CONTINUITY IN EVOLUTION - ON THE NATURE OF TRANSITIONS

To distinguish continuous from discontinuous evolutionary change, a re lation of nearness between phenotypes is needed. Such a relation is ba sed on the probability of one phenotype being accessible from another through changes in the genotype. This nearness relation is exemplified by calculating the shape neighborhood of a transfer RNA secondary str ucture and provides a characterization of discontinuous shape transfor mations in RNA. The simulation of replicating and mutating RNA populat ions under selection shows that sudden adaptive progress coincides mos tly, but not always, with discontinuous shape transformations. The nat ure of these transformations illuminates the key role of neutral genet ic drift in their realization.