DNA-DNA rehybridization kinetics and delta melting temperature studies
don't compare base sequences base-by-base. They compare the "composite"
base sequences--they are moderately sensitive to base substitutions, i.e.,
they can detect differences of one base out of 20 or so in a long sequence.
The exact percentage mismatch will depend on the reaction conditions used.
Even when we do have all the DNA sequences, it won't be easy to make
definitive statements about sequence similarities. Insertions and
deletions will make it hard to identify all similarity.
Sibley and Alquist made a statement about humans and chimps back in the
early 80's based on the delta Tm data. They had compared the sequence
similarities between species of birds before. When they found that ape
and chimp DNA bound together (hybridized) more tightly than some sister
species of birds in the same genus, they made the statement that if we
were doing the taxonomy of humans now, we might conclude that chimps and
humans were sister species within the same genus.
Jim Behnke, Asbury College, Wilmore, KY 40390 jimbeh@ms.uky.edu