Discontinuity in evolution: how different levels of organization imply preadaptation

Orazio Miglino* Stefano Nolfi+ Domenico Parisi+*Department of Psychology, University of Palermo, Italy+Institute of Psychology, National Research Council, Rome, Italy.fax:0039-6-824637e-mail:orazio@caio.irmkant.rm.cnr.itstefano@kant.irmkant.rm.cnr.itdomenico@gracco.irmkant.rm.cnr.it1. INTRODUCTION acts as a major determinant of the direction taken byevolution. The real questions then are the relativeimportance of adaptive versus non-adaptive changes and abetter understanding of the mechanisms underlying bothtypes of change.Although sequences of nucleotide bases in DNA and aminoacids in proteins appear to mutate at approximately constantrates, evolution seems to have been anything but steady. Infact, most dramatic changes in evolution occurred ratherabruptly (Eldredge and Gould; 1972; Gould and Eldredge,1993).Using a computational model we will try to show that non-adaptive evolutionary changes may largely outnumberadaptive ones. This fact appears mainly due to thehierarchical organization of the simulated organisms. Wewill also show how some of these non-adaptive (neutral)changes may subsequently become the basis for furtherchanges which do prove adaptive and therefore howpreadaptation phenomena may arise producing suddenevolutionary changes in the behavior of organisms.Gould (1991) claims that in order to explain such radicalchanges one should abandon the adaptationist or "ultra-Darwinist" paradigm. The idea that all change is gradualand continuous is in fact a consequence of interpretingphenotypic changes as mostly adaptations. Gould (see alsoGould and Vrba, 1982) proposed to classify phenotypictraits in three different categories: adaptations (traits thathave a well-defined function), preadaptations (traits built forone function and then adapted to another), and exaptations(traits not built as adaptations at all but later adapted forsome function) and he claims that the last two categoriesmust greatly exceed adaptations in number and importance.2. DESCRIBING ORGANISMS AT MULTIPLELEVELSIt is obvious that biological systems can and should bedescribed at various hierarchical levels. For example, anorganism can be described at the genetic level, at the neurallevel, at the behavioral level, and at the level of fitness. Adescription of the organism at the genetic level is adescription of the genotype of the organism, that is, of thegenetic material that the organism has inherited from itsparent(s) and that directs the construction of the phenotypicorganism. A description at the neural level is a descriptionof the nervous system of the organism, which is just oneaspect of the phenotypic organism. A description at thebehavioral level is a description of the behavior which isexhibited by the organism, given its nervous system, in aOf course, no biologist has ever advocated a completeequivalence between retained changes and adaptations.Adaptive changes in some trait may entail correlated but notnecessarily functional alterations of other traits (e.g. thehuman chin appears to be a by-product of functionalchanges in other parts of the human face duringhominization). The architecture of genetic andembryological systems defines channels of possible change.Selection may be required to push an organism down achannel but the channel itself, though not an adaptation,