Punctuated equilibrium theory
Darwin repeatedly emphasized that evolution (i.e. anagenesis) is basically gradualist, and progresses alternately at greater and lesser rates through the accumulation of minor evolutionary changes. On the basis of these theoretical principles, evolutionary biologists long assumed that most evolutionary changes could occur at any time during the existence of the species, i.e. that anagenesis should not be bound to cladogenesis within the individual phylogenetic lines. Where it seems on the basis of the paleontological record that anagenetic changes occur in sudden jumps, that a new species with fully developed traits appeared instantaneously in the record and that there were no transition forms between it and its predecessor, then this is only a consequence of the imperfectness of the paleontological record, in which the transition forms were not preserved from the key period of the greatest anagenetic changes. It was only in 1972 that N. Eldredge and S. J. Gould pointed out that the absence of transition forms between the individual species is a practically universal phenomenon that cannot be explained by imperfectness of the paleontological record and lack of data (Eldredge & Gould 1972). They contrasted the gradualist concept of evolution with the nongradualist – punctualist concept (Fig. XXVI.8) and demonstrated that the available paleontological data tends to confirm that evolution occurs in the vast majority of cases in a punctualist manner, i.e. the species was formed very rapidly, acquired its characteristic phenotype and then practically did not change throughout the rest of its existence. Like every new theory, this theory of discontinuous progress of anagenesis, mostly termed the punctuated equilibrium theory, encountered considerable skepticism amongst evolutionary biologists and paleontologists. Data was repeatedly collected to test the theory. At the present time, it seems that, in cases where sufficient data is available, these data tend mostly to correspond to the punctualist model. New species appear in the paleontological record completely developed during a period of the order of 10,000 years and then do not change throughout their existence, which is usually 100 – 1000 times longer. They either force out the parent species or coexist for a long time with them until their extinction. The greatest amount of empirical evidence for the punctuated character of evolution is available for marine invertebrates as the best (most numerous and most complete) paleontological data is available for them; this is also confirmed by the available data relating to marine and terrestrial vertebrates (Benton & Pearson 2001). In contrast, the available data related to Foraminifera tend to support the gradualist character of their evolution.