Divergent Evolution

Divergence is defined as an evolutionary event in which two morphological or molecular traits arose from a common ancestor, and were initially identical, but became dissimilar during evolution. Divergence is, of course, extremely common and the basis for most evolution; without divergence, all lineages would remain the same.

Divergent evolution is usually apparent at the molecular level from significant similarities between the amino acid sequences of proteins and the nucleotide sequences of DNA or RNA. There are so many such sequences possible that it seems most improbable that two sequences could become similar by chance. Similar sequences are said to be homologous, and the differences between them are assumed to be related to the time since they diverged from their last common ancestor. The molecular mechanisms of evolutionary divergence of the genetic material include nucleotide substitution and deletion/insertion; chromosomal recombination, transposition, and inversion; gene duplication, and gene conversion; exon shuffling, and domain shuffling; and horizontal gene transfer. The number of nucleotide substitutions is a simple and useful measure of the degree of divergence between two sequences. In fact, there are a dozen methods available for estimating the number of nucleotide substitutions, using one to six parameters (1-5). Once the number of nucleotide substitutions has been estimated, a phylogenetic tree can be constructed by using those numbers, to display the pathway that the divergence has followed during evolution (6).

 Divergence is contrasted with convergent evolution, which is much more rare. Convergence is an evolutionary event in which two morphological or molecular traits become similar during evolution, even though the ancestor is totally different. In the case of divergence, there must be a certain degree of similarity between two traits to suggest that they had a common ancestor. For convergence, on the other hand, a certain degree of dissimilarity must exist if the two traits originated from independent ancestors. Therefore, the distinction between divergence and convergence can be difficult to judge unless additional evolutionary information is available to indicate whether the similarities or the differences are the more significant. In the case of nucleotide or amino acid sequences, any similarities greater than expected from random are usually taken to indicate divergence. For example, the divergence of nucleotide sequences between human and chicken is shown in Figure 1, where the asterisk (*) indicates a site where nucleotide differences exist, and where 20 nucleotide sites are different and the remaining sites are the same. The probability that this similarity occurred by mere chance is extremely small. Therefore, it can be reasonably concluded that the difference in nucleotide sequences between these two species is due to divergence. Thus, it follows that divergence from the common ancestor has taken place as a result of the accumulation of nucleotide substitutions. 

Figure 1. Divergence of nucleotide sequences between humans and chickens.

During the process of molecular evolution, the number of nucleotide differences increases but probably never decreases. Therefore, in most cases, the evolutionary event can be explained by divergence only. Considering the tertiary structures of proteins, however, in some cases the structures are quite similar, even though their amino acid sequences, as well as their biological function, are very different. For these cases, convergence is a possibility, although it could also be simply a case of extreme divergence masking their original sequence and functional identity.

References

1. T. H. Jukes and C. R. Cantor (1969) in Mammalian Protein Metabolism, H. N. Munro, ed., Academic Press, New York, pp. 21–132. 

2. M. Kimura and T. Ohta (1972) J. Mol. Evol. 2, 87–90. 

3. M. Kimura (1980) J. Mol. Evol. 16, 111–120. 

4. N. Takahata and M. Kimura (1981) Genetics 98, 641–657. 

5. T. Gojobori, K. Ishii, and M. Nei (1982) J. Mol. Evol. 18, 414–423. 

6. M. Nei (1987) Molecular Evolutionary Genetics, Columbia Univ. Press, New York.