CROTCHETS & QUIDDITIES

We Hold These Truths to Be Self-EvidentKENNETH WEISS

Evolution is a pervasive cultural aswell as scientific metaphor of our age.An evolutionary worldview is as fer-vently held by biologists today as werethe prevailing views that evolutiondisplaced. Ultimately everything iscredited to genes, and given a genes-for evolutionary explanation: genesfor upright posture, dominance hier-archies, language, IQ, heart attacks.We’ve seen a century of spectacularprogress in biology but this does notmean the foundations on which evo-lutionary explanations rest are unam-biguously established. It is not en-tirely clear how well some of ourvenerable concepts jibe with new ge-netic and developmental data pouringout of laboratories today.

I thought it would be timely to try toidentify aspects of traditional ideasthat still hold up well and areas thatdon’t do as well. Crotchets & Quiddi-ties will attempt to do that. Crotchetsare eccentric or idiosyncratic opin-ions, and this will certainly reflectmine. A quiddity, a term I first learnedfrom W.V. Quine1 (1987), is a philo-sophical essence or trifle, from theLatin quid, or ‘what.’ It can refer to thesometimes-intangible quirks, or nice-ties, of a thing or idea (which I hopereaders will not equate dismissivelywith today’s “Whatever!”). I hope notto be too crotchety, but “the intensity

of the conviction that a hypothesis istrue has no bearing on whether it istrue or not” (Medawar, 1979). In thiscolumn I want to examine some of ourassumptions, to stimulate thoughtabout them.2

The prevailing cosmology thatgreeted Darwin’s Origin of Species in1859 rested on the theologically basedassumption that the universe was cre-ated at a single point in time by apurposive intelligence who selected abestiary of species designed to beadapted to their environments. Thiswas assumed to be given truth ratherthan something one had to infer fromobservation. By comparison, in biol-ogy we believe we are practicing a rig-orous, objective, empirical method-of-knowing that does not rest on wishfulthinking. Yet much of our work restson axioms—conventional wisdom orlaws of Nature, if you will—that weassume to be true, but cannot actuallyprove.

THE FORMAL BASIS OFMODERN BIOLOGY

We can quibble (“quiddit”?) overwhat should be included or excluded,but among the basic principles of dar-winian biology are (1) all life ultimatelycame from a point source in the prime-val molecular slime; (2) descent withheritable modification of biologicaltraits; (3) the origin of biological formand of species by adaptation due to nat-ural selection via differential repro-duction; and (4) a uniformitarian ex-trapolation of these phenomena backto the origin of life.

Although descent with modificationis directly observable today, evolu-

tionary biology insists on its univer-sality, which is an assumption. Theother three principles are clearly as-sumptions. Nobody taped the start oflife. We do not really know from directexperience how speciation occurs(and have some difficulties even defin-ing species). And “uniformitarian” istoo big a word to be anything butmade up (assumed).

The success of physics in the 19th

century, with its formal theoreticalbasis (e.g., Newton’s laws) had wide-spread impact; anything less is “soft,”“descriptive,” “natural history.” In the1930’s and 40’s the neodarwinian syn-thesis provided biology with its ownuniversal, axiomatic theoretical foun-dation, in the form of population ge-netics. That formalized the basic prin-ciples of evolution into a theory thatgives theoretical cover for evolution-ary inference and is the pervasiveframework for designing and inter-preting biological (including anthro-pological) studies. The theory wassubsequently augmented with its ownatomic units—genes—and the addi-tional axiom of the Central Dogma,that a gene is a discrete molecularstructure that codes for one protein bya unidirectional information transferfrom DNA through RNA to protein.

Population genetics begins by posit-ing a Platonic kind of idealized infinite,freely mixing population, with no mu-tation, no migration, and no differentialreproduction. This still-life would notgenerate evolution as we know it, sowe relax each of the ideal conditionsin an orderly way, to analyze the con-sequences. Relaxing the idealistic as-sumptions makes things realistic, butat the price of making evolutionarybiology probabilistic in theory,3 be-cause the modifications to the ideal-ized population are all stochastic: ran-dom mutation, random change in

Ken Weiss is Evan Pugh Professor of An-thropology and Genetics at Penn StateUniversity.

Evolutionary Anthropology 10:199–203 (2001)

Evolutionary biology rests more deeply on axioms than we may wishto believe. What are they and how sound is their logical standing?

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frequency due to sampling across gen-erations in finite populations, randomeffects of migration, random mende-lian segregation and recombination,and the randomness of changing en-vironments relative to existing geneticvariation.

The assumption of a point source isimportant to the entire theory, so weinsist upon spontaneous generation inthe beginning, but must forbid it atany other time. Under the assumptionthat genes don’t “blend” in the Dar-winian sense each generation, the ac-cumulation of mutation change leadsto divergent evolution. Mutationalchanges have accumulated from DayOne, so that genomes today reflect thesurviving sequences of their entirepast history. The extrapolation ofthese processes over long time periodsprovides theory for the directedchange in frequencies of randomly oc-curring genetic variants in response toselection, and hence adaptive diver-gence and speciation. Under theseconditions, the biosphere is con-nected, contingent, and hence unique.

HOW TRUE ARE THE AXIOMS?

It’s nice to know that population ge-neticists are on guard (even if they’re abit too geeky for our department), buttruisms can become vacuous or leadto uncritical thought. The core no-tions of evolution may be basicallyright, but they do have problems, notall of which are trivial. As we look atsome of them we should keep in mindthat modern evolutionary theory as-signs biological meaning ultimately togenes. The theory provides no explicithelp in understanding the evolution ofphenotypes unless we separately spec-ify the genotype-phenotype relation-ships that apply, and these (whenknown) usually have a probabilisticelement of their own. The probabilis-tic aspects of the theory are somewhatstrange: they don’t refer to repeatedscenarios, and we can’t specify or con-firm their actual values or distribu-tional properties, because of the ax-iom that life has evolved but oncefrom a single origin. Yet many biolo-gists have an assured, deterministicworldview, and this certainly charac-terizes much of biological anthropol-ogy.

A Point Source For All Life

The grandeur of Darwin’s view was“of life having been breathed by theCreator into a few forms or into one.”But is there a Tree of Life all de-scended from a single beginning? Ig-noring those who argue that life isseeded from space,4 earthly inheri-tance itself does not go strictly verti-cally from parent to offspring, but reg-ularly occurs across species. Deepbranches of gene phylogenies for themajor classes of life are inconsistent,in part because of the transfer of en-tire bacterial operons (clusters of func-tionally related genes), plasmids, mito-chondria, chloroplasts, viral DNA,and even perhaps the basic recombi-national component of our immunesystem. We have no way to be surethat there was not a diffuse ratherthan single point source of life in theprimal soup, because traces of any ini-tial diversity may have been replacedby subsequent forms.

Most horizontal transfer may haveoccurred before the evolution of mul-ticellularity, and hence be peripheralto the evolution of later species (suchas primates). Nobody suggests we de-rive our genes for limb developmentby horizontal transfer from frogs, say,but a percent or more of our entiregenome comprises exogenous (mainlyviral) genes, and about 35% is com-prised of repeated sequence elementsshuffling around within the genomebefore being transferred to the nextgeneration. These are not trivial frac-tions of our entire genome.

We routinely seek homologies byconstructing trees of gene sequences,or gene families, but the origin ofgenes by duplication or recombina-tional mechanisms sometimes playshavoc with that, because individualgenes don’t always “coalesce” in a sim-ple way back to gene-specific, or evenconsistent species-specific Adams andEves. Different fragments of eachgene, ultimately individual nucleo-tides, go back to different genes anddifferent common ancestors, that mayeven be in different ancestral species(Adam and Trog?). When we try toreconstruct the ancestry of presentlife, we have to be aware that it isreticulated rather than a Tree with asingle trunk.

Random Mutation

The critical evolutionary pointabout mutation is that it must be ran-dom relative to function; specifically,the needs that arise during an organ-ism’s life do not induce mutations tosatisfy those needs. We rightly strug-gle mightily against any Lamarckianchallenge to this fundamental tenet,which at present seems relatively se-cure; but there are a few cracks. Muta-tions and the equivalent variation-gen-erating processes of gene conversionand recombination are not randomalong the genome. The reasons are notall known, but there are suggestionsthat genes may sometimes be ar-ranged in ways that protect some re-gions of the genome from change,leaving others specifically more vul-nerable to change, for functional rea-sons. We know of systems in somespecies, especially bacteria, in whichrelevant parts of the genome are ac-tively made more mutable in the pres-ence of stressful environments, whichallows the organism to generate diver-sity that may enable it to survive.“Adaptability” systems of that sortdon’t lead to purely Lamarckian in-heritance of specifically acquiredtraits, but they approach it, and theassumption of the randomness of mu-tation relative to function is so funda-mental to evolutionary theory thatany experience-directed phenomenaneed to be looked at seriously.

Population History

Evolutionary change at the genelevel is affected by population size andstructure, migration and mating pat-terns, and the time and place of eachmutational event. When we attempt toreconstruct evolution from patterns ofpresent genetic variation, we have tomake assumptions about the unknow-able details of that history. We areunable to write an equation for theexact trajectory of, say, the evolutionof sparrow flight the way we can forarrow flight. Instead, we create as-ifmodels. Real populations have allsorts of structure, which changes ev-ery generation. Since we don’t knowthe actual details, we estimate the as-if“effective population size,” Ne, of a ho-mogeneous population that over verylong time periods would generate the

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same amount of genetic variation thatwe observe today. That’s where thecommonly used value of 10,000 forthe size of source population for mod-ern humans comes from. We makesimilar assumptions about averagemutation, recombination, and migra-tion rates, and selective coefficients.The resulting theory has built-in non-identifiability issues, because differ-ent sets of plausible parameter valuescan yield the same result. We face ad-

ditional problems when we try to re-construct the evolution of complexmorphological or behavioral traits,because we typically have no idea howmany genes are involved or the selec-tive and stochastic forces that actedon them. Yet it is routine to describemorphology as-if it evolved in adap-tively deterministic ways, and as ifthat makes the explanations true be-cause they are consistent with popu-lation genetics principles (genes-for

upright posture were favored in this-or-that environment, etc.).

We construct evolutionary scenar-ios as-if conditions were simple,though we know they weren’t and thatwe usually can’t specify a very believ-able uncertainty range.3 This is differ-ent in kind as well as in degree fromthe approximations introduced inNewtonian theory in making predic-tions of an arrow’s trajectory. Too of-ten, the result is over-determined,largely uncheckable Just-So recon-structions. We may choose to believeour favorite scenario (and of coursemight be right), but that is not as dif-ferent as we might fancy from the patnature of other cosmologies, becausewe can always reconstruct a plausiblestory given our axioms. This is onereason we have so much trouble arriv-ing at consensus even about relativelysimple, recent events at the core ofour discipline, like modern human or-igins.

Extrapolation, Speciation, andAdaptation

We treat population genetics as ameans of explaining darwinian evolu-tion of species, but population genet-ics does not really deal with the for-mation of species per se; we have toadd them deus ex machina to our the-ory. The theory itself is uniformitarianin that it specifies microevolutionaryprocesses that we extrapolate indefi-nitely over time and space. It may beright to assume that speciation in-volves genetic divergence produced bypopulation genetics processes actingover long time periods, but that’s ba-sically as speculative as when the Syn-thesis was proclaimed 70 years ago.Genetic divergence can occur withoutspeciation (you and I are different),and speciation can be uncaused bygenetic divergence (e.g., behavioralisolation). Surely, also, species canarise by drift rather than adaptive di-vergence.

Whatever species are, they may ormay not arise at a single point in time.Most theory is darwinian (gradualisticand adaptational), but it may not re-quire much genetic change to estab-lish a mating barrier; which anywayneed not involve adaptive change. Therole of adaptation has if anything

Figure 1. Is any explanation ‘true’? A drawing by Ernst Haeckel of natural diversity inseashells. What could be the source of this variation? Does evolutionary theory really“explain” it?

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been reopened by recent discoveriesof homeotic mutations in laboratoryexperiments (e.g., the production via asingle mutation of extra digits, limbs,teeth, etc.). These experiments are rel-evant to anthropological concerns likethe evolution of primate dental orlimb morphology, but there is littleuseful theory for it.

There are deeper difficulties associ-ated with modeling adaptive evolutionthat may not be widely perceived. Se-lection means genetic change becauseof particular traits. Selection is typi-cally treated as a deterministic“force,” in contrast to the randomnessof genetic drift. But what could makeit a force? A force should have a con-sistent magnitude and direction, butdifferential reproduction among ge-notypes is probabilistic at best and ex-cept in the extremes it is difficult tosay why it happens. Those clear-cutextreme effects (like lethal mutations)are typically rare or biologically unin-teresting, and don’t appear to explainmost variation or adaptation. It is no-toriously difficult to discriminate be-tween selection and drift, based indi-rectly on existing patterns of geneticvariation or on direct observation ofdifferential reproduction.

We face the additional problem thatwe don’t know how many genes areinvolved or the genotype-phenotyperelationships for traits whose adaptivehistory we would like to understand.If homeotic mutations do occur—forexample to change the number of dig-its or teeth—they could easily rise tohigh frequency by drift alone. Adap-tive scenarios usually assume stableenvironmental forces over very longtime periods (strong enough to over-come genetic drift), which we usuallymodel as a kind of average, at best,but may be pure guesswork. For ex-ample, the human brain may be anadaptively selected structure, but it isno joke to try to reconstruct the eventsthat were responsible over the pastmillion years. Brain size may havegrown by an average of 1 mm3 pergeneration, but how is such changemanifest in terms of the actual fit-nesses of the beasts that competed dayto day on the African plains? Probablyby selection at the extremes in eachgeneration, but that’s just my guess.

Many biologists (and biological an-

thropologists) see the world throughselection-colored glasses, treating se-lection as prescriptive or energeticallyefficient for almost any trait. I thinksuch strong adaptationism leads to ex-cessive genetic determinism—theubiquitous invocation of genes-for ex-planations. We’re surrounded bymanifest evidence that selection is notso prescriptive, because there is sub-stantial variation and inefficiency inmost traits, in most organisms, mostof the time. Genes are important, andsince we develop from a single cellthere must in some sense be genes-foralmost anything. But traits are not al-ways tightly determined by specificgenotypes. Given this, it may actuallybe more difficult to explain the deepphylogenetic stasis that we see insome genetic mechanisms (e.g., Pax6and eye or Hox genes and axial andlimb development) and morphology(e.g., horseshoe crabs and cockroach-es).

WHEN IS FITTING TOO WELLNOT FITTING WELL ENOUGH?

There are no theory police, but the-ory does help science by placing con-

straints on what are acceptable asser-tions. A problem with issues like thoseI’ve raised here is that they allow evo-lutionary theory to fit too well, en-abling us to evade some of the coreconstraints by which we judge scien-tific statements. Evolutionary recon-structions are probabilistic and in asense not clearly falsifiable. This al-lows us to cling to favored hypotheses(like genes-for disease or various be-havioral traits) in the face of repeatednegative results. Even to specify thelikelihood of our reconstructions weuse statistical methods (like signifi-cance tests) that assume samplingfrom replicable phenomena, yet weknow that evolution produces uniqueobjects by unique events. Though his-torical sciences can’t claim the predic-tive powers of experimental sciences,we counter that at least we can “ret-rodict” the past. But in making (up?)the kinds of plausibility stories de-scribed above, all we usually can as-sert is that observed data are compat-ible, via the axioms of evolution, withsome scenario we can imagine. Inthese senses, evolutionary theory fitsthe data too well. But in some sensesit does not fit well enough.

In Ptolemaic astronomy, the heav-enly bodies circled the Earth. Occa-sionally, however, one was seen tomove backwards. Retrograde motioncould be explained by hypothesizingad hoc “epicycles” in individual orbits,as illustrated in Figure 2. This workedfor a while until too much of it wasneeded, Ptolemy was replaced by Co-pernicus’ Sun-centered model (Kuhn,1957), and many exceptions fell intoplace. The realization of evolutionsimilarly changed how we view life,explaining many things at once. Be-cause its essential ideas about com-mon ancestry seem doubtlessly to beright, evolutionary theory has resisteda century and a half of assault,5 butsome of our basic notions about theprocesses involved are becoming a bitfrayed at the edges in light of recentgenetic data, and evolutionary modelsare loaded with ad hoc tinkering andexceptions in order to make them fitwhat we observe. Are there placeswhere we need to get off our epicycles,because questions we haven’t an-swered can’t be answered adequatelywith our current premises? Or is the

Figure 2. Scientific theory going around incircles? Hypothetical path of an object ro-tating about the Earth, with an epicycleadded to explain apparent retrograde mo-tion as viewed from the Earth. Top shows thehypothesized orbit, bottom the apparent di-rection of motion. (see Kuhn, 1957).

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real truth that only a very permissive“theory” will accommodate the facts,because evolution is messy at its core?If so, by what are assertions to be ad-judicated other than shouting con-tests, ever-recycling the same alterna-tive scenarios? Good for employment,not so good for understanding Nature.

One can debate the exceptions, re-visions, or uncertainties in these tra-ditional notions about how evolutionworks. The importance of things likehorizontal transfer of genes may bechallenged because they are rare. Butmutation is also rare, and muchhinges on the relative effective rarityof each evolutionary factor. If we startsaying that rare things are unimpor-tant in evolution, do we also have todismiss selection because most of it isvirtually impossible to detect directly?If so, what we’re left with is drift.

The formal axiomatic theory of evo-lution is about 80 years old. In manyways, recent discoveries simply revisitissues we already knew about. Butnew data show clearly that genomesare more fluid, complex, and dynamicthan the tightly programmed, simply-evolving, one-way semiconductors ofprotein-coding information wethought them to be. It is healthy to beskeptical even of truths we hold to beself-evident, and to ask: suppose itisn’t true—what would follow? Do weneed a theory of evolutionary biology?

What beyond shared ancestry is invi-olate? Forthcoming columns will lookat where some of these issues stand atthe verge of a new century, to see wherethey still seem sound, and where theymay need changing, if we wish to un-derstand evolution more clearly.

NOTES

I would welcome comments on thiscolumn: kmw4@psu.edu.

I thank Anne Buchanan, as usual,for doing her best to make this as un-derstandable and responsible as shecan get me to be. John Fleagle has beenmore than forbearing during the itera-tions needed to get this off the ground.

1. I also liked the word for the sen-timental reason that Quine, who was aleading philosopher of logic, precededme as a math major at Oberlin College(but he was capable enough to makesomething of it).

2. My purpose is to stimulate ideas.That means being declarative ratherthan guarded, short on caveats, omit-ting countless credits to others whoalready thought the same things. Idon’t claim originality.

3. This is as distinct from probabi-listic (statistical) means of evaluatingdata. Evolutionary theory has prob-lems in both.

4. Not all their arguments are en-tirely trivial (whether they’re right or

wrong). For example, evolutionary re-constructions assume common ances-try on Earth.

5. Given a spate of recent anti-evo-lutionary books, I feel compelled tostate that nothing in this column in anyway questions the fact of evolution,which seems indubitably established,nor supports creationist accounts (onecannot call them “explanations”) for thediversity of life. See Crews references.

TO READ

Most things discussed here can beprofitably explored by web searching.

Crews, F The new attack on evolu-tion. NY Review of Books XLVIII(15,16), 2001.

Haeckel, E (1899) Art Forms in Na-ture. (available in reprint from Dover).

Kuhn, T (1957) The Copernican Rev-olution. Cambridge: Harvard Press.

Medawar, PB (1979) Advice to aYoung Scientist, Basic Books; NY.

Moore, JA (1992) Science as a Wayof Knowing. Cambridge; HarvardPress.

Provine, W (1971) The Origins ofTheoretical Population Genetics. Chi-cago, University of Chicago Press.

Quine, WV (1987) Quiddities: an In-termittently Philosophical Dictionary.Cambridge; Belknap.

© 2001 Wiley-Liss, Inc.DOI 10.1002/evan.10006

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