Evolution__3rd_Edition

..
. . . and levels of polymorphism, ...
. . . or nucleotide diversity
CHAPTER 7 / Natural Selection and Random Drift 161
Table 7.1
Rates of evolution for amino acid changes in proteins, and for nucleotide changes in DNA.
Rates are expressed as inferred number of changes per 10 9 years for an average amino acid site
in the protein, or an average nucleotide site in the gene. Calculated using data in Li (1997).
Rate of amino acid Rate of nucleotide
Gene evolution evolution
Albumin 0.92 6.08
a-globin 0.56 4.92
b-globin 0.78 3.36
Immunoglobin V H 1.1 5.87
Parathyroid hormone 1.0 4.57
Relaxin 2.59 8.98
Ribosomal S14 protein 0.02 2.18
Average
(45 proteins and genes) 0.74 4.25
proteins evolve at different rates. Here we are just looking at the approximate figures.
An approximate, memorable figure suggested by Table 7.1 is that amino acids are
substituted at a rate of a bit less than one per billion years at each amino acid site in a
protein.
Another important figure is for the amount of genetic variation within a species at a
particular time. The amount of variation can be described by two main indexes. One is
the chance that two randomly drawn alleles differ at an average locus, or heterozygosity
(H, see Box 6.3, p. 149); we previously met H as a property of one locus. H can also be
measured for a number of loci, and then expressed as an average for all of them. The
other measure is the percentage of polymorphic loci. If, say, 20 loci are studied by gel
electrophoresis, and 16 show no variation and four have more than one band on the
gel, then the percent polymorphism would be 4/20 × 100 = 20%. Gel electrophoretic
evidence suggests that about 10–20% of loci are polymorphic in species in nature
(Table 7.2).
Genetic variation has been measured at the DNA level in fewer species, because
it requires sequencing a stretch of DNA in many individuals within one species.
DNA diversity within a species is expressed as the “nucleotide diversity” (π), which is
mathematically equivalent to heterozygosity. In humans, is about 0.001. Thus, two
randomly picked human DNA molecules (including two within any one human body)
differ at about one in a 1,000 sites. Human DNA may be less diverse than that of many
other species (Box 13.2, p. 365). Drosophila DNA has a nucleotide diversity almost
10 times higher than human DNA.
Kimura (1968, 1983) thought that the rate of molecular evolution, and the amount
of molecular variation, was too high for a process driven by natural selection. His arguments
are now mainly of historic importance and are outlined in Box 7.1.