Extremeenvironmentalchangea ndevolution: stress-inducedmorphologicalvariationi sstrongly concordantwithpatternsof evolutionarydivergence inshrewmandibles AlexanderV .Badyaev *{ and KerryR. Foresman Division of Biological Sciences,The University of Montana, Missoula, MT 59812-1002,USA Morphologicalstructures oftenconsist ofsimpler traits whichcan be viewed as either integrated(e.g. correlateddue to functional interdependency) or non-integrated (e.g. functionally independent) traits. Thecombination of along-term stabilizingselection onthe entire structure witha short-term directional selection onan adaptively important subset oftraits shouldresult inlong historical persistence of integratedfunctional complexes, with environmentally induced variation and macroevolutionary change con¢ned mostly to non-integrated traits. Weexperimentallysubjected populationsof three closelyrelated species of Sorex shrews toenvironmental stress. Aspredicted, wefoundthat most ofthe variationin shrew mandibularshape was localized between rather thanwithin the functionalcomplexes; the patterns of integrationdid not change between the species. Thestress-induced variationwas con¢ ned to non- integratedtraits andwas highly concordant with the patterns ofevolutionarychange öspecies di¡ered in the same set ofnon-integrated traits whichwere most sensitive tostress withineach species. Wesuggest that lowenvironmental and genetic canalization of non-integrated traits mayhave caused these traits to bemost sensitive notonly to the environmentalbut alsoto genetic perturbations associated with stress. Thecongruence of stress-induced andbetween-species patterns ofvariation in non-integrated traits suggests that stress-induced variationin these traits mayplay an importantrole in species divergence. Keywords: environmentalstress; canalization;functionalintegration ;mosaicevolution; Sorex

gration(and the associatedenvironmental canalization) 1.INTRODUCTION canstrongly facilitate adaptive evolution by reducing the Understandingthe evolutionof complex morphological environmentalvariance in each of the integratedtraits structures is acentralquestion in evolutionary biology and,thus, minimizingenvironmental variance ortho- (e.g.Bonner 1988; Ra¡ 1996) .Complexstructures often gonalto the adaptivedirection of morphological change consist ofsimpler morphologicaltraits whichcan be forthe entire structure (Lande1 980,1 984;W agner1988) . viewedas either integratedor non-integrated, e.g. func- On the otherhand, morphological integration could have tionallyintegrated traits arehighly dependent on each beenproduced by historically invariant developmental otherbecause they are involved in the same organismal patterns (e.g.Riedl 1978);the highgenetic canalization of function(such asattachment ofa muscle).Incontrast, integratedtraits canreduce the expressionof genetic non-integratedtraits arefunctionally and developmen- variationand slow or bias morphological responses tothe tallyindependent of each other (reviewedin Cheverud present selection (Lande1 979;Stearns 1993;W agner et al. 1996). 1997). Quantitativegenetics theorypredicts that long-term Attempts toresolve the constrainingand facilitating stabilizingselection andcorrelated mutations will facilitate roles ofintegration have led to the developmentof a integratedtraits evolvinggenetic correlations consistent conceptof mosaic evolution (Simpson 1953; W agner1996) . withtheir functionalor developmental relationships and, Thetheory of mosaic evolution assumes the existence of ultimately,evolvingin a correlatedmanner compared to relativelyindependent morphological units (e.g.functional the more independentevolution of non-integrated traits complexes)within a structure;such relativelyinvariant (Lande1979, 1980; Cheverud 1 982,1984) .Indeed,many complexesare subject toa long-term stabilizingselection studies havedocumented strong in£ uences ofdevelop- whichpreserves the basiclevel of integration necessary mentaland functional integration on the structure of across environments.Directional selection exerted by phenotypicand genetic covariation (Cheverud ( 1996)and changingenvironments can favour rearrangements of references therein). these complexes,thereby producing changes in the overall However,it is unclearwhether the correlatedevolution structure, whilemaintaining integration within each ofintegratedtraits constrainsor facilitatesadaptive modi- complex.Thus, the combinationof along-term stabilizing ¢cationsof morphologicalcomplexes (reviewed in Arnold selection onthe entire structure withstrong short-term 1992;Wagner et al.1997).On onehand, morphological inte- directionalselection onasubset ofunits withina structure shouldresult inhigher variation between complexes of integratedtraits, butlower variation within these * Author forcorrespondence (abadyaev@selway .umt.edu). { Present address:331 F unchessHall, Department of Biological complexes(Simpson 1 953;Berg 1960; Cheverud 1 984; Sciences,Auburn University,Auburn, AL36849,USA. Wagner1996; W agner& Alternberg1996).

Proc. R.Soc.Lond. B (2000) 267, 371^377 371 © 2000The RoyalSociety Received 2September1999 Accepted 1November1999 372A. V.Badyaevand K. R.Foresman Stress and mosaic evolution

Here wetest twopredictions of the mosaictheory of (untreated)plots and plots where the overstorey vegetation was evolution.First, persistent stabilizingselection on removed.W edesignedour study within theseplots to minimize complexesof integrated traits favourslow sensitivity of thee¡ ects of immigration and emigration following treatments the integratedtraits toenvironmental and genetic pertur- (seeBadyaev et al. (2000)for details of theexperimental design bations(i.e. environmentaland genetic canalization; andtrapping schedules) .The vegetationremoval treatment Schmalhausen1 949;Cheverud 1 982;W agner1988; createshighly stressful conditions for shrews, as evidenced by Wagner et al. 1997).Highcanalization of integrated largeincreases in thedevelopmental instability of shrew complexesreduces the heritablephenotypic variation embryosand by a decreasein theindividual conditions of adults availableto natural selection and,thus, canlead to low (Badyaev et al. 2000).Weexamined56 0two-to three-month- rates ofevolutionarychange within integrated complexes. oldindividuals of threeclosely related Sorex species(F umagalli et Thus,we can predict alonghistorical persistence of al. 1999;see F oresman( 1999)for details of the species biology): complexesof integrated traits (Lande1979; BjÎ rklund Sorexcinereus (control,46 males and 43 females, and vegetation 1994)and high similarity of recently divergedspecies in removal,74 males and 44 females) , Sorexmonticolus (control,37 integratedtraits (Wagner1 988;Schluter 1996 a). On the malesand 37 females, and vegetation removal, 39 males and 38 contrary,non-integratedtraits whichare relatively free of females)and Sorexvagrans (control,46 males and 52 females, persistent stabilizingselection maybe less environmen- andvegetation removal, 46 males and 58 females) .All sex- tallyand genetically canalized (Stearns &Kawecki1994; relatedvariation was removed from the data in generallinear Stearns et al. 1995).Thisshould lead to greater variability modelsand standardized residuals were used for all analyses. innon-integratedtraits inresponse toboth environmental Leftand right shrew mandibles were positioned on a slide andgenetic perturbations and to faster evolutionin these andthen photographed under 7.5magni¢ cation using an £ traits (Wagner1996; W agner et al. 1997).Thus,we can OlympicSZH stereo photomicroscope and a videocapture predict that recently divergedspecies shouldbe the most board.The resultingimages were further magni¢ ed 2 using £ di¡erent innon-integrated traits. Second,if lower envir- MochaImage Analysis software ( JandelScienti¢ c) .The data onmentalcanalization of non-integrated traits is asso- were the x- and y-coordinatesof 1 7homologousmorphological ciatedwith their lowergenetic canalization (Gavrilets & landmarks(¢ gure 1 a).Instudies such as ours, the variance Hastings 1994;W agner et al. 1997),then the variation resultingfrom the positioning of amandibleunder a microscope betweenrecently divergedspecies shouldbe similar tothe istypically much higher than the variance from repeated environmentallyinduced variation within each species ö measuresof already scanned images. Thus, we usedthe reposi- species shoulddi¡ er the most inthe same traits that are tioningerror as ameasurementerror term in ouranalysis. Each the most sensitive toenvironmental variation within mandiblewas repositioned and digitized three times. High species. magni¢cation and three replicates for each measure allowed us Threeclosely related species of Sorex shrews providea tominimize and estimate the measurement error. Repeated goodsystem inwhich to examine these predictions measureswere separated by several measurement sessions. Both experimentally.First, existingwork on the anatomyof the mandiblephotography (by a laboratorytechnician) and mand- shrew mandibleallowed us toassign mandibular areas a iblemeasurements by A. V.B. wereconducted without prior priori tointegrated (i.e. participatingin the attachment of knowledgeof the treatment, sex or species of the measured the same muscle) andnon-integrated units (e.g.Kindahl animal.Thus, we assumedthat the measurement error contrib- 1959;DÎ tsch 1982).Second,previous work on the sensi- utedequally to each treatment category . tivityof the shrew mandibleto environmental conditions Todescribethe functional integration in theshrew mandible, (Badyaev et al. 2000)allowedus todesign an appropriate we selectedlandmarks associated with muscleattachment points experiment forthe studyof environment-induced varia- andfunctionally related units of dentition (after A « rnbÌck- tion.Finally ,the morphologicalvariation in the shrew Christie-Linde1907; Kindahl 1 959;DÎ tsch1982; Dannelid 1 998; mandible(particularly in the muscle attachments which Reumer1998; shaded areas in ¢gure1 a).Weidenti¢ed land- determine bite force;Carraway& Verts 1994)is strongly marksas being`integrated’ when theywere associated with the associatedwith ¢ tness andis understrong current selec- attachmentof thesame muscle (e.g. integrated landmarks 6 and tion(Badyaev et al. 2000).Underour predictions, (i) the 7areassociated with attachmentof themuscle musculi pterygoi- shrew species shouldbe most similar intheir structure deusmedialis) .Functionallyand developmentally independent andposition of integrated complexes (i.e. muscle attach- tissuesof the mandible were identi¢ ed as non-integrated ment groups)and di¡ er mostlyin non-integrated traits (¢gure 1 ).Patternsof integration between landmarks were (i.e. mandibletissues notassociated with muscle attach- evidentin thestrong covariation between landmarks (¢ gure 1 b; ments),and(ii) non-integratedtraits shouldbe more see } 2(c));these patterns were highly congruent to those sensitive tothe experimentalincrease inenvironmental described a priori fromthe literature. variationthan integrated traits andthe patterns ofenvir- onmentallyinduced variation, and the between-species (b) Dataanalysis variationshould be similar innon-integrated traits. Herewe focusedon the variation in mandibleshape ;thus, allvariation in mandiblesize was eliminated before the analysesby scaling all specimens to unit centroid size. W e 2.MATERIAL AND METHODS applieda singleProcrustes superimposition (generalized ortho- (a) Datacollection gonalleast-squares ¢ t;Rohlf & Slice199 0)to align the land- The studywas carried out in June ^Julyof 1994^1 996on markcon¢ gurations from all species, treatments, individuals, eightexperimental plots ( ca.17haeach) located in fourstudy bodysides and replicas simultaneously (after Klingenberg & siteswithin 32km of one another in theSwan River V alleyof McIntyre1998) .Beforethe superimposition, left mandibles westernMontana, USA. Eachstudy site contained control werere£ ected to their mirror images by assigning a negative

Proc. R.Soc.Lond. B (2000) Stress andmosaic evolution A.V.Badyaevand K. R. Foresman373

(a) (c) 1 1

3 2 3 4 6 2 7 15 5 8 17 14 4 15 9 16 16 11 13 5 6 17 10 7 14 12 8 11 13 10 9 12

(b) (d) 1 1

3 2 3 2 4 4 6 6 5 5 7 7 17 16 8 8 15 9 17 16 15 14 9 14 11 13 11 13 10 10 12 12

Figure 1. (a)Landmarksre£ ecting functional integration in theshrew mandible and principal components of the covariance patternsin landmarkdisplacement due to variation( b)betweenindividuals, ( c)betweentreatments and ( d)betweenspecies. Shadedareas indicate functionally integrated units and the numbers show homologous landmarks (integrated, solid lines and non-integrated,dashedlines; see } 2).The PCcoe¤cients are shown as avectororiginating at themean con¢ guration of the landmark,and the length and direction of thePC coe¤cients. The PC1sfor each e¡ ect and the accounted variation in thecorre- spondingProcrustes mean squares are ( b) 23.2%, (c) 73.2% and (d)89.1%.The lengthof thevectors is proportionallymagni¢ ed byafactorof tenfor better visibility. signto their x-coordinates.W ecalculatedthe overall consensus Klingenberg& McIntyre1998) .Toexaminethe patterns of ofsuperimposed con¢ gurations with theSAS/ IML(interactive jointvariation in thelandmarks due to each e¡ ect, we analysed matrixlanguage) code provided by C. Klingenberg. thecovariance matrices of the Procrustes coordinates. Based on The variancein theset of optimallyaligned landmark con¢ g- theexpected mean squares, we computedseparate matrices of urations(hereafter Procrustes coordinates) was then partitioned thesums of squares and cross-products for the between- species, in ANOVAmodels(Goodall 1 991).The variationin eachset of between-treatment,between-individual, between-sides and Procrustescoordinates was assumed to be due to a combination between-replicatesvariations (after Klingenberg & McIntyre ofthe e¡ ects of species, treatment, individual, body side and 1998).Tovisualizethe patterns of covariation in thelandmarks measurementreplicate. In this ANOV Amodel,the individual dueto each e¡ ect, we representedthe principal components andtreatment were entered as random e¡ ects and body side as (PCs)of eachof thematrices graphically as displacement of the a¢xede¡ ect. The individuale¡ ects were nested within treat- landmarksfrom their consensus position. The vectorassociated mentgroups. Degrees of freedom for the Procrustes ANOV A with eachlandmark represents the direction and intensity of werecalculated following Goodall ( 1991)andKlingenberg & displacementof thislandmark due to an e¡ ect. McIntyre( 1998). Toexaminethe similarity between the between- species, between-treatmentand between-individual patterns of land- (c) Localization andvisualization ofe¡ ects markcovariation, we calculatedthe angles between corre- Topartitionthe e¡ ects of eachlandmark on the overall varia- spondingPC 1vectors.F orthe PCs oftwo groups, the angle tionin mandibleshape, we ¢rstsummed the x and y mean betweenthem is the arc cosine of the inner product of the two squaresof each landmark (Klingenberg & McIntyre1 998).We vectorelements. The statisticalsigni¢ cance of the vector angles computedthe variance components of these mean squares wasassessed with resamplingof the within-sample PC coe¤- accordingto the expected mean squares for each of the e¡ ects cientsfor each e¡ ect. W especi¢cally compared the correlations (e.g.species, treatment and individual) .This allowedus to andcorresponding angles for the vectors of displacement of the localizein£ uential landmarks for each e¡ ect separately (see integratedand non-integrated landmarks due to each e¡ ect.

Proc. R.Soc.Lond. B (2000) 374A. V.Badyaevand K. R.Foresman Stress and mosaic evolution

(a) p50.0001).PC1accountedfor only 23.2% of all varia- 1.0 tionand the ¢rst ¢vePCs accounted for 62. 7%. Environmentalstress hada largee¡ ect onmandible shape (MS 3.51, F 6.43 and p 0.0004),butmost of 0.8 the stress-inducedˆ variationˆ occurred ˆ in the non-integrated landmarksresulting inhighly localized patterns of landmarkdisplacement (table1 and¢ gure1 c). The PC1 0.5 forthe e¡ect ofstress accountedfor 73.2 %ofall the variationand most ofthe stress-induced variation occurreddue to the displacement ofnon-integrated 0.3 landmarks(landmarks 1 ,10^13,16 and 1 7;table 1 and

n ¢gure 1c). o i t Mandibleshape di¡ ered stronglybetween species a l 0.0 e

r (MS 233.96, F 66.57 and p 0.003),butthe between-

r integratednon-integrated

o ˆ ˆ ˆ

c species variationwas limited toonly a fewlandmarks and (b) r

o 1.0

t nearlyall (89. 1%) ofthe between-species variationin 1 7 c

e landmarkswas accounted for by PC 1(¢gure 1 d). Simi- v larlyto the stress-induced variationwithin each species, 0.8 the between-species variationwas mostly due to variation innon-integrated landmarks 1, 10, 1 1,16 and 1 7(table1 and ¢gure 1d).Moreover,the directionand magnitude of 0.5 the displacement ofnon-integrated landmarks was similar forthe species andstress e¡ects (¢gure 2 b), i.e. species weremost di¡erent inthe same non-integrated 0.3 landmarksthat weremost stronglydisplaced by stress in eachspecies. 0.0 integratednon-integrated 4.DISCUSSION

Figure2. Vector correlations (mean and the bootstrapped Theconstraining and facilitating roles offunctional standarderrors of theestimates) of integratedand non- integrationin the evolutionof complex structures are integratedlandmark displacements for ( a)between-species addressed bythe conceptof mosaic evolution (e.g. versusbetween-individua lvariation( t 4.73 and p50.001) Simpson1953; W agner1 996).Thisconcept predicts that ˆ (speciesare more similar in patternsof displacementof the combinationof a long-term stabilizingselection on integratedlandmarks than in non-integratedlandmarks),and the entire structure withstrong short-term directional (b)between-speciesversus between-treatme ntvariation selection onan adaptively important subset oftraits (e.g. (t 2.49 and p 0.01)(species di¡ er themost in thesame set functionalcomplexes) should result inparcellation öthe ofˆnon-integratedˆ landmarks which aremost strongly displacedby stress in eachspecies). evolutionof relatively autonomous complexes of traits withincomplex structures. Suchcomplexes may be capableof mutual rearrangements favouredby direc- tionalselection, whilemaintaining high integration 3. RESULTS withineach complex (Berg 1 960;V ermeji 1974;W agner Thebetween-individual variation in landmarks 1996;W agner& Altenberg1 996). revealeda strongpattern ofcoordinated displacements Here weshowed that macroevolutionarychange in the (¢gure 1b).Thejoint landmark displacement washighly shrew mandibleis concordantwith interspeci¢ c predic- concordantwith the functionalrelationships in the shrew tions ofmosaic evolution. W efoundstrong support for mandible(¢ gure 1 a),indicatingthat our a priori designa- bothof our predictions. First, weshowed that shrew tionof landmark integration is biologicallymeaningful. species weremost similar intheir relativeplacement of Landmarkswithin the integratedunits covariedwith integratedlandmarks (e.g. muscle attachments; ¢gures 1 d otherlandmarks within the same functionalunit, but not and 2a)andwere most di¡erent intheir placementof withlandmarks in di¡ erent units orwith non-integrated non-integratedlandmarks (¢ gures 1 c and 2b).Thisresult landmarks(¢ gure 1 b).Integratedlandmarks 4^5, 6^7 ,8^9 isexpectedif persistent stabilizingselection onintegrated and1 4^15variedin highly coordinated patterns notonly traits leadsto higher environmental and genetic canaliza- betweenindividuals (¢ gure 1 b),butalso between treat- tionof these traits (e.g.Gavrilets &Hastings 1994; ments (¢gure 1 c)andbetween species (withthe excep- Wagner et al. 1997);such canalizationultimately results in tionof landmarks 4 and5, ¢ gure1 d ).Thehistorical alonghistorical persistence ofintegrated functional persistence offunctional integration is illustrated bythe complexes(e.g. T ucic& Avramov1 996;¢ gures 1 d and 2b). similarityof the between-individualand between-species Theabsence of strong stabilizing selection onnon-inte- variationin integrated landmarks (¢ gure 2 a).Thee¡ ects gratedtraits results intheir lowerenvironmental canali- ofthe between-individualvariation were relatively zation(Stearns 1993;Stearns et al. 1995)and, possibly ,in evenlydistributed amongall landmarks (except fora lowergenetic canalization (e.g. W agner et al. 1997). Low largee¡ ect oflandmark 1 ;table1 )(Procrustes ANOVA environmentalcanalization may explain the high forthe e¡ect ofindividual, MS 0.54, F 4.30 and sensitivity ofnon-integrated traits tothe e¡ects ofstress ˆ ˆ

Proc. R.Soc.Lond. B (2000) Stress and mosaic evolution A.V.Badyaevand K. R. Foresman375

Table 1. Variance components (% variance) for each landmark for the e¡ects inthe Procrustes ANOVAofmandible shape for three Sorex species (All variancesare multiplied by 10 6.) landmark species treatment individual side side individual error £ 1 3562 (35) 935 (9) 2432 (24) 427 (4) 2076 (20) 871 (9) 2 342 (29) 190 (16) 198 (17) 40 (3) 246 (20) 180 (15) 3 144 (9) 0 (0) 390 (24) 88 (5) 655 (40) 350 (22) 4 129 (11) 1 (0) 193 (17) 131 (11) 417 (36) 301 (26) 5 595 (26) 57 (3) 439 (19) 205 (9) 669 (29) 355 (15) 6 246 (20) 87 (7) 297 (25) 87 (7) 310 (26) 179 (15) 7 53 (7) 25 (3) 155 (20) 123 (16) 278 (36) 142 (18) 8 313 (26) 79 (6) 332 (27) 6 (1) 305 (25) 193 (16) 9 501 (20) 137 (6) 702 (29) 105 (4) 576 (24) 432 (18) 10 904 (17) 658 (13) 257 (5) 512 (10) 1883 (36) 1031 (20) 11 781 (19) 739 (18) 549 (13) 62 (2) 1144 (27) 910 (22) 12 52 (3) 213 (11) 405 (22) 4 (0) 761 (41) 431 (23) 13 51 (5) 139 (13) 152 (14) 0 (0) 530 (48) 226 (21) 14 174 (13) 8 (1) 261 (19) 50 (4) 575 (42) 317 (23) 15 356 (7) 52 (1) 609 (12) 17 (0) 3044 (62) 857 (17) 16 249 (11) 176 (8) 136 (6) 192 (9) 952 (43) 495 (23) 17 275 (21) 192 (15) 155 (12) 15 (1) 505 (38) 181 (14)

(¢gure 1c).Reducedgenetic canalization of these traits population¢ tness landscape,thus creatingan opportu- mayaccount for their stronger response tonatural selec- nityfor new directions ofadaptive divergence (Arnold tionand genetic drift (Stearns 1993;W agner et al. 1997); 1992).Inaddition, stress isoftenassociated with a strong faster evolutionarychange in non-integrated traits is decrease inpopulation size anda correspondingincrease evidentin the strongdivergence of relatedspecies inthese inthe e¡ects ofgenetic drift (Ho¡mann & Parsons( 1997) traits (¢gures 1 d and 2b). andreferences therein).Itis possiblethat less canalized, Theoverall pattern ofvariation in the shrew mandible non-integratedtraits aremore sensitive toboth the canbe described bylow phenotypic variation and low disruptivee¡ ects ofstress duringdevelopment and the macroevolutionarychange within functional complexes, e¡ects ofselection andgenetic drift associatedwith stress but highvariation and more frequent macroevolutionary (e.g.Bryant & Me¡ert 1988;W agner et al. 1997), thus changebetween functional complexes (table 1 and explainingthe parallelismbetween stress-induced varia- ¢gures 1and2) .Thiscombination could provide a neces- tionand evolutionary change in the shrew mandible sarybalance between the requirements forthe basic (¢gures 1d and 2b). levelof integration across environments(e.g. maintaining Wefoundthat functionalintegration strongly in£ uences muscle attachment groups)and the needfor an adaptive the phenotypicvariation in mandible shape in shrews; response tochanging conditions (e.g. rearranging juxta- the between-individualand between-species variation positionof muscle groups)(e.g. Simpson 1953; Berg 1 960; (¢gure 1b,d)werehighly concordant with a priori Cheverud1 984;W agner1 996;W agner& Alternberg described patterns offunctional relationships. F or 1996).Similarly,Churchill( 1996)found strong evidence example,landmarks 8 and9, which consistently covaried formosaic evolution of recent humans;a model witheach other (¢ gure 1 ),areattachments ofthe musculi combiningthe integrationof some traits andparcellation masseter muscle alongthe lateralaspect ofthe angular ofother traits most closelyaccounted for by the available process (¢gure 1 a;DÎtsch 1982).Landmarks1 4and1 5 fossil data. alsoshowed correlated variation among individuals and Lowenvironmental canalization of non-integrated species andthese arethe attachment pointsof the musculi traits maybe associated (either directlyor indirectly) digastricmuscle atthe mandibularramus (¢gure 1 a; withlow genetic canalization of these traits (Gavrilets & DÎtsch 1982).Concurrently,landmarkswhich did not Hastings 1994;W agner et al. 1997).Thus,non-integrated showcorrelated variation were not associated with func- traits maybe highly sensitive notonly to environmental tionallyrelated tissues (e.g.landmarks 1 2and1 3located butalso to genetic pressures (such asselection andgenetic aroundfunctionally and developmentally independent drift).Inthis case, the patterns ofstress-induced variation incisors; Kindahl1 959). withina species shouldbe similar tothe between-species Inshrews, periodsof intense environmentalstress are variation.W efoundthat the shrew species di¡ered from oftenassociated with increased food competition caused eachother inthe same set oftraits (non-integratedland- bymajor habitat alterations (e.g. Badyaev et al. (2000) marks) thatwere most sensitive tostress withineach of andreferences therein) orextreme population£ uctuations the species (¢gure 2 b). (e.g.Zakharov et al. 1991).Stress-induced modi¢cations of Periodsof intense stress caninduce morphological the shrew mandibletraits maybe bene¢ cial if the changeby disrupting normal development and thereby resulting di¡erences inbite forceor chewing patterns increasingmorphological variation (reviewed in Ho¡ - allowconsumption of newprey items (Carraway& Verts mann& Parsons1 997).Stress alsocan alter the 1994).Smallpopulations which are separated by

Proc. R.Soc.Lond. B (2000) 376A. V.Badyaevand K. R. Foresman Stress and mosaic evolution unsuitablehabitats and increased competition for Carraway,L.N.& Verts,B. J.1 994Relationship of mandibular similar resources canfavour rapid divergence and morphologyto relative bite force in some Sorex fromwestern increase reproductiveisolation among new forms NorthAmerica. In Advancesin the biology of shrews (ed. J. F. producedby stress (Schluter 1996 b).Interestingly,the Merritt,G. L. Kirkland Jr & R.K.Rose),pp.201^210 . traits inthe shrew mandiblethat weremost sensitive to Pittsburgh,P A:CarnegieMuseum of Natural History . Cheverud,J. M. 1982Phenotypic, genetic, and environmental stress (e.g.T -traits inBadyaev et al. 2000;landmarks integrationin thecranium. Evolution 36, 499^516. 10^14,16 and 1 7,this study) arealso closely associated Cheverud,J. 1 984Quantitative genetics and developmental withchewing patterns andthe diet (Carraway& Verts constraintson evolution by selection. J.Theor.Biol. 110, 1994;Dannelid 1 998).Moreover,these traits commonly 155^172. di¡er amongclosely related shrew species (Dannelid Cheverud,J .M. 1996Developmental integration and the evolu- (1998)and references therein;¢gure1 d).Forexample, tionof pleiotropy. Am. Zool. 36, 44^50. Sara¨ (1996)found that twoclosely related species ofthe Churchill, S.E.1996 Particulate versus integrated evolution of genus Crocidura weremost di¡erent intheir positionof theupper body in latePleistocene humans :atestof two the lowermandibular branch (landmarks of 10 and 1 1; models. AmJ.Phys.Anthropol. 100, 559^583. ¢gure 1a),the areaassociated with di¡ erences in Dannelid,E. 1998 Dental adaptations in shrews.In Evolutionof chewingpatterns. shrews (ed.J .M. Wo¨jcik& M. Wolsan),pp.1 56^174.Mammal ResearchInstitute, Polish Academy of Sciences,Bialowieza. Insummary ,ouranalyses of the morphologicalvaria- DÎtsch,C. 1982Der Kauapparat der Socicidae (Mammalia tionin the shrew mandiblehave produced two principal Insectivora).FunktionsmorphologischeUntersuchungen zur results. First, the phenotypicvariation and macroevolu- Kaufunktionbei SpitzmÌ usen der Gattungen Sorex L., tionarychange was mostly localized between rather than NeomysK., and Crocidura. Zool. Jb. (Anat.) 108, withinthe functionalcomplexes, as predicted bythe 421^484. mosaictheory of morphological evolution. A longhistor- Foresman,K. R.1994Comparative embryonic development of icalpersistence offunctional complexes was evident in the theSoricidae. In Advancesin the biology of shrews (ed. J. F. species similarities intheir patterns ofintegration. Merritt,G. L. KirklandJr & R.K.Rose),pp.24 1^258 . Second,the stress-induced variationwas highly concor- Pittsburgh,P A:CarnegieMuseum of Natural History . dantwith the patterns ofevolutionary change ;the shrew Foresman,K. R.1999 Thewild mammals of M ontana .University ofIdaho Press. species di¡ered inthe same sets ofnon-integrated traits Fumagalli,L., Taberlet,P .&Vogel,P .1999Molecular phylo- whichwere most sensitive tostress ineach species. We genyand evolution of Sorex shrews(Soricidae: Insectivora) suggestthat lowenvironmental and genetic canalization inferredfrom mitochondrial DNA sequence data. Mol. ofnon-integratedtraits mayhave caused these traits tobe Phylogenet.Evol. 11, 222^234. most sensitive notonly to environmental pressures, but Gavrilets,S. &Hastings,A. 1994A quantitative-geneticmodel alsoto the changesin selection andgenetic drift asso- forselection on developmental noise. Evolution 48, ciatedwith stress. 1478^1486. Goodall,C. 1991Procrustes methods in thestatistical analysis of shape. J.R.Statist.Soc . B 53, 285^339. Wearegrateful to GÏ nterW agner,DolphSchluter, Leary Ho¡mann, A. A.&Parsons,P .A.1997 Extremeenvironmental Leamy,DouglasEmlen, Cameron Ghalambor and Paul Martin changeand evolution. CambridgeU niversityPress. forstimulating discussion, detailed reviews of this study and Kindahl,M. 1959Some aspects of the tooth development in manyhelpful suggestions. W ealsothank F redAllendorf, Kelly Soricidae. ActaOdontol. Scand. 17, 203^237. Corll,Bill Etgesand W endyParson for thorough reviews of ear- lierversions of this manuscript. W ethankRex McGraw and Klingenberg,C. P.&McIntyre,G. S. 1998Geometric morpho- GregParkhurst for help with collectionof the shrews, Celeste metricsof developmental instability :analyzingpatterns of Fiumarafor patiently digitizing thousands of shrew mandibles £uctuatingasymmetry with Procrustesmethods. Evolution 53, andChris Klingenberg for providing the SAS algorithmfor 1363^1375. Procrustestransformation. Lande,R. 1979Quantitative genetic analysis of multivariate evolution,applied to brain:body size allometry . 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