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

Individual, frequently even closely related species of organisms can differ very substantially in the sizes of their genomes. The genetic complexity (C-value), i.e. put simply, the total amount of DNA recalculated to the haploid genome, differs more than 80,000-fold for eukaryotes, 5800-fold for protozoa, 250-fold for arthropods, 350-fold for fish, 5000-fold for algae and 1000-fold for angiosperm plants (Cavalier-Smith 1985; Petrov 2001). Such large differences cannot be caused by differences in the number of genes in the genomes of the particular species and are certainly not correlated much with the complexity of the individual organisms (Fig. VI.3). Consequently, this phenomenon is called the paradox of genetic complexity – the C-value paradox.

Fig. VI.3. The DNA content in the individual groups of organisms. The individual taxons of organisms differ substantially in the average DNA content in their nuclei. However, even within a single taxon, there are large differences between quite closely related species. For example, it is not clear why most plants have substantially more DNA than most animals, or why some species of fish and tailed amphibians have an order of magnitude more DNA than others. The horizontal axis of the graph is plotted on a logarithmic scale.

            A frequent explanation of the C-value paradox could consist in the tendency of certain species or groups of species towards (repeated) polyploidization of the genome or part thereof. Another quite probable explanation is that mutations of the insertion type predominate in the genomes of some species of organisms, while mutations of the deletion type predominate in the genomes of other organisms. This hypothesis has been tested by comparing the frequencies of the individual types of evolutionarily fixed mutations in the genomes of drosophila and in crickets of the Laupala genus (Petrov et al. 2000). The genome of crickets is approximately 50 times larger than that of drosophila. In agreement with the expectations following from the tested hypothesis, it was found that the mutations in drosophila contain a greater number of deletions and fewer insertions than those of crickets. Very marked differences have also been observed in the range of the relevant mutations; the average length of deletions in drosophila equals 24.9 nucleotides, while that in crickets equals 6.0 nucleotides. On the other hand, the length of insertions was larger for crickets. The results of this study did, of course, not demonstrate that the cause of the different sizes of the genomes lies in mutation bias. The initial data do not permit determination of whether the discovered differences in the sequences of the individual species of crickets and individual species of drosophila are caused by differences in the probability of the individual types of mutations or differences in the probability of evolution fixation of the individual types of mutations. In any case, the action of mutation bias remains a highly probable explanation of the existence of the complexity paradox. Alternative explanations of this phenomena are, however, provided by other, basically different hypotheses, some of which assume that noncoding DNA in the nucleus can have functional importance for the cell – e.g. it permits maintenance of a constant ratio between the size of the nucleus and the volume of cytoplasma (Beaton & Cavalier-Smith 1999).