Evolution__3rd_Edition

186 PART 2 / Evolutionary Genetics
Bird
Convergence is evidence that
selection has operated
14 lysine
21 lysine
75 aspartic acid
87 asparagine
126 glutamic acid
14 lysine
21 aspartic acid
75 aspartic acid
87 asparagine
126 lysine
Cow
Other
ungulates
Other
primates
Langur
Figure 7.7
Convergent evolution of stomach lysozymes in langurs and
ruminants. In the evolutionary lineages leading to langurs
and cows, changes have occurred at the same five sites in the
lysozyme protein, and the changes have been similar or identical.
The numbers refer to amino acid sites in the protein.
7.8.4 The gene for lysozyme has evolved convergently in
cellulose-digesting mammals
Lysozyme is a widespread enzyme, used in defense against bacteria. The enzyme breaks
open the bacteria cell wall, causing lysis of the bacterial cell. Lysozyme is found in body
fluids such as saliva, blood serum, tears, and milk. In two mammalian groups, ruminants
(such as cows and sheep) and leaf-eating colobine monkeys (such as langurs),
a new version of lysozyme has evolved in addition. Both taxa use lysozme to digest
bacteria within their stomachs. The stomach bacteria themselves digest cellulose from
plants, and the cow or langur obtains nutrients from cellulose by, in turn, digesting
the bacteria.
Ruminants and colobine monkeys secrete lysozyme in their stomachs, which is a
more acid environment than found in normal body fluids. When the sequences of
stomach lysozymes in ruminants and colobines is compared with the sequence of
standard lysozyme, we see that several identical amino acid changes have occurred
independently in the two lineages (Figure 7.7). The amino acid changes allow the
lysozyme to work better in acid environments, as well as providing other advantages.
The lysozymes of ruminants and colobine monkeys are a molecular example of
convergent evolution (Section 15.3, p. 429). Convergence is usually due to adaptation
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