Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Most biologists tend to favor this alternative at the present time; nonetheless, there is so far no direct evidence for it, similarly as for the opposite alternative.The subsequent insertion of introns into the originally continuous gene is supported, e.g. by the fact that there are a number of types of introns that mutually differ quite fundamentally, in the mechanism of their splicing, from the RNA and mechanism of their horizontal proliferation in the genome (Belfort 1993).It is thus apparent that the individual types of introns originated independently and that thus at least some of them originated at a later stage in evolution.The distribution of introns in the phylogenetic tree is also indirect evidence for the relatively late insertion of introns into the originally continuous genes.It holds for a number of introns that, from the viewpoint of the maximum parsimony principle, insertion of an intron into the originally continuous gene is a more probable explanation of their distribution in the phylogenetic tree than repeated independent disappearance of the relevant introns from the genes in several different taxa (Logsdon 1998)[11092].On the other hand, it is apparent that, in the vast majority of eukaryotic branches, losses of introns predominate over their insertion.The opistoconta – bilateria evolutionary branch is an exception; here 48 losses and 1466 insertions of introns have been recorded [11830].So far, the best evidence for later insertion of introns into the originally continuous genes is provided by comparison of groups of homologous genes that branched off from a common ancestor in the far distant past (old paralogues).In contrast to the group of genes that branched off later, the introns occur here in various places, from which it can be derived that they were inserted into the individual paralogues later and independently.The authors of the study think that groups of originally continuous genes were “infected” at a certain moment by self-splicing introns of type II, from which present-day introns evolved over time.

            Hypotheses of subsequent insertion of introns into continuous genes can be basically divided into two types.According to hypotheses of the first group, introns are not useful, but organisms are not capable of getting rid of them (Hickey & Benkel 1986).According to the second group of hypotheses, introns have functional importance for organisms and are somehow useful for them (Colston & Davis 1994; Duret 2001).