75% rule for subspecies

Michael Gochfeld gochfeld at eohsi.rutgers.edu
Mon Apr 1 20:22:19 EST 2002

Kenelm Philip wrote inter alia:
       By the way, it was my understanding that, in olden times, a
subspecies was considered valid if around 75% of the individuals could
be distinguished from other subspecies. This means, conversely, that

> around 25% can _not_ be so distinguished. Do people these  days admit
> that--or is it now believed that any individual of ssp A can be reliably
> distinguished from any individual of ssp B? And if so, why is it a sub-
> species rather than a species (in ignorance of what happens when you
> breed them)?
>                                                         Ken Philip

I think that in the 1940's or early 1950's Dean Amadon (and perhaps others,
though solo authored papers were the rule) wrote an AMNH bulletin on the 75%
rule for subspecies.  I believe it had to do with mensural characters
primarily (or possibly only).  I haven't heard anyone refer to it in years.
In answer to Ken's last question, I think that the rule arose in the context
of a biological species concept where it was assumed that the forms in
question were not reproductively isolated (as if we knew that).

I believe also that it was assumed that where populations of a species had
been isolated by some physical barrier for a period of time, they would
diverge genetically and morphologically.  If upon secondary contact they
interbred (without evidence of selection against offspring), then they would
be considered subspecies.  In that case, subspecies might not be stable over
time as introgression gradually obliterated the differences, at least in a
band along the zone of contact.

It would be nice if things really worked that way, but it certainlyl had
great heuristic value for a generation of us.

Mike Gochfeld


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