Biology and Philosophyll: 67_88,1996. O 1996 Kluwer Academic publishers. printed in the Netherlancls. E_volutionary Plasticity in prokaryotes: A panglossian View Evolutionary P lasticity in p rokaryotes MARCEL WEBER Bictzentrum Universityr;f Basel Basel,Switzerland und [' uchgr u ppe Philosophi e Universityo.f Konstanz Kon.rtan7., ()ermuny I -e mar ' :alroni , 'ecorl :A SIUC]' - 3rkrnd ;,:r d o : - s e d rl :'e vcI ':s Thr : - s o f a ' :-wrdr 'a " ews Abstract' finzyme-dirccted gcnetic mechanisms causing random DNA sequence alterations are ubiquiklus in both eukaryotcs and prokaryotes. A nurnberof moleculai geneticists have invoked .adaptation through natural seiection to account fbr this läct, however,alternative c'xplanations havealsoflourished. The population gencticist G.C. williams hasdismissed the possibility of sclecti.n for mutat.r "ciiuity on a"prirtri grounds. In this paper, l attempt a retutation.of williams' argurnent. In acldition, I discuss some conceptual problems relate6 to recent claims madeby microbiologists on the adaptiveness of "molecular variety generators" in the e.volution of prokaryotes- A_distinctionis proposed betweenselectiontbr mutations <'a use d by a nrutator activ ity and sclection fbr thc m utator activity proper. The latterrequires a concept of fitness cliff'erent from the one commonly useain microbiology. Key words: Microbiarevolution, genetic diversity, genome prasticity, mutation, selection, adaptation, function, etiological c.nception, fitness, änvironmentar näi".og"n",ty, expected timeto extinction, clone, individual, clan, subclan, bacteriophage, DNe inveÄion, site_specific recombination, secondary crossover sites, transpitsitiolr. :'i:';;: Introduction : slracts The question of whether gene products thatcreate genetic diversityby causing , : otog random DNA sequence alterations are biologically adaptivein the narrori .lanti sense seems not to havebeen resolved to this date (Maynard Smith, lggg). ? catio Several population geneticists have claimed that the production of muta- 'servic tions by so-called "mutator genes" is at leastone of the specific functions of these genes and that there is something like an optimal mutation rate for eachorganism (e.g., Darlington,1g5g;Kimura, 1g60; Ives, lg50). on the other hand, G.c. williams (1966) has advanced a remarkabre a priori argu_ ment against this view. The function usually attributed to mutatorgenes and other genescausingDNA sequence varietiesis to ensureplasticity of the I