof!he The bookedited bySwine demonstrates. chapters lack definite conclusions. FurtherThe &mulatlan of ecological ivloras doesmuch of thecurrent :iteratore in the thoseautharswith s%~peciflcstudies mation at the taxonomic level is not making field of tropical seedling ecology, that there rarely had a clearly-dehned pnenomeno~~ to our understanding (our ability to predlci are lew II any generalizatiurls emerging. The isvestigate or problem to answer. They saw change) any clearer. In the past decade we morestudies done. the more lragmentary is that their Ior& had several species and then havecome to realize that every forest site is our understanding. Take, for example, light tried to eluetdate in which ways their seedunique in the details of its species cnmposiwithin the forest. The varying irradiance m hogs differed to explain complementation tinn, and that reeking generallzaHons at tcw ceived by any one seedling (or part oi it) broad a scale lsnot sensible (or particularly and coexistence. Surely, we have moved on changesthroughthe day. changes as it grows lrom such an approach: we should be more useful). Environmental stochasticity. his(if it does) and changes as the vegetation aware 01 the Rolex of history and chance. tory, regional inlluence and chance factors

more direct

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