On divergent and progressive evolution

Abstract

The idea already conceived by Lamarck, namely, that there are two kinds of evolution, which may be called ‘divergent’ and ‘progressive’ respectively, has been borne out by modern phylogenetic classification. With two kinds of evolution two different mechanisms are required. Basing the argument on the axiom of competitive exclusion it is concluded that the prerequisite for divergent evolution is isolation. Isolation may depend upon factors extrinsic to the organism themselves, in which case it is random; when it depends upon intrinsic factors it is non-random. Isolation may lead to taxonic divergence, that is, the origin of a new taxon, but it may also simply serve the purpose of taxonic survival, because it prevents the access of potential rivals. Taxonic divergence depends upon reproductive isolation. This may happen fortuitously when a few individuals become isolated spatially, in which case they may with the course of time form a new taxon. Non-random taxonic divergence may occur if reproductive isolation is established by chromosome re-arrangements and a variety of other factors. Random taxonic survival may occur as a consequence of comprehensive geological events, as exemplified by the marsupials in Australia. Non-random taxonic survival follows from ecological isolation, due to the possession of habits or properties which protect the organisms from competitors. It may be presumed that sooner or later taxonic survival leads to taxonic divergence. Progressive evolution is the result of the interspecific competition which takes place when two taxa happen to occupy the same niche. When this occurs, the axiom of competitive exclusion predicts that one of the taxa must succumb, and the other one, the dominant taxon, prevails. As this has occurred repeatedly in the course of time, the survival of ever more dominant forms has ensued, resulting in progressive evolution. It is shown that upon the adoption of certain simple criteria and a particular rule, the establishment of a phylogenetic classification may lead to the following three results: the polarity and the dominant phylogenetic lineage is established, and it becomes possible to test and to corroborate the ecological theory, which predicts that each nondominant taxon will be isolated, randomly or non-randomly, as the case may be. Since an unadapted organism cannot survive, the concept of ‘adaptation’ does not have the importance usually ascribed to it. Organisms do not survive because they are adapted, but because they are isolated.