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26.6 Similar Molecular Evolution Can Take Place in Separate Lineages

Foregut-fermenting mammals such as the gray langur (a) have been evolving independently from the hoatzin (b) for more than 100 million years, but each has evolved similar modifications to the enzyme lysozyme.

(a) Presbytis entellus

26.6 Similar Molecular Evolution Can Take Place in Separate Lineages

Foregut-fermenting mammals such as the gray langur (a) have been evolving independently from the hoatzin (b) for more than 100 million years, but each has evolved similar modifications to the enzyme lysozyme.

(a) Presbytis entellus inferring phylogenies from data on single molecules can be very misleading. Identical molecular changes in this protein are not evidence of common descent.

of which lie on the surface of the lysozyme molecule, well away from the active site (see Chapter 6). Several of the shared substitutions involve changes from arginine to lysine, which makes the proteins more resistant to attack by the pancreatic enzyme trypsin. By understanding the functional significance of amino acid substitutions, molecular evolutionists can explain the observed changes in amino acid sequences in terms of changes in the functioning of the protein.

A large body of fossil, morphological, and physiological evidence shows that langurs and cows do not share a recent common ancestor. However, langur and ruminant lysozymes share many amino acid residues that neither animal shares with the lysozymes of their own closer relatives. The lysozymes of these two animals have converged on a similar sequence despite having very different ancestry; in other words, they are homoplasies. The amino acid residues they share give these lysozymes the ability to lyse the bacteria that ferment leaves in the foregut.

An even more remarkable story emerges if we look at lysozyme in the crop of the hoatzin, a leaf-eating South American cuckoo, the only known avian foregut fermenter (Figure 26.6b). Many birds have an enlarged esophageal chamber called a crop. Hoatzins have a crop that contains bacteria and acts as a fermenting chamber. Many of the amino acid changes that occurred in the adaptation of hoatzin crop lysozyme are identical to the changes that evolved in ruminants and langurs. Thus, even though the hoatzin and the foregut-fermenting mammals have been evolving independently from one another for more than 100 million years, they have each evolved a similar molecule that enables them to recover nutrients from their fermenting bacteria in a highly acidic environment. The lysozyme story also illustrates why

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