Convergent Evolution

Convergence is defined as an evolutionary event in which two morphological or molecular traits become similar during evolution due to a similarity in environment or selection pressure, even though these traits have independent ancestors or origins. In the case of amino acid or nucleotide sequences, there is little known evidence for convergence; the probability of two sequences becoming similar by convergence seems vanishingly small, and any similarities in sequence are attributed to divergent evolution. Yet even the absence of sequence similarity cannot be taken as evidence of convergence, as this will also result from extensive divergence between two sequences that originated from a common ancestor. In many such cases where proteins related functionally have no detectable sequence homology, their three-dimensional protein structures are very similar; protein structure appears to change more slowly than does the primary structure, and such cases seem to be further examples of divergent evolution.

There are, however, some examples of convergence that are apparent in the protein structure and the active sites of enzymes. For example, there are several kinds of serine proteinases. One family of these proteinases, represented by chymotrypsin, has very similar tertiary structures and active sites, containing a catalytic triad of serine, histidine, and aspartic acid residues. The serine proteinase subtilisin also contains an extremely similar triad structure, even though the rest of its tertiary structure and sequence is unrelated to the chymotrypsin family (Fig. 1) (1). Moreover, the three residues constituting the catalytic triad occur in different orders in the primary structures, making it virtually inconceivable that the two families of proteins arose from a common ancestor. Furthermore, serine carboxypeptidase II also contains a catalytic triad at its active site, even though its catalytic mechanism is entirely different. It is considered that the very similar triad structures of these different protein families emerged from their respective and independent ancestors by positive natural selection. This is one of the best characterized examples of convergent evolution.

Figure 1. Convergent evolution of the active site residues in subtilisin, chymotrypsin, and serine carboxypeptidase II. Al relative positions of active-site residues differ in the amino acid sequences among subtilisin, chymotrypsin, and serine carboxypeptidase II, they have a common catalytic triad consisting of Ser 221, His 64, and Asp 32 in subtilisin; of Ser 19 and Asp102 in chymotrypsin; and of Ser 146, His 397, and Asp 338 in serine carboxypeptidase II. Thus, they are consider examples of convergence or convergent evolution (2).

There are other examples of enzymes with the same functions but sufficient differences to make it clear that they have not arisen from the same ancestor, but that the same function has arisen independently by a type of convergent evolution (Table 1) (2).

Table 1. Some Enzymes that Have Evolved Independently on More Than One Occasion, by Convergent Evolution

References

1. D. Voet and J. G. Voet (1995) Biochemistry, 2nd ed., Wiley, New York.

2. A. Doolittle (1994) Trends. Biochem. Sci. 19, 15–18.