Oecologiu (\990) 82:408-413 Oecologia © Springer-Ver!ag 199Q
Host-associated differences in fitness \vithin and between populations of а seed beetle (Bruchidae): effects of plant variabllity
David Н. Siemens and Clarence Dan Johnson Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011-5640, USA
Received December 19, 1988/ Received in revised form September 25,1989/ Accepted November J, 1989
Summary. Experiments were co11ducted with the sexually Кеу words: Fitness in bruchid beetles - Specialization reproduci11g seed beetle Stator limbatus a11d its lюsts i11 to specific hosts - Seed beetles - Bruchidae - Plant vari 11orth-ce11tral Arizona to determine if it was sнbstrнc aЬility tured i11to нnits, each specialized for higher fitness 011 а specific host species. Unlike many studies, we incorpo rated scale, I.e., conducting experiments betVv·een and within beetle popнlations 011 seeds from withiп апd be Several authors have focused on the genetic substructur tween plant species. Of particular interest ;vas whether iпg of species that feed оп mot·e thaп one species of intraspecific plant variability preYented beetle specializa host plant (e.g., Futuyma апd Peterson 1985). For infer tion vvithin beetle populatioпs. Resнlts sнggest that S. ences about the evolution of specialization to specitic limbatus is specialized to certaiп hosts. 011 the palo verde hosts it is important to bave corresponding measures Cercidium f/oridum, beetles origiпally reared from this of fitпess for these species. lп this study we tested for host had significantly higher emergence compared to host-associated differences in fitness within and between beetles transferred from other hosts. We did not test populations of the sexually-reproducing seed beetle Sta directly for а genetic basis for this. Alternative hypothe tor limbatus (Horn). For sexually reproduciпg insects, ses of variation in symblotic microorganisms in larval most of the evidence fог host-associated geпetic differ gнts and maternal effects were assessed. Essentially по ences in fitness сошеs fгom populations s~parated geo bacteria, yeast or protozoa were found, and maternal graphically оп locally abundant hosts (Hsiao 1978; effects as expressed Ьу varyiпg egg weights were not Rausher 1982; Scriber 1983; Т:J.bashnik 1983). There are detected; however, other шicroorgaпisшs might have по coпviпciпg examples of geпetic host-associated differ ences in fitпess for sexually reproducing iпsects within Ьееп present and maternal effects through iпdнciЬle eп zymes was possiЬle. Caution, theп, is needed in any ge local populatioпs, even though theorists сопсеrпеd with speciatioп nettc шterpretatioпs of our resнlts. The differences on sympatric have predicted it (Maynard Smith С. floridum were detected from tests between апd withiп 1966; Bush 1975; Endler 1977; White 1978; Udovic beetle populations. Evidence for specializatioп was поt 1980; Felsenstein 1981; Lande 1982; Kondrashov 1983; detected on the other hosts, Ce,·ciclium microphyilum and Rice 1984). Acacia greggii. On the other hosts. beetles performed Evolution iп local host utilization might Ье better well regardless of their source. Significant differences understood Ьу iпcorporati11g fiпe scale eпvironmental were detected amoпg individual plants of С. f/OJ'idшn factors into empirical studies. Coпspecific plaпt variaЬii} 1ty IS such а factor and has been shown to Ье significant as to the suitaЬility of their seeds Гоr developmeпt оГ S. limbatus. No such differences were detected among in preveпting insect species from adapting to а plaпtJ species (Pimeпtel апd Bellotti 1976; Vап der Plank 1984; the other host plaпts. These patterns of conspecific plant Whitham апd Slobodchikoff 1981 ), but which has not variaЬility are opposite of what is expected if plant vari been iпcorporated into studies of sympatric speciatioп. aЬility preveпts specialization of beetles to particular Неге we were concerned with conspecific variaЬility of species of hosts. Thus, the data suggest seed variaЬility plaпts in relation to the aЬility of S. limbaшs to become amoпg plants does поt preveпt specialization to host specialized with host species. We also co11sidered the pos species in this systeш. We discuss how the patterпs of host use in this study relate to the hypothesis of sympat siЬility that S. /imbatus could Ье specialized on certain ric host race formation. conspecific plants varying in quality. This гesearch was stimulated Ьу the above and specif ically Ьу the knowledge that Stator limbatus, which has at least 40 host plaпts (Johпsoп апd Kingsolver 1976), Offprint requests to: D. Siemens has а greateг mortality rate iп seeds of the palo verde. ~~~~r· 409 ТаЬiе 1. Expcrimcnts conducted to detcrminc if diffcrcnccs in litncss cxist bctwccn populations оГ Stator !imhatus. Тlн~sс cxperimcnts "crc also desigпcd to detect variation among conspccific plaпts, а t~н:tor \Vhich might prcvent spccialization оГ bectles. Bcetles were. tг.1nsfcrred onto sceds of individual plants thcy werc not originally rcarcd from. Thcse transfers wcrc comparcd to bcctlcs that werc oriJ;Iin;.11ly rearcd from those plants (controls). Ifthe controls outperformcd (see tcxt) thc tr;:шsfcrs. wc took this as cvidcncc tOr spccialization
Expt:riment Source of beet\es Seeds uscd
Location Host location Host
Verde А. gгeggii •.'"' P<~.radise C.jlщ-idum .11 tr;шst'crs .4.~. ~С'.+,
Cercitlium jloгidum, than it does in many of its other ТаЬiе 2. Experimcnts conducted to determine if differences in fit h'"" (Johnson 1981). Janzen (1980) hypothesized that пess exist within populations of S. limbatus. The experiments are thoro "·ere sympatric host strains of S. limbatus. If there similar to those bet\\·een populations (ТаЬ\е 1) except that seeds arc indeed specialized strains of S. limbatus, the beetles апd beetles \vere from one site reared from their original hosts should perform better Experiment Source Seeds used on these hosts. In particular, we hypothesized this for of beetles Sraror limbatus on Cercidiumjlд1·iclum. Thus our primary questions were: (1) Do some а) Roosevelt Lake groups of S. limbatus have higher fitness in the seeds transt'ers А. greggii C.jloгitlum ot' some of their hosts in an experimental en\·ironment? controls С. jloridum С. flмidum 12) variaЬility and !s there intraspecific in the protective Ь) Paradise Valley mochanisms of the seeds of the hosts to protect them transfers С. mici'Opiiyllum С. jlщ·idum !'r(·!:. the depredations of S. !imbaшs? controls C.jloridum C.jloridum
с) Paradise Yalley transfers C.jloridum .\\а terials, methods, and study sites с microplty!lum controls С. miaop/Iyllum с microp/Iyllum ln gcncral, our experiments involved rearing Sraror !imbatus from sceds collected in the wild and then ц11owing them to oviposit ТаЬiе 3. Experiments conducted to determine if fitncss diПerences and develop in seeds from which they \Vere reared and in seeds exist between and within populations of Stшor limbatlls occurring from \""hich they wcrc not reared. Each experiment \\·as two-factori on Cercidium jloridum. Experiment а tests differences bet\•;een a! in design and was analyzed Ьу t\VO-\.,.·ay ANOV А. The factors groups of S. limbatus from geographically separate stands оГ С "'~rc: 1) Groups of S. limbatus designared Ьу seed source and jloridum (Paradise Valley and Roosevelt Lake). This experiment ~~ ~..:eds of conspecific plants. Thus \\ie were <~.Ые to determine \Vas aJso designed to test differences between the seeds t'rom the di:":~~:::nccs between beetlc groups, which invo\Yed symp livc loпgcr whcn fcd. а small spongc soakcd in solution of sugar 10 1.0 water was placcd in cach pctri dish. Thc bcctlcs \vcrc allowcd to о 9 .9 \ау approximately four eggs per seed then removed aftcr 24-4~ h. <( ..J Eggs i11 cach dish were counted and as offspring cmergcd from .8 ,.._ .в seeds they were removed and counted. The number of offspring 1 "' 7 't 4 .7 to emcrge per egg laid Ьу adults was calculated for each dish. "' '+ t Each data point was the result of one petri dish. 'о .. .б Two field collections of seeds were made for each of the ра\о "' 5 1 Со. .5 verdcs: an early collection for uninfested secds which wcre uscd "' "' .4 1 in experiments. and а late collection from which bcetles wcrc rearcd "' .4 and also used in experiments. Early col1ections were made in early " г 1 r "' .3 .3 July 1986 and late collections were made in mid-August 1986. In "'.... -<:>- 1 ..J 2 some cases beetles were reared from early collections of С. miao :::> '-~>- .2 о pliyllшn because most plants of this species drop their pods in <( '+ .1 i .1 early July making thcm unavailaЬJc for collection in August. 1 There are other species of bruchid beetles on palo verdes that о ~ ~ ~ ~t о А 8 с D Е -Al present а proЬlem in sorting uninfested seeds if only late collections PL< are made. These other species (Mimosestes amicus and М. uikei) JNDIVIDUAL PLANTS lay their eggs on pods. not seeds. therefore some seeds removed и С. flocid"m from pods in the laboratory have no obvious marker (e.g., empty egg chorions, entry holes of larvae) indicating that they contain beetles. Stator iimbatus oviposits on seeds, thus the infested seeds have conspicuous eggs on them and can Ье sorted. Fig. 1. Evidence that Stator limbatus has higher fitness on а Sorting uninfested seeds from А. greggii collections was not from which it was originally reared compared to beetles fror а proЬlem. Seeds of А. greggii were collected in mid-August. other host species and location. These are results from experi Egg weights were measured to see if they were responsiьte а in ТаЬlе 1. On Cercidium floridum seeds from Paradise V for any differences in offspring survival. We did this Ьу weighing beetles originally reared from this host performed best com1 seeds before and after they were o\·iposited upon. to beetles transferred from Acacia greggii from the Verde V Beetles from Cercidium floгidum are represented Ьу closed ci those from А. greggii Ьу open circles. Numbers next to syr Resu\ts are the sample sizes (number of petri dishes, each containing; 50 seeds with about 100 eggs). Vertical bars are ranges. е Hosts separated geograp!Jically in the summary graph (all plants) where they are standard t of the mean. There was also significant variation among the vidual plants of С. floridum (named А, В. С. etc.) which w~ In experiments between beetle populations, Stator !imba expected in the presence of evidence for specialization tus performed best on а host from which it was originally reared (Fig. 1). Beetles from Cercidium floridum (Para dise Valley) had an emergence of 68 ± 6% (mean ± stan о dard error) on C.floridum, v;hile those from Acacia greg .. 1.0 _j gii (Verde Valley) had an emergence of 23 ± 5% on С. .9 oridum. In the experiments on the other two host spe "~ .8 TRL cies (С. microphyl/um and А. greggil) there was no indi UJ .7 cation of host-associated dift'erences in fitness in S. !im 'о UJ ~~======TRL1 .б ++ +-+ batus. ~ PV а:: PV Significant variaЬility in the suitaЬility of seeds to UJ .5 S. limbatus was detected among individual plants of С. UJ А floridum (Fig. 1). А multiple comparisons test (Studeпt "' U) .3 Newman-Keuls; SNK) among the individual plants 1- _j .2 ranked them as follows: D=A=G>B=F. In experi :::> о ments on the other two host species (С. microp!Jyl/шn .. .1 and А. greggii) we did not detect significant variaЬility о among individual plants. THEODORE -" PARADISE In an experimen t between t\vo geographically sepa ROOSEVELT VALLEY rate stands of C.jloridum (Paradise Valley and Roosevclt LAKE Lake), we did not detect significant differences in our STANDS OF !;_ floridum experiments between S. limbatus popнlatioпs or C.jlm·i tium seeds from the two stands (Fig. 2). Fig. 2. Therc werc no differences between thc bcetle popu from the two stands о!· Cercidium floridum. This was dete Host occurring sympatrica/~1' Ьу the non-significant interaction in the ANOY А. There ~ significant dift'ercnces in seed suitability betwcen the two As with the results between poplllu.tions, the experiшcпts Beetles originaHy from the Theodore Rooscvelt Lakc site а within populations suggest the specialization of Stator cated with the abbrevia.tion TRL and beetles from Paradise limbatus \Vith the host Cet·citlium j/01·idum. At the Roose- PV. Vertica\ bars are standard errors of the meaos .j 11 ROOSEVELT LAKE PARADISE VALLEY а 10 ,ol 1.0 9~ 1 9 9 1 .9 < 1 ~ 6 ~ ~ ' 1 '• ,11 8 ш 1 ' ' 6 i 6 ' ~ 1 t о с '1':'.Н '• ,, .5 il 4 i '-о- -о- ~ , .1 .4 ~ . '-9- 3 ~ ~ .i,. w ·~~~: о 1 ' ' ~ 2 о о з! о w о i :~;;;· < о 5 \ 'о- i w ~ ~ ~~ " о , с в D Е F 6 ш 1' PLANTS"' / :/~ INOIVIDUAL PLANTS ~ ' с ___ ~ ' ' <;_~ ~ о 3 ' !', 4 1 1 ' и 1 ' Zj ' PARADISE VALLEY ь lj ''l 10 ~ '~ !------< ,....____..... ,____._...... , >------4 ~ ~ 1 9 J~ о '"1 в D 6 с е; ' ' " 6 ' о 'll INDIVIOUAL PLANTS о ,j 't '+ ! w '+ '+· 7 ... о б"'' 2 1 6 w '.О. Fig. 4. There was no indicatioп that beetles were specializing оп о 1 '• 5 о '" conspecific plants although signilicant variability in susceptiЬi!ity о- ' 't 't 4 among CNcidium .fioridum p!ants was detected. Sma!l case letrers 3~' t indicate \\"hich plant the beetles were originally reaгed Гrom Zc ,2 о 't ,с о ТаЬlе 4. There \\'ere no significaпt difГcrcnces in weights of eggs 1 ос~ ~ ~ ~ ~ among the beerle populations that peгfoгmed diffcrently on Cerci ~ 4 в с D - G о dium .fioridum (Figs. 1. 3 Ь). The weight о[ eggs was deteгmined PLANTS INOIV 1DUAL PLANTS "' , Ьу weighing the seeds апd eggs together. then total egg Vieight per pctri dish was diYided Ьу the number of" eggs Host-associated Number оГ Egg weights populations petri dishes {SE mean) Н2.. За, Ь. Evidence for specialization to the host Ceгcidium jlon· ;.,,n \\·ithin t\vo populations о{ Starm· !imhatus. At the Roosevelt Cercitlium jloridum 16 2.47Е-04 !" .. :..: site (а) beet!es originally f'rom С. jloгi{/um (clvse(i ciгcles) (5.16Е-05) г..:r!'ormed bctter on this lюst t!шп bcctlcs orizina!lv reared t'rorn С. microphyl!um 10 2.90Е-04 Jcucia greggii (open circ!es). Similarly at the Paradise Valley site (1.11Е-04) !Ь). beet!cs originally from С. jlmblum (ciosecl ciгc!es) per!Ormed better on C.jloridum than beetles tr::шsferred from С. micmplп·/lum Acacia greggii 16 1.23Е-04 Uщerte(/ triangles). The design оГ thcsc cxperiments is cxp(ained (2.83Е-05) in ТаЬ!е 2. Numbers next to svmbols апd vertical bars through symbols сне as in Fig. 1. Variability among С. /lon"{/um р\шнs w:1s not significant at tl1e Rooscvelt Lake sitc In another experiment there was no indication that beetles were specialized on conspecific С. jloriclum ·,eJt Lake site, significantiy higher emcrgence restiited plants. Beetles did not perform betteг оп the C.jlm-iclum ")ffi С. jlon·dum seeds when eggs were laid Ьу bcetles plant from which they were originally гeared. compured ,,,.,~inally collected from this host (Fig. За). On C.jlo,-i to beetles transferred from other С. Jlm·i,/um plaпts (/шn. beetles from С ..floгblum had an emergence of 62 ± ( Fig. 4). In addition, beetles reared f"roш resistant plaпts 6% while those transferred t"rom А. greggii to С. jlшi (see below) did not perf"orm better оп these hosts com dum had 30±6°/о emergence. Results were similar at pared to beetles from other plants. the Par<:tdise Valley site, when beetlcs were transt'crrcd There was significant variaЫ\ity detected among in onto C.jlori,/um from С. miaopln·iillm (Fig. 3 Ь). Bcetles dividual plants о!" C.jlшiclum in this cxperiment (Fig. 4). from С . .floridum had an emeгgence of 68 ± б'Уо wl1ile А multiple coшparisons test (SNK) among these plants those from С. micгophyllum had 38 ± 11 1 У.1 cmergeпce. ranked thcm as follows: G = D =С= А= F >В. Plant 'l ~~ 412 Н is clearly most resistant, but there were some empty and sнrvivc on locally abundant host spccics has becn cells in the experimental design, so it was not included sllO\YП in the beetle Leptinotarsa Jcccmlineшa 'Hsiao in the analysis. 1978) and in many spccies of Lepidoptcra (Raushcr 1982; Scriber 1983; Tabashnik 1983). Rausher (1984) slюvYed genetic differences in fecundity in а chrysomelid Weiglzt of eggs beetle with different species of Ipomoea separated Ьу 6.5 km. With S. limbatus, we detected differences in fit There were no significant differences in the weights of ness betvveen and within popнlations, but further studv eggs between groups of S. limbatus that showed fitness n1ust Ье done to deterшine whether there is а geneti~ di!Terences on С. jloriclum (ТаЬ!е 4). On С. floriclum, basis to these patterns. eggs laid Ьу beetles originally from А. greggii and С. \\'е also asked if S. !imbarus was specialized for feed microplzyllum did not weigh significantly less than eggs ing in conspecific plants. We asked this question for both laid Ьу beetles originally from С. floz·iclum. betvv-een and within beetle populations. Between popula tions of S. limbatus we conducted experiments between geographically separated stands of С. floridum. \Ve de Discussion tect~d no significant differences betweeп seeds tfom the t\VO stands апd as expected found no stand-associated The first major question addressed in this study was: differences in beetle fitness. do sоше groнps of Stato,. limhatus hfl'/C higher fitness \Vithin populations, only Edmunds and Alstad in the seeds of certain hosts in an experimental environ (1978) claim specialization in an insect with conspecific ment? Our data suggest that S. limbatus associated with plant variability. They argнed that the low vagility of С. flшianimals, and may Ье manifested in the different sizes gestive evidence of specialization, С. floricfum, \V'C found of offspring at birth. This size diffcrence is in turn dне significant variation among conspecific plants (Figs. 1, to variation in the amottnt of material channelled into 4). This suggests that conspecific plant variation does eggs or emb!'yos (Falconer 1960; Bakker 1961; Welling- not preveпt specialization in S. limbatus to this host spe ,,,,~· ton 1976; Travis 1980). То the extent that egg weights ctes. ~;,." '~ ше а measure of this etTect. there were no maternal еГ- Variation amoпg conspecific plants, hov..-ev·er, may • .r.. _ .~. • t'ects ш our study.1Tl1is ts because there were по SJgшti- Ье important iп preventing beetles from overcoming the .' . -\,··:·· · \ cant dtfferences ш egg \Vetghts between the popнlatюns defenses of а single plant. We found а lack of specializa 1 (, ·-~ h r ,.,- · that showed performance dif'ferences (ТаЫе 4). Alterna tion iп the presence of significant plant v·ariation V~/~'. ~.. .:-·· ,},t tively, it is possiЫe that maternal effects were preseпt (Fig. 4). fч" ~··Q,.. ,...• :." . as expressed Ьу inducible eпzymes. which woнld поt ncc- Весанsе brttchids in general are known for being host f( х.}~·~. 'i.J\.1( essarily Ье detected in \V'eights of offspring. Altlюugh specific, Janzcn (1980) hypothesized the existence of .J.·\~11' ., · the fcmales used in онr expcriments wcre trcatcd tl1e "sympatric strains" for S. /imbatus. Janzen stressed the same once they emerged t'rom seeds, they spcnt time importance of seed defense chemistry in the evolution in the seed in nature under different conditions. of host specifk.1ty in bruchids. Our data support this For sexually rcpro<.:iuciпg insccts. gcnetic variatioп hypothesis iп that we have evidence suggestive of а strain a'mong geographic populations in the aЬility to grovv with the one host (C.jloridum) that appears to Ье unique 413 in its suitaЬility to S. /imbatus (Johnson !98! ). Оп the Fe!scпstciп J (19Х1) Skcptici::;m towards Saпta Rosalia. or \\·hy other two hosts (С. miaopl1ylium and А. gt·e.~.~il). how arc tl1cтc so t'c\\' kiпds of'aпimals'? Evoiution 35:124-138 e,er. beetles perfonned well without any apparent spe Futнyma DJ. Pctcrsoп SC (1985) Gcпctic variatioп in lile нsс tЭt' rcsoнrccs Ьу insccts. Лпn Rcv Entomol30:217-238 cialization. This suggests that host use iп bruchids is Ho\\'ard DJ, Bush GL. Brcznak JA (1985) Thc cvolutionary sigпifi not determined Ьу the digestiЬility of seeds alone. cance of bacteria associated \Yith Rfщкo!etis. Evolutioп 39:-1-05- The data in this study do not fit some of the predic 417 tions for sympatric host race formation. In the sympatric Hsiao ТН ( 1978) Host plaпt adaptations among gcograp!1ic popu h'rothesis, one predicts reproductive isolation with spe !atioпs оГ the Colorado potato bcctlc. Entomol Ехр Appl ci;clization in host use. That is. with specialization to • "4:437-447 Jапzсп ОН (1980) Speciricity ot' sccd-attacking bcct\es iп а Costa one host comes the inaЬility to use otl1cг hosts, thus Ricaп deciduous forcst. J Е со! 68:929-952 sclection against the use of other hosts causing reproduc Johпsoп CD ( 1981) Host pгefercnces of Stшvr (Co!eoptcra: Bru ti,·e isolation (Maynard Smith 1966; Bush 1975; chidac) iп nonhost sceds. Eпviroп Eпtomo\10:857-863 Rausher 1984). The putative specialization to one host Johnson СО. KiпgsolYer JM (1976) Systematics of" Statoг ot' North in this study does not prevent the beetles fгom feeding and Ccntral America (Coleoptcra: Bruchidac). U.S. Dept Agric \\·ell on the other hosts in the laboratory. We found this Tecl1 Bнll1537:1-101 Koпdrashov AS (1983) Multilocus modcl of sympatric speciation to Ье so between and within populations. If genetic, one II. Two charactcrs. Thcor Рор Biol24: 136--144 "·ould predict that the trait for specia!ization would Lande R (1982) Rapid origin of sexшtl iso!atioп апd character srread to populations on other hosts, resulting in only divergence in а cliпe. Evolution 36:213-223 ot:-: genotype among populations. As suggested most Mayпard Smith J (1966) Sympatric spcciatioп. Am Nat 100:637- rec·ently Ьу Bernays and Graham (1988), it is conceivaЬ!e 650 tlыt factors other tlшn the digestiЬility of plant tissue Pimcпtel D. Belloнi АС (1976) Parasite-host population systems апd geпetic staЬility. Am Nat 110:877-888 (e.g., predators) could promote reproductive isolatioп Rausher MD (1981) Popнlatioп diffcrentiation iп Eupllylf'·yas in sympatric distributions of herЬivores. eriitlю butterflics: larval adaptatioп to dit'fcrcпt hosts. E'-·olu tion 36:581-590 Acknoн'ledgments. We are grateful to R. Balda. D. Howard. Р. Rausher MD (1984) Tradeof"fs iп performaпce оп differeпt hosts: Price. and Т. Whitham for reviC\Ying earlier drafts of the manu Е videпce f"rom \v·ithin- апd bct\veen-site variatioп iп the beetlc ->eripr; to К. МоЬlеу t'or analyzing insect larv·ae tOr microorgan Deloyala guttata. Evolнtion 38: 582-595 i~ms: and to NSF Grant BSR82-11763 (to CDJ) t'or partiai sнpport Rice WR (1984) Disruptive se\ection on haЬitat prcf'erence <Шd t"or this research. the evolutioп of reproductivc iso1ation: а simulation study. Evolнtion 38:1251-1260 Scriber JM (1983) Evolution of" Гeediпg specialization, p11ysiologi cal cf"ficieпcy апd host races iп selected Papi!ioпidae and Satur Rrferences пiidae. Iп: Denno RF, McC!ure MS (eds). VariaЬle Plants und HerЫvores in Natural n.пd Maпaged Systems. Academic Press. B<.tkker К (1961) An analysis ot' factors \vhich determine sнccess New York in competition t'or food among laryae of Drosopllila me!ano Tabashnik ВЕ (1983) Host raпge evolution: the shift from пatiYc :;::aster. Arch Neerl Zool14:200-281 legume hosts to alf'alfa Ьу the butterfly, Cvlias plzi!otlice eri Benson L, Darrow RA (1944) А manual of soнth\vestern dcsert pl1J-'le. Е volution 37: 150-162 trces and shrнbs. Bulletiп No. 6. У. 15. UniYersity of Arizoнa Tra vis J ( 1980) Phenotypic variatioп and the oнtcome of interspeci Press, Tucson, Arizona fic competition in hylid tadpoles. Evolution 34:40-50 Bcrnays Е. Graham М (1988) On the evolution of host specificity Udovic D (1980) Freqнency-depcndent sclection, disruptive selec in phytophagous arthropods. Ecology 69: 886--892 tioп. and the e\'olнtioп of reproductive isolatioп. Am ]'.;at ЙtJ~h GL (1975) Sympatric speciation in phytophagous parasitic 116:621-641 :пsccts. In: Price PW (ed), Evolнtionary Strategies of Parasitic Van der Plank JE (1984) Disease Resistaпce iп Plaпts. Secoпd Edi !nsccts and Mites. Plcnum, Nc\Y York. рр 187-206 tioп. Academic Press, New York Carter АМ (1974) The genus Cercitlium (Leguminosac: Cacsa!pin Wellingtoп WG (1965) Some materпa\ iпflнences оп progeny qнali ioideae) in the Sonoran Desert ot' Mcxico and thc United States. ty in the western tent caterpi!lar, k!alacosoma pluvialc (Dyar). Proc Calit' Acad Sci 4th Ser 40: 17-57 Сап Eпtomo\97: 1-14 Endlcr JA (1977) Geographic Variation, Speciation. and Clines. White MJD (1978) Modes of Spcciation. Freemaп апd Соmрапу. Princeton University Press, Priпcetoп San Fraпcisco Edmunds GF, Alstad DN (1978) CoeYo\utioп iп iпsect hcrЬi,,ores Whitham TG, Slobodchikoff CN (1981) Evolution Ьу iпdiYiduuls. and coпifers. Scieпce 199:941-9-1-5 plant-herbivore interactions. and mosaics of genctic variaЬility: Falcoпer DS (1960) Introductioп to Qнaпtitatiye Genctics. Roпald. о the adaptive sigпiricance of somatic mutatioпs iп plaпts. Occo ;'\;е\\' York logia 49:287-.:::92