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Perhaps someone can enlighten us on how DNA is used to distinguish one 
species from another. I assume that not every difference in DNA makes two 
individuals belong to different species. For example, there must be some 
variations of DNA between recognizable subspecies -- i.e. if the larger, 
paler Ipswich Sparrow hatched on Sable Island did not have somewhat 
different DNA from a smaller, darker mainland Savannah Sparrow their 
appearances would identical. The old fashioned method of distinguishing 
between species was to ask how often they interbred in areas of sympatry. 
If we are to go by DNA alone, what amount (or what kind) of difference in 
DNA is now sufficient to separate species (and to predict that they will not 
interbreed)?

Do we now call the White-tailed Ptarmigan on every mountain top different 
species from those on every other mountain top because they have been 
isolated from each other for many generations? Or is there some way of using 
DNA to identify a genetic isolating mechanism, which would keep individuals 
of one population from breeding with individuals of another even population 
were they to occupy the same area?

Bob Fisher
Independence, Missouri
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