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THEORY
UPC
Codes
The Universal
Product Code system developed by the industrial sector
to brand retail items employs 10 options at each of 11
positions to create 100 billion alternates.
DNA Codes
Just like UPC barcodes, the
DNA sequences within each species are unique. A run of
15 nucleotides, with 4 options at each position, creates
the possibility of 1 billion codes, a hundred-fold excess
over the estimated number of animal species. Of course,
specific nucleotides are fixed at some positions by selection.
However, this constraint can be overcome by focusing on
protein-coding genes, where every third position is generally
free to vary because of the degeneracy of the genetic
code. As a result, by examining a stretch of 45 nucleotides
in these genes, one has the prospect of close to 1 billion
alternates.
DNA
Analysis
In practice,
there is no need to constrain analysis to such short stretches
of DNA, because sequence information is easily obtained
for DNA fragments 10 times as large. This ability to inspect
longer sequence arrays is desirable because the likelihood
of detecting diagnostic differences between species rises
with the number of nucleotide positions examined. Moreover,
since the incidence of diagnostic characters depends upon
species age and rates of evolution, there is no simple
prescription as to the number of nucleotides that must
be examined to ensure species recognition. However, given
a modest rate (2% per million years) of sequence change,
one expects to discover 12 diagnostic differences in a
600bp comparison between species with a million year history
of reproductive isolation. It is certain that most species
possess much longer histories of evolutionary independence
than this. In fact, it is true that even the most closely
allied species, those belonging to a single genus, usually
have longer histories of reproductive isolation than this.
As a result, it follows that the sequence analysis of
a 600 bp segment of the genome will permit the reliable
diagnosis of most species.
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