CONSERVATION OF ARABIAN GAZELLES - Nwrc.gov.sa

It appears that protein electrophoresis is still a powerful technique for taxonomy. For
example Nei (1987) showed that the resolving power of mitochondrial DNA, judged on the number
of nucleotides assayed by both techniques, is not necessarily higher: 33% of nucleotide substitutions
result in amino acid changes, and of these, 25% are detectable by electrophoresis, a total of 8.3 %
overall. There are 400 amino acids in an average protein (1 ,200 nucleotides in the gene), therefore
electrophoresis will survey about 1,200 x 0.083 ~ 100 nucleotides per locus. If 30 loci are examined,
protein electrophoresis is equivalent to studying 3,000 nucleotides, which is larger than the number of
nucleotides sequenced in many mitochondrial DNA studies.
DNA methods
Animal cells contain two types of deoxyribonucleic acid (DNA), and both may be used for taxonomic
purposes: 99.9% of the cell's DNA is found in the nucleus and the rest is found in the mitochondria
(mtDNA). The latter evolves (i.e. accumulates mutations) at a faster rate than nuclear DNA: about
five to ten times faster in mammals. This, coupled with its maternal, haploid mode of inheritance,
small size (it is typically 16,000 - 17,000 base pairs in length, compared to some three billion base
pairs in nuclear DNA), and ease of preparation make mtDNA very useful for addressing problems in
systematics and population genetics, especially in closely related animal groups such as the gazelles.
Mutations accumulate in a stochastic (random) manner. This is an essential concept for
understanding evolutionary processes and an illustration is instructive: assume that stochastic events
occurring at a rate of five per minute are observed and watched for three one minute periods (it could
be radioactive decay in a sample, or cars passing a point on the road). Although on average about
five events per minute are observed. a measure over only one minute can give a very inaccurate
estimate, for example 4, 2, or 8 events per minute (Figure 6.1). However, if the same process is
observed over three ten minute periods, figures such as 41 , 58, and 52, i.e. estimated rates of 4.1, 5.8,
and 5.2 events per minute respectively may be obtained, which is a marked improvement in accuracy.
As can be seen, the stochastic error is inversely proportional to the number of events measured.
1 II
2
3 III I I I
Figure 6.1 Graphic representation of three sequences of stochastic events occurring at a rate of five per minute.
There are many places in the DNA where mutations do not alter the way in which it is
expressed. The mutations measured by DNA techniques are therefore all essentially phenotypically
neutral (Kimura, 1983) since the phenotypically deleterious are eliminated by selection, and the
advantageous are rare. This gives them the useful property (unlike morphological characters) of
being uncoupled from selective pressures. Thus measurements of divergence between the DNA of
any two taxa gives a useful estimate of the time since they diverged from their common ancestor,
provided enough mutational events are recorded to reduce the stochastic error to an acceptably low
value. This will be on a relative time scale unless appropriate calibration of the mutation rate (the
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