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Influence of Generation Time on the Rate of Response to Selection

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Influence of Generation Time on the Rate of Response to Selection Vol. 137, No. 4 The American Naturalist April 1991 INFLUENCE OF GENERATION TIME ON THE RATE OF RESPONSE TO SELECTION JAY A. ROSENHEIM* AND BRUCE E. TABASHNIK Departmentof Entomology,3050 Maile Way, Room 310, Universityof Hawaii, Honolulu, Hawaii 96822 SuibmittedSeptember 11, 1989; Revised Januat-y30, 1990; Accepted March 13, 1990 Abstract.-We examined the influenceof generationtime on the rate of evolution of a trait underintense natural selection: pesticide resistancein arthropodpests. Previous empiricaland theoreticalanalyses supporteda positivelinear relationship between the numberof generations per year and the rate of evolutionof pesticideresistance. To testthis relationship, we assembled a data base that integratedinformation on resistance evolution, generationtime, and other biological parametersfor 682 NorthAmerican arthropod pests. The data did not supporta linear relationshipbetween generationsper year and the evolutionof resistance,revealing instead a nonlinearand highlyvariable relationship,with peak rates of resistance evolution for species withintermediate generation times. This resultwas independentof the differencebetween intro- duced and native species and of differencesamong the major arthropodtaxonomic orders in abilityto evolve resistance.A reevaluationof evidence fromanalytical and computer-simulation models of resistanceevolution suggests that the linearrelationship between generations per year and resistanceevolution is also withoutfoundation in theory.An extensionof a simpleanalytical model of resistanceevolution suggests instead that the rate of resistanceevolution is independent of generationtime. We also findlittle support for the suggestionthat species withmany gener- ations per year are regularlysubject to elevated levels of selection for pesticide resistance. Per-generationfitness values for genotypes conferringresistance to pesticides or genotypes conferringincreased fitnessin response to any density-independentselective agent are related exponentiallyto generationtime, resulting in the independenceof generationtime and the rate of response to selection in the simplest-casemodel. Generationtime can influencethe rate of resistanceevolution; however, ratherthan actingin a simple,uniform manner, generation time interactswith a varietyof genetic,ecological, and operationalfactors to produce a multitudeof effects. The molecular-clockhypothesis has spurreda continuingcontroversy regarding the rate of evolutionarychange in lineages withdifferent cohort generation times. Most studies attemptingto resolve thisissue have been conductedat the molecu- lar level and have examinedgenetic changes thatare arguablyselectively neutral (Sarich and Wilson 1973; Wu and Li 1985; Easteal 1988; Graur et al. 1989; see also Palumbi 1989). The degreeto whichthe rateof molecularevolution of neutral traitsis constantis importantbecause rate constancymust be assumed to recon- structevolutionary trees with correct topologies and to inferthe phylogenetic divergencetimes of taxa of known molecularsimilarity. The influenceof generationtime on the evolutionof nonneutraltraits is also a * Presentaddress: Departmentof Entomology,University of California,Davis, California95616. Am. Nat. 1991. Vol. 137, pp. 527-541. ? 1991 by The Universityof Chicago. 0003-0147/91/3704-0005$02.00.All rightsreserved. This content downloaded from 128.120.194.195 on Tue, 23 Sep 2014 01:11:58 AM All use subject to JSTOR Terms and Conditions 528 THE AMERICAN NATURALIST theoreticallyimportant question. Variation in generationtime is one of several factorsthat could cause the rateof adaptiveevolution to varyin differentlineages. If the fitnessvalues of given alleles are independentof generationtime, then lineages witha largernumber of generations per year (GPY) exhibitgreater annual changes in allele frequencies.Rate constancyof evolutionfor selectivelyimpor- tant traits would argue against the claim that a constant rate of evolution is consistentonly with a neutraltheory of evolution (Hartl and Dykhuizen 1979; Gillespie 1986; Zuckerkandl 1987). The effectof generationtime on the rate of adaptationof populationscould also influencethe evolutionof life-historystrate- gies. If a largerGPY increases evolutionaryplasticity, shorter generation times could be favored over longer generationtimes. Shortergeneration times might thereforebe favored evolutionarilyfor one of the same primaryreasons that sexual reproductionhas been hypothesizedto be favoredover asexual reproduc- tion (Maynard Smith 1984; Nunney 1989). Nevertheless,few studies have at- temptedto assess the influenceof generationtime on the rate of adaptive evolu- tion. In a seminal study,Hartl and Dykhuizen(1979; Dykhuizenand Hartl 1981) found that adaptation of Escherichia coli to a novel habitat was a functionof absolute timerather than the numberof elapsed generations.These authorswere able to change generationtime by manipulatingthe rate of nutrientflow into bacterial cultures in a chemostat. Similar experimentaltests of the influenceof generation time on adaptive evolution have not been performedwith higher eucaryotes. The evolution of pesticide resistancein arthropodpests provides an opportu- nity to test the influenceof generationtime on the evolution of a selectively importanttrait. The economic importanceof pesticide resistancehas resultedin the documentationof resistance in a large numberof arthropodspecies (Geor- ghiou 1981); these data reveal the results of a large-scale natural experiment in organic evolution,observed at the organismalrather than the molecularlevel. Generationtime has been identifiedas one of the few factorsthat appears to influenceresistance evolutionin a strongand consistentmanner. Empirical and theoreticalanalyses have uniformlysupported a positive linear relationshipbe- tween GPY and the rate of resistanceevolution (Comins 1979; Georghiou 1980; Tabashnik and Croft 1982, 1985; May and Dobson 1986; Hartl 1988; Tabashnik 1990). Here we present an empiricalanalysis of resistance evolution in North American arthropodpests thatchallenges this view. We reevaluate existingthe- oryin lightof our resultsand show thatit failsto supporta consistentrelationship between generationtime and the rate of resistanceevolution. Finally, we extend our conclusions to a broad array of traitsinfluenced by selective forces that operate in a density-independentmanner. Elsewhere we present the results of computer simulationselucidating the various influencesof generationtime on resistanceevolution (Rosenheim and Tabashnik 1990). METHODS Data-Base Compilation Our analysis is based on a data base thatintegrates information on biology and resistanceevolution for North American arthropod pests. We used Davidson and This content downloaded from 128.120.194.195 on Tue, 23 Sep 2014 01:11:58 AM All use subject to JSTOR Terms and Conditions GENERATION TIME AND RESPONSE TO SELECTION 529 Lyon's (1987) extensive list of pest species as the foundationfor the data base. All pests of agriculture,stored products,and human or animal health were in- cluded. Pests of ornamentalshrubs and trees, rangelands,and foresttrees were included only if subject to pesticidal control. For each pest species, we then sought five pieces of information:(1) a measure of abilityto evolve resistance, (2) an estimateof GPY, (3) a measure of pest severity,(4) whetherthe species was introducedor native, and (5) the species' taxonomicorder. An index of each species' abilityto evolve resistancewas generatedby count- ing the number of insecticide/acaricideclasses to which at least some North American field populations had been reported as resistant(Georghiou 1981). Thirty-sixpest species reportedas resistantby Georghiou(1981) but not present in Davidson and Lyon (1987) were added to the data base. Pesticides were grouped into six classes followingGeorghiou (1981): (1) DDT and analogues, (2) benzene hexachloride and cyclodienes, (3) organophosphates,(4) carbamates, (5) pyrethroids,and (6) other miscellaneous compounds, includingbinapacryl, chlordimeform,ovex, propargite,quinomethionate, sulfenone, and tetradifon. Resistances to inorganic,botanical, and microbialinsecticides, which are rela- tivelyuncommon, were excluded. Althoughwe feel that our measure of ability to evolve resistance is unbiased with respect to species GPY, the index is not withoutlimitations. Most important,the exposure of differentpests to pesticide selection pressures probably varied withinand between differentmanaged eco- systems. We have attemptedto accommodate variable selection intensityby in- cluding in the analyses a measure of pest severity.We analyzed the data base firstas a singleunit and thengrouped species by pest typeand crop attacked (see below). Estimates of each species' GPY were compiled fromthe literature.For cases in which a range of GPY associated with latitudewas cited, or when different studies produced differentGPY estimates,the arithmeticmean of the observa- tions was used. No attemptswere made to estimateGPY fromlaboratory studies or egg-to-adultdevelopment rates. We estimatedGPY for 90 species for which literaturereferences could not be found by using the mean value reportedfor congeners. This approximationwas not applied to mosquitoes, which demon- stratedlarge within-genusvariation in GPY. In thisarticle, we use the termsGPY and generationtime to referto the same biological parameter;however, because many arthropodsundergo diapause forpart of the year, the inverse relationship between GPY and generationtime will not be exact. This distinction,although
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