Mystery of Mystery

320 Pages·1999·0.43 MB·English

by  Michael Ruse

T
hat mysteryof mysteries,
y
thereplacement of extinctspeciesby others.
JohnF.W.Herschel,astronomer-philosopher,
inalettertoCharlesLyell,geologist,February2p0,1836
o
C

m
a
x
E
Copyright © 1999 The President and Fellows of Harvard College
ReligionwithoutRevelation 85
For Wright, the key to evolutionary change was the breaking of
large populations into small subpopulations, genetic divergence within
y
the latter, and then a violent shaking down as the subgroups rejoin
(Wright 1931). Very signi~cant for Wright was the fact that, if one has
a subgroup suf~ciently small and long-lasting, the purely contingent
effectsofmatingandtherandomeffectsofMendelianptransmissioncan
outweigh the force of selection. This essentially accidental and nondi-
rected form of change has come to be known as genetic drift, or the
Sewall Wright effect(Provine 1986; see box).
o
ThefactsofnaturewerenottotallyirrelevantforWright,anymore
than they were for Fisher. For ten years, the American worked for the
U.S.DepartmentofAgriculture,andanintensivestudyofcattlebreeding
C
convincedhimthatthemosteffectivemethodsrequiredfragmentation,
change within smallgroups,and thenareturntothewholepopulation.
But ultimately,thephilosophicalandculturalfactorswereassigni~cant
for Wright as they were for Fisher. Behind Wright’s theorizing lay the
teaching of his Harvard profess or of chemistry, L. J. Henderson (1913,
1917).Followingtheolderman,WrightwasanenthusiasticSpencerian,
m
believing in particular that nature tends perpetually toward a state of
moving or dynamic equilibrium: balance and homogeneity are forever
being disturbed by external factors; these disruptions provoke forces
trying to return the system to equilibrium; and in the overall process,
heterogeneityincreasesandnatureprogressesupwardtoabetterstate.
a
Wrightcalledhistheoryofevolutionthe“shiftingbalancetheory,”
meaning that one got an uneasy or ever-moving balance between the
forces for homogeneityand thoseforheterogeneity:divisionofpopula-
x
tionsandgeneticdriftbreakthingsupandcreatediversity,andthenthe
subsequent rejoining of populations and the natural selection that now
kicks in creates more uniformity. “The type of moving equilibrium to
E
be expected, according to the present analysis . . . agrees well with the
apparentcourseofevolutioninthemajorityofcases”(Wright1931,154).
VeryimportantherewasWright’smetaphorofan“adaptivelandscape,”
which saw populations of organisms as sitting on the tops of adaptive
peaks; then, thanks to disruptive forces, these organisms are driven
down to the less-adapted valleys and so up the sides of new peaks
(Wright 1932; see ~gure).
Copyright © 1999 The President and Fellows of Harvard College
86 JULIANHUXLEY
PopulationGenetics
Mendel’s law of segregation applies to individuals butycan be readily
generalizedtogroups.Supposeforsimplicitythatonehasjusttwoalleles,
Aanda,atsomelocusinalargepopulationandthattheratioofAtoa
is p to q (where by de~nition p + q = 1). Then the so-called Hardy-
Weinberg law states that, if breeding is at randomp, the ratio p to q will
stay the same and that, after the ~rst generation, the distribution of
genotypeswillbe~xedbyasimpleformula.
o
FPO
A – –A A – –a a– –a
p2 2p q2
C
Thislawimpliesthat,allotherthingsbeingequal,evenifageneisvery
rare,it(anditsphenotypes)willpersistinde~nitelyinthepopulation.As
Newtonian mechanics uses the First Law (bodies remain at rest or in
uniformmotion unlessacteduponbyaforce)asabackgroundassump-

tion against which one can introduce agents of change, so population
genetics uses the Hardy-Weinberg law as a background assumption
m
againstwhichone can introduceagentsof change,for example mutation
and natural selection. Onecan studytheeffectsofsuchcausesovertime,
con~dentthatthingswillnotjustfollowinahaphazardfashion.
Thelawholdsonlyforlargegroups.Insmallgroups,chancefactors
become signi~cant. Even though selection may favor a particular allele,
chance effects might swamp its virtues and driveit to extinction within
a
the group (or, conversely, establish as universal an allele not favored by
selection over its fellows). At any time, one might expect to ~nd some
variationnotunderthetightcontrolofnaturalselection.Thisisthekey
toSxewallWright’sconceptofgeneticdrift.Justhow smallgroupshaveto
beandhow manygenerationsarerequiredforcertaineffectstobecome
likelyorprobablecanbedemonstratedmathematically.
E
It was certainly not a necessary part of the picture, but Wright
interpreted all of this in a progressionist fashion. Indeed, this was a
major attraction of the metaphor. For all that the environment might
change, and peaks might no longer be so very peakish, or new peaks
might be lower than old peaks, Wright thought that the overall effect
over timeisupward.Indeed,we know thathehad ametaphysicalview
Copyright © 1999 The President and Fellows of Harvard College
ReligionwithoutRevelation 87
y
p
FPO
o
C

Abird’seyeviewofanadaptivelamndscape(fromWright1932).
of “panpsychic monism,” something which he got from the English
philosopher W. K. Clifford (1879) via the philosopher-statistician Karl
Pearson(1892). Wrightbelievedthatall oflifeis a combinationof both
a
mind and body and that there is a progressive scale whose summit is a
universal mind/body, a kind of naturalistic, post-Hegelian Absolute.
Thisbelief was certainly not partofWright’sscience—likehis teacher,
x
Henderson, he believed that one should keep one’s science and one’s
metaphysics strictlyseparate—butsucha beliefwasthe framework into
which Wrightwas happy to mold hisscience.
E
Justasoneshouldbewaryofassumingthatthepopulationgeneti-
cists single-handedly upgraded (or wanted to upgrade) evolutionary
thought to the level of what we (or they) would consider fully mature
science, so also should one take care in assessing the relationship be-
tween these men and those who came later and took up their thought.
By and large the biologists of yesterday were not very mathematically
inclined. Indeed, one can put matters more strongly and say that they
Copyright © 1999 The President and Fellows of Harvard College
88 JULIANHUXLEY
tended to _ee from ~gures. Hence, one doesnot ~nd thatotherevolu-
tionists simply took the formalisms of the population geneticists and
y
clothed them with empirical ~ndings. As far as the mathematics is
concerned,itfunctionedmoreasaninspirationandencouragementthat
evolutionary thought could be put on a formal basis if one was so
inclined. At least as important, the very existenpce of the formal work
gave a weapon to turn on those doubters and critics who sneered that
evolution deserves its second-rate status as a science for the museums.
By the 1930s, an increasing number of young biologists, ardent in
o
their evolutionism, wanted such a weapon. They were growing frus-
trated with the low academic status of evolution—its exclusion before
such subjects as genetics and experimental embryology and physiol-
C
ogy—and were forming the resolve to upgrade evolution as a science
(Cain 1992, 1994; Smocovitis1992). These people arethe main focusof
thisandthenextchapter.Ichoosetworepresentatives:forthischapter,
JulianHuxley,anEnglishmanwhowasultimatelyunsuccessful;andfor
the next chapter, Theodo sius Dobzhansky, a Russian-born American
who was ultimatelysuccessful.
m
Julian Sorell Huxley (1887–1975)
JulianHuxleywastheoldestgrandsonofThomasHenryHuxley,“Dar-
win’sbulldog.”EducatedatEtonandOxford, he was, like hisgrandfa-
a
ther,abiologist,aproli~cwriter,and,bytheendofhislife,averypublic
~gure (Huxley 1970, 1973; Waters and van Helden 1992). However,
unlike his grandfather, Julian Huxley disliked university life, and after
x
short-term spells atRiceInstitute(nowRiceUniversity)inTexas,back
at Oxford, and in London, he left academia to live by his pen and
through such other work as he could obtain. This included a period as
E
secretaryoftheLondonZoo,as~rstdirector-generalofUNESCO,and
(~rstonradioandthenontelevision)asamemberofaverypopularquiz
show, TheBrainsTrust.
Huxleywasalwaysakeenevolutionist,andmuchofhiswritingwas
on and around the subject. His magnum opus, Evolution: The Modern
Synthesis, was published in 1942, but it was the culmination of many
years of similar writings and was followed by many more years of
Copyright © 1999 The President and Fellows of Harvard College
ReligionwithoutRevelation 89
continuedwork.Asthetitlesuggestsandashiscareerpatterncon~rms,
Huxley was not so much a frontline empirical worker as a synthesizer.
y
But givenhiswideinterests,hismanycontacts,and hisgreatskillsasa
professionalwriter(Huxleyhad collaboratedwithH. G. Wellsand son
onamassivepopulartreatiseonbiology),this wasprecisely therolefor
whichhewas suited.Inthisgreatsyntheticwork,hepardentlyintended
toprovideanessentialfoundationonwhichtobuilda~rst-classprofes-
sional evolutionary biology.
Evolution: The Modern Synthesis (14) begins with a discussion of
o
natural selection,followingclosely the argument given intheOrigin:
Darwin based his theory of natural selection on three observable
facts ofnatureandtwodeductionsfroCm them.The~rstfactisthe
tendency of all organisms to increase in a geometrical ratio. The
tendency of all organisms is due to the fact that offspring, in the
early stages of their existence, are always more numerous than
their parents; this holds good whether reproduction is sexual or

asexual, by ~ssion or by budding, by means of seeds, spores, or
eggs.Thesecondfactistmhat,inspiteofthistendencytoprogres-
siveincrease, thenumbersofa given speciesactuallyremainmore
orlessconstant.
From these two facts, we can deduce the struggle for existence. Then,
we addinthethirdfact:theexistenceofwidespreadheritablevariation.
a
And with this and the struggle, we make our second deduction, to
natural selection: “a higher proportion of individuals with favourable
variations willon the averagesurvive, a higherproportionofthosewith
x
unfavourable variations will die or fail to reproduce themselves.” Over-
all,andgiventime,evolutionwillensue.Moreover,onbalance,thiswill
be evolution inthe directionof adaptive improvement.
E
Next,Huxleyturnstogenetics,showinghownaturalselectionand
Mendelism are complements rather than rivals: how Mendelian genes
canbepassed onentirefromgenerationtogeneration,andsoselection
can take effect and not be swamped out by blending of the units of
heredity (or their effects); how mutations—that is, random vari-
ations—occur regularlybut can be suf~cientlysmallas not toupset the
effects of selection but suf~ciently big as to have an effect over the
Copyright © 1999 The President and Fellows of Harvard College
90 JULIANHUXLEY
generations,astheymountup; howselection can acttopromotediver-
sity in populations as well as uniformity, thus collecting up ever-more
y
variation; and much more.
AlwaysHuxley,theoft-timespopularwriter,eschewsmathematics
and undue technicalities. He makes a point and then backs it with a
p
series of judiciously chosen supports; no less than ten supports, for
instance,underpintheexistenceandimportanceofsmallmutations.He
begins with the fact that one can produce them arti~cially with x-rays,
he works through such things as the polyomorphism in natural popula-
tions,andheendswiththewaysinwhichdifferencesinpopulationscan
be correlated with the different conditions (and hence selective forces)
encountered by subpopulations.
C
Afterpresentingthecentralmechanismsofevolution,Huxleynext
turns(verymuchinthespiritofDarwin)totheapplicationofhisideas.
Ofparticularinterestisthewholequestionofspeciation:thebreakingof
interbreeding groups into daughter groups, isolated from their former

fellowspeciesmembers.Muchtimeisspentontheissueofwhetherone
always needs geographical isolation of populations, one from another,
m
before one can get the formation of new species. Unlike the German-
AmericanornithologistErnstMayr(1942),whoatthetimewaswriting
thatgeographicalisolationiscriticaltospeciation,Huxleywasinclinedto
think that on occasion ecological isolation (where organisms overlap
geographicallybutoccupydifferentthoughadjacenthabitats)willsuf~ce.
a
Whatabouttheother majorarea ofevolutionary concern,paleon-
tology? Unlike Darwin, Huxley had no ~rsthand experience of this
subject; buthehadalwayskept himselfwell-informed,beginningwhen
x
he was young with discussion and information from Henry Fair~eld
Osborn,theleaderamongAmericanpaleontologistsandanoldstudent
of his grandfather. Of particular interest was the question of trends in
E
thefossilrecord,aswhenorganismsgetbiggerovertheyearsordevelop
morebaroqueforms.Asarule,Huxleysawlittleornoproblemin~tting
them into a Darwinian picture (494):
The trends . . . would appear to present no dif~culties to the
selectionist, and it is hard to understand why they have been
adducedasproofofnonadaptiveandinternally-determinedortho-
Copyright © 1999 The President and Fellows of Harvard College
ReligionwithoutRevelation 91
genesis.Whenever they are trulyfunctional and lead toimprove-
ment in the mechanical or neural basis for some particular mode
of life, they will confer advantage on their possessors anyd will
come under the in_uence of selection; and a moment’s re_ection
will show that such selection will continue to push the stock
further andfurtheralongthelineof developmentunptil alimithas
beenreached.
One point stressed by Huxley was that directed changes in time
which do confer ever-greater adaptive advantage often come less
o
throughstrifeagainstthebruteelementsandmorethroughcompetition
between evolvinglines (495):
The evolution of the ungulates is noCt adapted merely to greater
ef~ciencyinsecuringanddigestinggrassandleaves.Itdidnottake
place in a biological vacuum, but in a world inhabited, inter alia,
by carnivores. Accordingly, a large part of ungulate adaptation is
relative to the fact of carnivorous enemies. This applies to their

speed,and, inthecaseofthe ruminants, to the elaboratearrange-
ments for chewing the cmud, permitting the food to be bolted in
hasteand chewedatleisureinsafety.Therelationbetweenpreda-
tor and prey in evolution is somewhatlikethat between methods
ofattackand defenceintheevolutionofwar.
Thisisanimportantanticipationofwhatweshallseetoday’sevolution-
a
ists calling “arms races,” a metaphor drawingon the similarity between
competition among animals and competition among nations that op-
pose one another.
x
We have now more than a _avor of Huxley’s thought. Therefore
letusturntothekindofanalysisthathasbeengivenofearlierthinkers.
E
Values of the Evolutionists
For all that I would qualify their contribution, I do not want to leave
the impression that the population geneticists were indifferent to the
epistemic necessities of good-quality science. That would be very far
from the truth. Their sins, if such they be, were those of commission
(addingthenonepistemic)ratherthanthoseofomission(neglectingthe
Copyright © 1999 The President and Fellows of Harvard College
92 JULIANHUXLEY
epistemic). Fisher had had good training both as a mathematician and
asatheoretical physicist,andfromthis base hebuilta deserved reputa-
y
tion as the leading statistical theorist of his age. He used evidence to
build predictions, he was concerned about consistency and consilience,
his work was fertile in the extreme, and (as one would expect from a
mathematician) he was extremely sensitive to simpplicityand elegance.
Whatever hismajormotivation,whenFisherturnedtoevolution-
ary biology he was not about to forget epistemic values. Even though
hisworkwas essentially theoreticalratherthanexperimental,epistemic
o
factors were crucial. Most obviously, of course, was the question of
consistency. All of this generation of theoreticians were eager to show
that Darwinian selection and Mendelian genetics, far from being con-
C
tradictory rivals, are complements. The one area of inquiry does what
the other leaves open and demands. At the beginning of his book, The
Genetical Theory of Natural Selection, Fisher was quite explicit on this
score, showing how selection is the only mechanism that can dovetail
smoothlywith the Mende liantheory.A mechanism wheretheunitsof
inheritance “blend” in each generation simply will not work. But more
m
than this, even as he showed consistency, he was pushing the way
toward other epistemic virtues—for instance, fertility—ashesuggested
that, for all that the synthesis creates or shows new gaps in our knowl-
edge, itpoints to fruitfulwaysto plugthem (20–21):
a
Thewholegroupoftheorieswhichascribetohypotheticalphysi-
ological mechanisms, controlling the occurrence of mutations, a
power ofdirectingthecourseofevolution,mustbesetaside,once
thxeblendingtheoryofinheritanceisabandoned.Thesolesurviv-
ingtheoryisthatofNaturalSelection,anditwouldappearimpos-
sibletoavoidtheconclusionthatifanyevolutionaryphenomenon
Eappears to be inexplicable on this theory, it must be accepted at
present merely as one of the facts which in the present state of
knowledge seems inexplicable. The investigator who faces this
fact, as an unavoidable inference from what is now known of the
natureofinheritance,willdirecthisinquiriescon~dentlytowards
astudyoftheselectiveagenciesatworkthroughoutthelifehistory
ofthegroupintheirnativehabitats,ratherthantospeculationson
thepossiblecauseswhichin_uencetheirmutations.
Copyright © 1999 The President and Fellows of Harvard College
ReligionwithoutRevelation 93
Then, later in the book, Fisher demonstrated againand again that
histheoryhasepistemicvirtues,forinstance,theabilitytomakepredic-
y
tions about what one should expect under certain speci~ed circum-
stances.Oneexampleishisdiscussionofsexratios,whereheshowsthat
theseshouldbalancethemselvesoutsothatparentsareputtingthesame
amount of effort into raising sons and daughters. If tphey do not, then
theimbalancewouldleadtoonegroupgettinganadvantageoverothers,
which would continue until they competed against each other, thus
puttingmatters torights:“Selection wouldthusraisethe sex-ratiountil
o
the expenditure upon males became equal to that upon females” (143).
Thisspeculationaboutsexratioshasbeenoneofthemostfruitfulideas
inthe historyofevolutionarytheory,withhugeamountsofpredictively
C
fertile (and con~rmed) work following from it. Whatever the cultural
elementsinFisher’swork—whichgenerallyreducetoday’sevolutionists
to embarrassed silence—its epistemic virtues have more than stood the
test of time.
Interestingly, when we co me to Huxley’s Evolution: The Modern
Synthesis,published some twelve yearslater,the epistemic factorswere,
m
if anything, in decline. I do not want to exaggerate. Huxley was eager
to conserve the advances made by Fisher and the other population
geneticists.Forinstance,hetoomakesmuchoftheconsistencybetween
natural selection andMendeliantheory,linkingthisindeed to a second
virtue, simplicity. “The rise of Mendelism, far from being antagonistic
a
to Darwinian views (as was claimed, notably by the early Mendelians
themselves,intheyearsimmediatelyfollowingitsrediscovery),makesa
selectionist interpretation of evolution far simpler” (55). Moreover, in
x
somerespects Huxleystruckoutonhisown,showingthefertilityofhis
ideas in directions not covered by Fisher. The work on arms races is a
case in point. And whereas Fisher admitted candidly that he did not
E
deal with (nor was he desperately interested in) evolution beyond the
causalmechanismsandtheir immediateeffectstoday, Huxley aimedto
give a broad sweep of the whole evolutionary picture, as Darwin had
done. After all, that is what a synthesis is all about: bringing many
different areas together under one causal process.
But when all is said and done, perhaps precisely because Huxley
does give a synthesis—a survey—rather than a report of frontline re-
Copyright © 1999 The President and Fellows of Harvard College