| Abstract: | Neutral mutation and evolutionary progress The process and causes of regressive evolution are still under debate. Contrary to DARWIN'S original assumption, Neo-Darwinian proponents make selection responsible for reduction. Biologically functionless structures like eye and pigmentation in cave animals deliver excellent material to study this problem. Comparison of regressive (eye, pigmentation, aggression, dorsal light reaction) and constructive traits (gustatory equipment, egg yolk content, feeding behavior) in epigean and cave fish (Astyanax fasciatus, Characidae) reveal a high variability of the regressive features in the cave forms. Contrary to this, the constructive traits are characterized by a low variability in epigean and cave fish. This difference is attributed to the lack of selection on regressive structures. The existence of an intermediate cave population between epigean and true cave fish of A. fasciatus makes possible the study of evolutionary rates. It is shown that the regressive traits do not evolve more quickly than the constructive ones do. On the contrary, constructive traits like egg yolk content are even more rapid because they are of great biological value in the cave biotope. Especially energy economy is claimed by Neo-Darwinists to play a decisive role as a selective force. Comparison of the development of epi- and hypogean larvae of A. fasciatus shows that the formation of a smaller and less differentiated eye in the cave specimens has no effect on body growth. Furthermore, even behavioral traits like aggressiveness, schooling, dorsal light reaction, or negative phototaxis, which all are not performed in darkness by the epigean ancestor, become genetically reduced in the cave fish. The principles of regressive evolution, loss of selection and increase in variability, play a central role in evolution in general. When biota with empty niches are colonized, stabilizing selection relaxes from the special adaptations to the niche inhabited before by the invading species. Variability may arise in these and is permitted as long as fitness is guaranteed. Such processes characterize adaptive radiation. Examples are given by the species flocks on isolated islands or in chemically abnormal lakes like those of the East African Rift Valley. Only secondarily, on the basis of the arisen variability, does directional selection promote the newly developing species into different niches. The loss of stabilizing selection is an important factor for the evolutionary process to be open for evolutionary progress. |
