Darwin's Principle of Divergence
Author(s): Ernst Mayr
Source: Journal of the History of Biology, Vol. 25, No. 3 (Autumn, 1992), pp. 343-359
Published by: Springer
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Darwin'sPrincipleof Divergence
ERNST MAYR
Museumof ComparativeZoology
HarvardUniversity
Cambridge,Massachusetts02138
One must avoid makingthe historyof science a hagiographyof
its greats.Even the greatestof the scientistshad their blind spots
and fell victim to various contradictions.In spite of my almost
unbounded admirationfor Charles Darwin, I must confess that
even he was human. This has recently been pointed out by
anothergreatadmirerof Darwin's,David Kohn, who showed how
often Darwin vacillatedand, in order not to hurt the feelings of
friends and membersof his family,sometimeseven concealed his
realviews.'
Darwin's principle of divergence and its application to the
process of speciationis an area where Darwinwas quite unableto
come to a clearcutsolution, and where his writingsshow that he
had not yet emancipatedhimselfcompletelyfrom "pre-Darwinian"
modes of thinking.2In his autobiography,he says:
But at that time [the 1840s- 1850s] I overlooked one problem
of great importance. ...
This problem is the tendency in
organic beings descended from the same stock to diverge in
character as they become modified . .. and I can remember the
very spot in the road, whilstin my carriage,when to my joy the
solution occurredto me; and this was long after I had come to
Down. The solution, as I believe [and this can be considered
Darwin'sdefinition of the principle of divergence]is that the
modifiedoffspringof all dominantand increasingforms tend to
1. David Kohn, "Darwin's Ambiguity: The Secularizationof Biological
Meaning,"Brit.J. Hist.Sci., 22 (1989), 215-239.
2. I have dealt with this subject once before (Ernst Mayr, "Isolationas an
EvolutionaryFactor,"Proc. Amer. Phil. Soc., 103 [19591,221-230), arrivingat
some tentative conclusions. A new situation, however, was created by the
publicationof Darwin's"Big Species Book" (CharlesDarwin,NaturalSelection,
ed. R. C. Stauffer [Cambridge:CambridgeUniversity Press, 19751), in which
Darwinpresentedhis reasoningand his evidence in far greaterdetail than in the
Originof Species.This permitsa new and indeedmoredefinitiveanalysis.
Joumal of the Historyof Biology,vol. 25, no. 3 (Fall 1992), pp. 343-359.
? 1992 KluwerAcademicPublishers.Printedin the Netherlands.
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344
ERNST MAYR
become adapted to many and highly diversifiedplaces in the
economyof nature.3
In his writings from the 1850s on, Darwin never failed to
emphasize the evolutionaryimportanceof this principle.In fact,
he stated in a letter to Joseph Hooker that together with the
principleof naturalselectionit was the most importantconcepthe
had ever developed: "the 'principle of Divergence,'which with
'NaturalSelection'is the keystoneof my Book & I have very great
confidenceit is sound."4
Among the students of Darwin's writings there has been
considerabledisagreementas to when Darwinactuallydiscovered
this principle, and even more disagreementas to why he considered it so important.Janet Browne thinks that it was discovered in 1857,5 but Kohn has advancedstrong argumentsthat
Darwinhad alreadydeveloped the new concept in 1854, although
it seems that he did not use the definiteterminology"principleof
divergence"until 1857, in correspondencewith Hooker and Asa
Gray.6On September5, 1857, Darwinwrote to Asa Gray:"One
other principle,which may be called the principleof divergence
plays, I believe, an importantpart in the origin of species. The
same spot will support more life if occupied by very diverse
forms:we see this in the many generic forms in a squareyard of
turf."n7
The basic point of the principle of divergence is simplicity
itself: the more the coinhabitantsof an area differfromeach other
in their ecological requirements,the less they will compete with
each other; therefore natural selection will tend to favor any
variationtowardgreaterdivergence.The reasonfor the principle's
importanceto Darwinis that it seemed to shed some light on the
greatestof his puzzles - the natureand originof variationand of
speciation.
To solve these puzzles Darwin asked himself two important
questions: (1) Where in nature do we encounter the greatest
3. CharlesDarwin, TheAutobiographyof CharlesDarwin,ed. Nora Barlow
(London:Collins,1958), pp. 120-12 1.
4. Frederick Burkhardtand Sydney Smith, eds., The Correspondenceof
CharlesDarwin(Cambridge:CambridgeUniversityPress, 1991), VII, 102.
5. Janet Browne, "Darwin'sBotanical Arithmetic and the 'Principle of
Divergence,'1854-1858," J. Hist.Biol., 13 (1980), 53-89.
6. David Kohn, "Darwin'sPrincipleof Divergenceas InternalDialogue,"in
The Darwinian Heritage,ed. D. Kohn (Princeton:Princeton UniversityPress,
1985), pp. 245-257.
VI, 448.
7. Burkhardtand Smith,Correspondence,
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Darwin'sPrincipleof Divergence
345
amountof variation?(2) How is this variationconnectedwith the
origin of new species? To answer the first question, Darwin
employed the "botanical arithmetic"so carefully analyzed by
Browne.8He finallycame to the conclusionthat there were three
kinds of genera:large genera (that is, genera with very numerous
species), and two kinds of small genera.The first categoryof small
genera contained similar species occurringin a rather restricted
contiguous area. The second category encompassedgenera with
ratherdistinctor aberrantspecies, and a ratherscattereddistribution. When calculatingthe numberof varietiesper species, Darwin
found that "on an averagethe species in the largergenera in any
countryoftenest present varietiesin some degree permanent,and
likewise a greater average number of such varieties,than do the
speciesof the smallgenera."9
From this Darwinconcludedthat genera went througha cycle.
At first they were small, with the species still rather similar to
each other and occurringin a relativelycompact area. By further
evolution they would become rich in species, and these species
would develop a tendency to have numerousnew varieties.And
finallythere were certainsomewhatsenescentgenerathat had lost
their abilityto producevariation,had a scattereddistribution,and
were likely to become extinct sooner or later. The question
whetherDarwin'scalculationswere truly relevant,and his results
statisticallyvalid, I shall leave unansweredbecause the answer is
not pertinentto my furtherarguments.Let us assumeinsteadthat
Darwinhad trulydiscoveredthe sourceof new varieties.
VARIETIESAND SPECIES
But why did Darwinconsider varietiesso important,and what
is their relation to the principleof divergence?To answer these
questions,it is necessaryto review Darwin'sconcepts of varieties
and species.Whatdid he meanwhenhe used the term"species"?
There was a drastic change in Darwin's species concept between the time of the TransmutationNotebooks (1837-1838)
and the 1850s. In the TransmutationNotebooks Darwin had an
amazinglymodem biological species concept, evidentlybased on
the observationof sympatricspecies of animals.It includes such
statementsas "mydefinitionof species ... is simply,an instinctive
impulse to keep separate,""The dislike of two species to each
other is evidently an instinct - & this prevents linterjbreeding,"
8. Browne,"Darwin'sBotanicalArithmetic"(above,n. 5).
9. Darwin,NaturalSelection,p. 235.
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ERNST MAYR
346
and "Hence species may be good ones & differ scarcely in any
externalcharacter."10
Almost no trace of this kind of thinkingabout species can be
found in Darwin'slater writings.There is no doubt that by the
mid-1850s he had shifted to a rather nominalist-typological
species concept. "Species," he said now,"...
[likel genera ... are
Naturally,
merelyartificialcombinationsmade for convenience."'II
the membersof a species were derived,as Alphonse de Candolle
said (followingJohn Ray), "froma common source after a conBut none of Darwin'slater
siderable number of generations."'I2
nominalist-typologicaldefinitionspermittedany demarcationbetweenspecies andvariety.
Darwin is quite inconsistentin what he calls a variety.At one
place he states that he calls a varietythat which is connected at
the present day by intermediategradations,'3but he also recognized discontinuousvariants.Varieties, so Darwin thought,were
on the whole less distinctthan species - or, as Browne has said,
they were "little species." Nor is ordinary individual variation
separablefrom the presence of distinctvarieties:"we should look
at all individualdifferences(independentlyof those producedby
crossing) as having the same nature & origin with those marked
14
by naturalistsas varieties."
What was far more injuriousfor Darwin'sanalysiswas that he
quite consistentlyused the same term, "variety,"for two entirely
differentnaturalphenomena:on the one hand, for geographically
delimited populations,currentlycalled geographicraces or subspecies; and on the other, for intrapopulationvariants- that is,
for individualsdifferingfrom the majorityof the populationby a
diagnostic character,which, as we now know, may be due to a
single gene difference.With respect to these two very different
categoriesof varieties,a definiteshift had occurredover the years
in Darwin'sthinking.Can we reconstructthe stages in his conversion froma biologicalto a moretypologicalposition?
The first stage occurredin March 1837 when the three island
populations of mockingbirdswhich he had considered to be
varietieswere declaredby John Gould to be three species.For the
10. P. H. Barrett,P. J. Gautrey,S. Herbert,D. Kohn, and S. Smith, Charles
Darwin'sNotebooks, 1836-1844 (Ithaca,N. Y.: CornellUniversityPress, 1987),
C, 161;B, 197; B, 213.
11. Charles Darwin, On the Origin of Species (Cambridge:HarvardUniversityPress, 1964), p. 485.
12. Darwin,NaturalSelection,p. 96 (translationmine).
13. Darwin,Originof Species,p. 485.
14. Ibid.,pp. 105-106.
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Darwin'sPrincipleof Divergence
347
first time Darwin realized that accepting degree of difference as
the species criterion might lead to differencesof opinion: what
one authorconsidereda species, anothermightconsidera variety,
and vice versa.More importantly,he learnedthat this was particularly true for what we now call "allopatric"(= geographically
representative)populations.
It is quite evident from the discussionsin his species book that
Darwin was ratheruncertainwhen to use the terms "subspecies"
and "variety."Of subspecieshe says that they are "thegeographical races of some Zoologists. But the term subspeciesis used by
some authors, to define ... very close species."'" In agreement
with prevailingzoologicalcustom,Darwinin the 1830s and 1840s
called geographicraces "varieties,"and referredto the situation
"whentwo varietiesinhabit two distinct countries as is often the
case and as is very generallythe case with the higher animals."'6
The problem of what to call geographicallyisolated populations
played a considerablerole in the workingout of Darwin'szoological Beagle collections, not only for the Galapagos,but also for
Chiloe and the FalklandIslands:"I was much struckhow entirely
arbitrarythe distinction is between varieties & species, when I
witnessed differentnaturalistscomparingthe organicproductions
which I brought home from the islands, off the coast of S.
America."'7
His work on the barnacleswas the next stage in the development of Darwin'sspecies concept. It did not produce any major
shift in his thinking,but it forced him scores of times to make
difficult decisions as to which forms to consider species, and
which others subspecies,in a highlyvariablegroup of species. In
lettersto his friendshe detailedagainand againhis difficultiesand
frustrations:"Afterdescribinga set of forms, as distinct species,
tearingup my M.S., & makingthem one species tearingthat up &
makingthem separate,& then makingthem one again (whichhas
happened to me) I have gnashed my teeth, cursed species, &
asked what sin I had committedto be so punished."'18
In the end
Darwin used varietiescopiously, particularlyfor allopatricpopulations. As a leading barnacle specialist (William A. Newman)
writes me: "mostof his varietiesare now recognizedas good species."'9For instance,of the ten varietiesof Balanustintinnabulum
15.
16.
17.
18.
Ibid., p. 99.
Ibid.,p.138.
Ibid., p. 115.
Burkhardt and Smith, Correspondence, V, 156.
19. WilliamA. Newman,personalcorrespondence,December17, 1991.
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ERNST MAYR
348
recognizedby Darwin,one is a synonym,while all the other nine
are now consideredgood species. Congenericspecies of barnacles
are morphologicallymuch more similar to each other than are
most speciesof birds.
Two points are evident from Darwin's barnacle work. The
enormous effort he made to arrive at the right conclusionin the
question "species or variety?"indicates (1) that at that time he
considered the species a real phenomenonof nature,and not an
arbitraryartifact of human sorting; and (2) that he considered
varietiesto be a stage in the process of speciation.He described
most of his barnaclevarietiesfrom wide-rangingspecies,which,as
Newman tells me, "showed variation especially when samples
This, indeed, was hinted
from distantlocalitieswere compared."20
at by Darwinhimself.
In contrast to the prevailing custom in zoology, the term
"variety"in the botanicalliteraturewas applied more often than
not to individualvariantswithina population.Furthermore,there
was rathergeneralagreementamongbotaniststhatthere is no real
difference between varieties and species, species simply being
more distinct than mere varieties. For instance, Darwin was
informedby one of his botanist friends that among the varieties
listed in the London Catalogue(of British Plants) there are 182
varieties that "have been ranked by some one botanist as species."21Quite logically,Darwin stated:"byour theory two closely
allied species do not differ essentially from a species and its
strongly defined variety."22Hence, "Accordingto the views discussed in this work, species do not differ essentiallyfrom varieties; - two closely allied species usuallydifferingmore from each
other than two varieties,& being much more constantin all their
Just how uncertain naturalistswere at that time
characters."23
about the natureof species is indicatedby the commentmade by
Darwinthat "morethan one-thirdof the varietiesmarkedby Asa
Gray are considered by him as possibly deserving to be called
species."24
THE ARBITRARINESSOF THE DISTINCTIONBETWEEN
VARIETYAND SPECIES
By the mid-1850s, and afterwards,Darwinfrequentlyexpressed
20.
21.
22.
23.
24.
Ibid.
Darwin,NaturalSelection,p. 137.
Ibid.,p. 139.
Ibid.,p. 165.
Ibid.,p. 137.
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Darwin'sPrincipleof Divergence
349
his convictionthat the distinctionbetweenvarietyand species was
purelyarbitrary:"itis no wonderthat there should be difficultyin
defining the difference between a species & a variety;- there
being no essential,only an arbitrarydifference... there is often a
wide neutral territoryin which the terms species & varietiesare
bandied about according to the state of our knowledge & our
ideas of the term species."25If one adopted Darwin'sclaim, then
the problem of the origin of new species was virtuallysolved:
every varietywas at the thresholdof becominga new species, and
it requiredonly a little push of naturalselection to complete the
process. Therefore,varietiesand species are smoothly connected
by ";agraduatedseries from the finest shades of individualdifferences, to well defined races, distinguishedwith great difficulty,if
really distinguishableat all, from sub-species & closely allied
species."26This observation was formulated by Darwin in the
assertionthat "speciesdo not essentiallydiffer from varieties,&
that varieties by further modification may be converted into
species."27Again and againDarwinstatesthat he looks at varieties
as incipientspecies.
One of the ambiguitiesin Darwin's thinking about species
stemmed from his uncertainty about the nature of variation.
Sometimes, when talking about the gradual change of a variety
owing to natural selection, he uses the language of population
thinking.On other occasions he sounds rathertypological,as if a
species consisted of a mosaic of varieties:"As each new varietyis
formed throughnaturalselection, solely from havingsome advantage over its parent, each new variety will tend to supplantand
exterminateits predecessor."28
WHATIS SPECIATION?
A species, as we now see it, is characterizednot only by being
different,but also by being distinctfrom other species (separated
by a gap, reinforced by isolating mechanisms). In his more
nominalistictreatmentof species in the later 1850s Darwin was
concerned only with difference, the first criterion of this dual
characterization.As a result he apparentlynever fully realized
that there was a fundamentaldifference between the phyletic
evolution of a lineage (the typologicalchange of a species in the
25.
26.
27.
28.
Ibid., p. 98.
Ibid., pp. 164-165.
Ibid., p. 164.
Ibid., p. 272.
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350
ERNST MAYR
time dimension)and genuinespeciation(thatis, the multiplication
of species).
Darwin'sdescriptionof the changeof a varietyinto a species is
a description of phyletic evolution. In a discussion of various
forms of the origin of varieties,he says that a new variety "will
tend in the long run to supplant& exterminateits parent-stock;
for its formationis due to some new advantagegained under the
conditionsto whichit is exposed, & it will generallylargelyinherit
the advantages of its parent."29He describes this process of
"supplanting"
as follows:"forof those living at one time & within
one area we should see only [in the fossil record]the parent-stock
and one or two varieties,which if destined to become triumphant
will increase in numbers & range & so ultimatelysupplantthe
parent;the parent,I may add, being rankedas the variety,as soon
as its range became less than that of the conqueringvariety."30
There is no question as to Darwin'sfocus on phyletic evolution
("verticalspeciation"),even though in a few passages (see below)
he also refers to the speciationof geographicallyisolated "varieties."
HOW DOES A VARIETY BECOME A SPECIES?
A species arises simply, says Darwin, by a variety gradually
becomingmoreandmore differentthroughnaturalselection:
I believe ... thatby far the most effectiveoriginof well marked
varietiesand of species, is the naturalselection or preservation
of those successive,slight,& accidental(as we in our ignorance
must call them) variations,which are in any way advantageous
to the individualsthus characterized:hence there would be a
better chance of varieties &species being thus formedamongst
common than amongst rare [species].I may add, to illustrate
what I mean, that a nurserymanwho raises seedling of a plant
by the hundredsof thousand far oftener succeeds in his lifetime in producinga new & valuablevariety,than does a small
amateurflorist.3'
When tryingto answerthe questionwhy new varietiesmightbe
successful in the struggle for existence, Darwin introduces his
principleof divergence.He points out that Man in the production
29. Ibid.,p. 263.
30. Ibid.
31. Ibid.,p. 136 (italicsadded).
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Darwin'sPrincipleof Divergence
351
of his domestic races has tended to select the most extremeforms;
and he cites examples from pigeons, fowl, horses, and so forth.
"Now in nature,I cannot doubt, that an analogousprinciple,not
liable to caprice, is steadily at work, through a widely different
agency;& that varietiesof the same species, & species of the same
genus, family or order are all, more or less, subjected to this
As illustration,Darwinmentionsvariablespecies.
influence."32
The importanceof the principleof divergencelies in the fact
that "asin the long run,more descendantsfrom a common parent
will survive, the more widely they become diversifiedin habits,
constitution& structureso as to fill as many places as possible in
the polity of nature,the extreme varieties& the extreme species
will have a better chance of survivingor escapingextinction,than
the intermediate& less modifiedvarietiesor species."33And it is
the principleof divergencethat is responsiblefor the observation
that "the average difference between two species of the same
genus, the parents of which by our theory once existed as mere
varieties,is greaterthan the averagedifferencebetween two such
varieties."34
Darwindoes not realize how close he is to a circular
argument,consideringthat he determines species status by the
degreeof difference.
Much of Darwin'sargumentsuffersfrom his failureto make a
distinctionbetween ecological segregation(occupationof a different niche) and geographicsegregation(occupationof an isolated
area). With reference to vacant ecological niches, he says: "The
expressionof variationin a right directionimplies that there is a
place in the polity of nature,whichcould be betterfilled by one of
the inhabitants,after it has undergone some modification:the
existence, therefore,of an unoccupied or not perfectlyoccupied
place is an all-importantelement in the action of naturalselection."35Unfortunately,under "unoccupiedplace" Darwin confounds ecological niche with a newly colonized geographicarea,
for he continues: "both Mr. [Thomas] Wollaston & Alph. de
Candolle have stronglyinsisted that isolated areas are the chief
scenes of what they consider, like most naturalists,as the actual
creation of new species & likewise of varieties."36Darwin and
Wollaston - de Candolle talked about two entirely different
things: ecological niches and geographic isolates. These two
32.
33.
34.
35.
36.
Ibid.,p. 228.
Ibid.,p. 238.
Ibid.,p. 249.
Ibid.,p. 252.
Ibid.
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352
ERNST MAYR
phenomena play utterly differentroles in the process of speciation, and it is impossibleto understandthe process of speciationif
one does not makea distinctionbetweenthem.
Equally misleadingis Darwin'sfailure to distinguishbetween
the ecological niche of a species and the slight ecological singularitiesof individualsin a population.His principle of divergence applies almost exclusively to different coexisting species,
and he produces no evidence whatsoeverthat it also applies to
members of a single population.If it did, then every population
would consist of ecotypicallyhighly different individuals- but
this is not what one finds. Highly polymorphicspecies are rare;
the individualsof most species, and certainlyof most populations,
resemble one anotherquite extraordinarily- like peas in a pod,
as the saying goes. Thereforeintrapopulationvariationdoes not
supply the material for a multiplicationof species. Darwin's
nominalistapproachwas unable to solve the problem of speciation. Perhapsnot surprisingly,one finds exactly the same confusion of ecological and geographic isolation in the writings of
Darwin'sdiscipleGeorgeRomanes.
THE ROLE OF GEOGRAPHICISOLATION
The Darwin of the Galapagos mockingbirdswas aware that
geographic isolation can play a role in converting a variety
(geographic race) into a species. He realized that there is a
difference between sympatric varieties and situations where "two
varietiesinhabittwo distinctcountries,as is often the case & as is
very generally the case with the higher animals."37 Not only
isolation as such but the degree of isolation is importantfor the
completion of speciation.Hence, on Madeira,strong flying birds
have not developed any notable endemism,whereaspoorly flying
beetles and even more so the sedentaryterrestrialmolluskshave
producedgreatnumbersof endemics.Incidentally,all the particular forms mentioned by Darwin in this discussion as being the
product of geographicisolation are animals.38This is not surprising since, beginningwith Pyotr Simon Pallas, there had been in
zoology a long traditionof the studyof geographicvariationof the
description of geographic varieties and subspecies, while there
was much less study of geographicvariationin plants,particularly
in Britain and America before the mid-nineteenthcentury.The
numerous difficultiesin the delimitationof sympatricspecies of
37. Ibid.,p. 138.
38. Ibid.,pp. 253-257, 269.
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Darwin'sPrincipleof Divergence
353
plants may have been the reason for the neglect of the study of
geographicvariationby botanists.
Darwin's main contact after the completion of the barnacle
monographswas with botanists,particularlywith Hooker, Hewett
C. Watson,and Asa Gray.This is well reflectedin his correspondence. The new informationthat he received from these botanists
made him change his thinkingin a number of ways. They convinced him, for instance,that sterilitywas not the secure species
criterion he had once thought. Also, they had a much more
morphologicalspecies concept than he had had earlier.If Darwin
wanted to convince his botanical friends of the validity of his
ideas on the origin of species, it was a good tactic to adopt a
theory that mightappeal to them - and this, he thought,was one
of the virtuesof the principleof divergence.
It is obvious that Darwin had trouble appreciatingthe importance of geographicisolation.The main reason for this is that he
could see only one kind of geographic barriers,oceanic ones.
Rises and falls of sea level seem to have been the only continental
barriersconceived by him. Thus he concluded:"inthe case of the
southern extremityof Africa, which is so extraordinarilyrich in
species, [one must assume]that it formedat no very remote epoch
a large archipelagoof islands."39(He later abandonedthis conjecture.)If only water barrierslead to isolation, and "considering
the whole world, from the fewness of the completely isolated
spots, & from the difficultyof the subsequentdiffusion of new
forms therein produced, such isolated spots, will probably not
have played a very importantpart in the manufacturingof species."40In other words, most new species, accordingto Darwin,
originatedin continuousareas:"I do not doubt that many species
have been formed at differentpoints of an absolutelycontinuous
area, of which the physicalconditionsgraduatefrom one point to
anotherin the most insensiblemanner."41
The only discontinuous
factoris the occurrenceof competitorswhose species borderdoes
not coincide with that of the species with which it competes.
Hence "I do not doubt that over the world far more species have
been producedin continuousthanin isolatedareas."42
It was this downgrading of the importance of geographic
isolation that was later attackedby Moritz Wagner.43Wagner,of
39. Ibid.,p. 265.
40. Ibid.,p.261.
41. Ibid.,p. 266.
42. Ibid.,p. 254.
43. Moritz Wagner, Die Darwin'scheTheorieund das Migrationsgesetzder
Organismen(Leipzig:Dunckerund Humblot,1868).
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354
ERNST MAYR
course, was quite right, even though Darwin answeredhim with
the words "Itwould have been a strangefact if I had overlooked
the importanceof isolation,seeing that it was such cases as thatof
the Galapagos Archipelago, which chiefly led me to study the
origin of species."44Evidentlyby then (1876) Darwinhad forgotten how adverse he had been in the 1850s to attributingan
importantrole to geographicspeciation.In spite of the opposing
claims by some historians,it would seem that Wagnerclearlyhad
the better of the argument.He demonstratedthat in the scores, if
not hundreds,of cases of ongoing speciationthat he had studied,
geographicbarriershad without exception isolated the incipient
species. That Wagner had a rather erroneous understandingof
naturalselection is irrelevantfor the speciationargument.Darwin,
by contrast,was unableto demonstrateeven a singlecrediblecase
of sympatric speciation. August Weismann'slater endeavor to
documentsympatricspeciationin the case of the Steinheimsnails
is at best inconclusive,since these snails have enormousphenotypic plasticityand it is most likelythat differentphenotypeslived at
the same period in different springsin the by-then largelydried
up SteinheimBasin.45
THE ALTERNATE:SYMPATRICSPECIATION
If geographicisolation is insufficientto account for the origin
of the majority of species, what then did Darwin imagine to
occur? He thought that all that was necessary was that natural
selection would enhance the differences among the coexisting
varietiesof a species until they had reachedspecies level. And this
is the point where the "principleof divergence"entered the
picture.Darwinsuddenlyconceived the idea that the more different the varietiesbecame from each other, and the more they used
differentresourcesand occupieddifferentnichesin orderto avoid
competitionwith each other,the bettersuch varietiescould coexist
in the same area and the more quicklythey would become different good species. Interestingly,in his big manuscript Darwin
44. Francis Darwin, Life and Lettersof CharlesDarwin (London:Murray,
1888), III,159, letterof October 13, 1876.
45. Ernst Mayr,"Weismannand Evolution,"J. Hist. Biol., 18 (1985), 295329; FrederickChurchill,"Weismann'sContinuityof the Germ Plasmin Histori87/88 (1985), 107-124; W.-E.
cal Perspective,"FreiburgerUniversitdts-blitter,
Reif, "Endemic Evolution of Gyraulus Kleini in the Steinheim Basin," in
Sedimentaryand EvolutionaryCycles,ed. Ulf Bayerand Adolf Seilacher(Berlin:
Springer,1985), pp. 256-294.
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Darwin'sPrincipleof Divergence
355
comparedthis process with "ourdomestic productions":here too
the breeder selects "the most extreme forms. He has made the
race-horseas fleet & slim as possible & goes on tryingto make it
fleeter: the cart-horsehe makes as powerful as he can," and so
forth.46As a consequence,"in any country,a far greaternumber
of individualsdescendedfrom the same parentscan be supported,
when greatlymodified in differentways, in habits constitution&
structure,so as to fill as many places, as possible, in the polity of
nature,thenwhennot at all or only slightlymodified."47
Darwinfound evidence for his principlealso in other areas, as
when he pointed out that "the view that the greatest number of
organic beings ... can be supported on any area, by the greatest
amountof their diversification... is in fact that of 'the divisionof
labour',so admirablypropoundedby Milne-Edwards."48
Indeed,
Darwinspecifiedof whatthis diversificationconsists:the diverging
varietiesare being kept apartbecause they have become "isolated
from hauntingdifferentstations,dislikingeach other, breedingat
different times &c, so as not to cross."49 I have shown previously
that the numerous putative instances of sympatricspeciation in
the literatureare highly vulnerable.50The populationalcharacteristicsmenticned by Darwin are clearly secondaryby-products
of the genetic divergenceof isolated populations;hence, they are
the result of ongoing speciationratherthan being the cause of it.
The "disliking,"the "hauntingof different stations," and the
"breedingat differenttimes"relateto differentpopulations,not to
differentindividualswithin a population.Indeed, such ecological
and behavioral properties in individualswould sexually isolate
themanddoom themto haveno offspring.
Darwin'sputativeevidence plays such an importantrole in his
thinkingthat it might be worthwhileto look at it in more detail.
Thirteen cases of incipient sympatric speciation are listed in
NaturalSelection.5'The first six cases involve domestic races of
mammalsand birds, where membersof one race are reportedto
have preferredto associate and mate with membersof their own
race ratherthanwith those of the other;the artificialisolationthat
resulted in the productionof these races evidently preceded the
46. Darwin,NaturalSelection,p. 227.
47. Ibid.,p. 228.
48. Ibid.,p. 233.
49. Ibid.,p. 269.
50. Ernst Mayr,Animal Speciesand Evolution (Cambridge,Mass.:Harvard
University Press, 1963), pp. 449-450. See also Ernst Mayr, "The Why and
How of Species,"Biol. Phil., 3 (1989), 431-441.
51. Darwin,NaturalSelection,pp. 258-259.
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356
ERNST MAYR
testingof the preferences.A sixth case, the story of the two kinds
of wolves in the Catskill Mountains,was never confirmed.The
next case is the two kinds of caribou,and here clearlygeographic
isolation was the primaryfactor.Not enough informationis given
about Gould'scase of two varietiesof a Tasmanianbird species to
analyze this case, but the occurrence of both migratory and
sedentary individuals in the same population of birds is well
establishedin both hemispheresand has never, so far as I know,
resulted in speciation in spite of Darwin's claim (tenth case).
Finally,Darwinlists three cases, one in crows, one in rollers,and
one in groundbeetles, in which two varietiesor species hybridize
in a zone of contact.All three cases have since been well studied,
and it is now quite clear that this zone of contact is the line at
which the two previouslyisolated incipient species met after the
breakdownof the extrinsicisolation. Thus, in none of the cases
cited by Darwinwere the behavioraldifferencesprimary,leading
to the isolation of the incipient species. It is obvious from this
analysis to what an extent he underestimatedthe importanceof
geographicisolation.
In the applicationof his principleof divergenceto the descendants of a set of parents,Darwin arguedas if every varietyduring
the process of divergent speciation were an asexual clone, not
interactingat all geneticallywith the other varietiesof the species
but subjectonly to divergentselection.But we are actuallydealing
with sexually reproducingspecies, and no mechanismis known
that would keep these divergingindividualsfrom interbreeding
witheachother.
If Darwin'sreasoningwere valid, all species ought to be highly
variable,consistingof ecological specialists.However, nothingof
the sort is found in nature.Highly variablepolymorphicspecies
are very rare.The uniformityof nearly all species, as well as the
frequencyof siblingspecies, documentson the contrarythat such
centrifugalvariationwithina populationhardlyever occurs.What
variationis found is mostlygeographicvariation,but this does not
at all illustratethe principleof divergencesince each geographic
variant(subspecies)is the only representativeof the species where
it occurs.
The discriminatingreader will notice that throughoutNatural
SelectionDarwintalks only about ecological specialization.Nowhere does he mentionthe aspects of species he had so emphasized
in his TransmutationNotebooks; for example, nothing is mentioned about the acquisition of the instinct of repugnanceto
intermarriage,or the instinctto keep separate,which at the time
of writing the Notebooks had been for him among the chief
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Darwin'sPrincipleof Divergence
357
characteristicsspecies.52Nothing is mentionedabout the originof
that which we now call the isolating mechanisms.In fact, in his
later controversywith A. R. Wallace about the origin of crosssterilityof species, Darwinvery specificallystatedthatit could not
have been acquiredby naturalselection under sympatricconditions; as he said in a letter to T. H. Huxley, "Naturenever made
species mutuallysterile by selection, nor will men."53It is quite
curiousthat Darwinseems never to have realizedthat his process
of sympatricspeciationby characterdivergencewould not work
unless he supplied simultaneouslyan explanationof the acquisition of isolatingmechanisms.As I showed above, all the suggested
incipient isolating mechanismsdescribed by Darwin are clearly
propertiesof populationsthathadbeen previouslyisolated.54
Perhaps the greatestweakness of his argumentis that Darwin
appliedthe principleof divergencenot only to species, where it is
indeed largelysupportedby modem ecological research,but also
to the offspringof a set of parents:"I considerit as of the utmost
importance fully to recognize that the amount of life in any
country, & still more that the number of modified descendants
from a common parent,will in chief part depend on the amount
of diversificationwhich they have undergone,so as best to fill as
many & as widely differentplaces as possible in the great scheme
of nature."55Actually, Darwin had no evidence for putative
ecological differencesamong the individualsof a single population; the use of the words "common parent"is equivocal and
misleading.And if the intrapopulationvarietiesdo not differfrom
each other ecologically, then the principle of divergencecannot
operate.
THE VALIDITYOF THE PRINCIPLEOF DIVERGENCE
It is now evident that Darwinfailed to prove that the principle
of divergenceplays a primaryrole in speciation.This failuremust
be attributedto
52. Barrettet al., Notebooks,B, 197 and 213; C, 161.
53. Francis Darwin, More Letters of Charles Darwin (New York: D.
Appleton, 1903), vol. 1, p. 277.
54. See Ernst Mayr, Evolutionand the Diversityof Life, (Cambridge,Mass.:
HarvardUniversityPress, 1976), pp. 120-128; M. Kottler,"CharlesDarwin's
Biological Species Concept and Theory of GeographicSpeciation,"Amer. Sci.,
35 (1978), 275-297; and FrankJ. SuHoway,"GeographicIsolationin Darwin's
Thinking:The Vicissitudesof a CrucialIdea,"Stud. in Hist. Biol., 3 (1979), 2365.
55. Darwin,NaturalSelection,p. 234.
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358
ERNST MAYR
(1) his typological-nominalistconceptualizationof species
andof the processof speciation;
(2) his mistake of not discriminatingbetween intrapopulation variantsand geographicsubspecies, calling both of them
varieties;and
(3) his failureto distinguishbetweenisolationin an ecological nicheandin a geographicallyisolatedarea.
It would be whiggishto criticizeDarwinfor his nominalistway
of looking at species and varieties, for this was the universal
attitudein his time. Indeed, he frequentlydid break away from it,
particularlyin his treatmentof naturalselection and the acquisition of adaptedness,wherehe introducedpopulationthinking.56
Typological thinking also plagued one of Darwin's contemporariesin a like manner:it contributedto a failureof Mendel's
later researches. Not making a distinction between two very
differentkinds of "hybrids"- those of intraspecificvariants,like
the pea varietiesMendel was crossing, and hybridsbetween real
species (such as in Hieracium,Aquilegia, Verbascum,Nicotiana,
etc.) - Mendel got conflicting results, which greatly frustrated
him. It may have given him the feeling that he had not really
solvedthe problemof the natureof hybrids.
Too many problemswere as yet unsolved in Darwin'sday to
allow a resolutionof the problemof speciation.Althoughseveral
naturalists(HenrySeebohm,EdwardPoulton,KarlJordan,Erwin
Stresemann)eventuallyapplied consistent populationthinkingto
the species problem and greatly clarified the issues, it was not
until the evolutionarysynthesisthat the problemof speciationwas
clarified to such an extent in the writingsof BernhardRensch,
Theodosius Dobzhansky, and myself that the main questions
could be considered as solved. It is now no longer necessaryto
invoke a principle of divergence for this particularprocess of
evolution.
It is ironic that of the two principles,which Darwinconsidered
equally important,that of divergencewas far less often criticized
than that of naturalselection. Any yet eventuallyit was natural
selection that was victorious, while it is now evident that the
principleof divergenceis invalid.
56. See Ernst Mayr, "Darwinand the EvolutionaryTheory in Biology,"in
Evolution and Anthropology:A CentennialAppraisal, ed. Betty J. Meggers,
(Washington,D.C.:AnthropologicalSocietyof Washington,1959), pp. 3-12.
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Darwin'sPrincipleof Divergence
359
Acknowledgments
I acknowledge with deep gratitude the valuable suggestions
received from F. J. Sulloway,John Beatty, and an anonymous
reviewer,and particularlyfrom WilliamA. Newman on Darwin's
treatmentof varietiesandspeciesof barnacles.
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