Mayr on "What is a Species"

Richard Pyle deepreef at BISHOPMUSEUM.ORG
Tue Apr 27 00:19:07 CDT 2004

> >Geographically-defined populations are, in a way, potentially
> >much less genetically cohesive than units we refer to as "species",
> I can't follow you here,

I knew what I meant when I wrote the above, but it didn't come out in a
coherent way.  Here is what I was thinking:

If we define "introgression" as an occurence of gene-merging (i.e., sexual
reproductive event) between two individuals that share a significantly more
distant evolutionary relationship than the "average" or "typical"
evolutionary "distance" between any two reproducing individuals within a set
of individuals -- then it could be argued that "populations" may be less
genetically cohesive than "species", in that populations are subject to
higher levels of (proportional) introgression.

Put another way, the ratio of [gene flow *between* two different
populations]:[gene flow *within* either population]; is probably higher than
the ratio of [gene flow *between* two different species]:[gene flow *within*
either species].  Viewed as a defined set of individuals, gene flow between
two different "species" would generally be proportionally much rarer than
gene flow between two different populations of the same species.  Thus, the
boundary between two "species" sets of individuals is likely to be more
identifiable/consistent/"discrete" than the boundary between two different
populations of what we would think of as the same species.  And if there is
any "Natural" reality to a species boundary; I reckon this would be it --
some sort of threshold beyond which gene flow across purported species
boundaries goes from being moderately rare, to extremely rare (a
discontinuity in the otherwise smooth transition; as I tried to explain in
an earlier post, many moons ago).

So...this is what led me to suggest that "populations" can be thought of as
"less genetically cohesive units" than "species".

But this defines "cohesion" in a proportional sense, rather than an absolute
sense.  A *ratio*; rather than absolute gene flow.

> >It seems we differ only in how we scope a "population".  If I understand
> >your definition/scope, I would tend to use words like "school", "herd",
> >"flock", "colony", etc. to circumscribe a set of directly-interacting
> >individuals.
> You understand quite well. But I learnt with my professor of population
> ecology that "a population is what you decide to define as a population
> given the needs of your ongoing research". Classification again, allbeit
> based on biologically relevant criteria. Populations are
> frequently defined
> as loose systems, the loosiest being "mere geographic proximity".

So, then, I'm not sure I understand why "populations" can be thought of more
as "individuals" than "species"; unless you define "populations" as
exclusive of the "dead ones".

> >   The word "population", to me, transcends the direct
> >interactions among simultaneously extant individuals, and neccessarily
> >includes the dead ones (and yet-to-be-born ones).
> Under this definition, it's clearly a class ("taxonomic", Ron would say),
> not a real biologically cohesive whole.

Then it seems we differ only in our implied meaning of the word
"population"; we seem in full agreement on the broader conceptual issues.


Richard L. Pyle, PhD
Ichthyology, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at

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