Evolution__3rd_Edition

176 PART 2 / Evolutionary Genetics
Hemoglobin is another example
...
. . . as is synonymous and nonsynonymous
evolution ...
. . . and pseudogenes
Table 7.5
Rates of evolution in the surface and heme pocket parts of the hemoglobin molecules. Rates are
expressed as number of amino acid changes per 10 9 years. Reprinted, by permission of the
publisher, from Kimura (1983). © 1983 Cambridge University Press.
Region a-hemoglobin b-hemoglobin
Surface 1.35 2.73
Heme pocket 0.165 0.236
generalization is that “house-keeping” genes, which control the basic metabolic processes
of the cell, evolve slowly. The ribosomal protein, for instance, performs much the
same function in the ribosome in almost all life forms. It evolves slowly. Other genes,
such as the globins and immunoglobulins, have more specialized functions and only
operate in specific cell types. They evolve more rapidly. The pattern is less clear-cut
than the pattern we have just seen within a gene for insulin and for hemoglobin.
However, the evidence does suggest that the degree of functional constraint is related to
the rate of evolution for a large class of genes. A basic house-keeping gene may be more
difficult to change during evolution than a gene with a more localized function.
The same relationships between functional constraint and evolutionary rate have
been found for DNA as well as for proteins. Two properties of DNA sequences are particularly
interesting: the relative rates of synonymous and non-synonymous changes in
the DNA, and the evolutionary rate of pseudogenes.
Synonymous base changes, which do not alter the amino acid, should be less
constrained than non-synonymous changes. Kimura had predicted, before DNA
sequences were available, that synonymous changes would occur at a higher rate. He
was right: evolution in fact runs at about five times the rate in synonymous, as in
non-synonymous, sites (Table 7.6).
A pseudogene is a region of a DNA molecule that clearly resembles the sequence of a
known gene, but differs from it in some crucial respect and probably has no function.
Some pseudogenes, for example, cannot be transcribed, because they lack promotors
and introns. (Promotors and introns are sequences that are needed for transcription,
but are removed from the mRNA before it is translated into a protein. The pseudogene
may have originated by reverse transcription of processed mRNA into the DNA.)
Pseudogenes, once formed, are probably under little or no constraint and mutations
will accumulate by neutral drift at the rate at which they arise. They will show pure
neutral evolution in the “pan-neutral” (see Figure 7.1) sense that all mutations are
neutral. The neutral theory predicts that pseudogenes should evolve rapidly. And they
do a they evolve even more rapidly than synonymous sites in functional genes. The
average rate of evolution at synonymous sites in Table 7.6 is 3.5 changes per 10 9 years. A
comparable set of pseudogenes has evolved at about 3.9 changes per 10 9 years (Li 1997).
A number of studies have shown the rate of pseudogene evolution to be about the same
as, or somewhat higher than, the rate of synonymous evolution. (Box 7.3 describes how
the rate of pseudogene evolution can be used to infer the total mutation rate in DNA.)
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