On 2004-02-07 10:42:13 -0700, Willi said:

I used to accept your definition because it seemed to make the most sense
and seemed concrete and absolute. However, through reading and thinking
about it, I've come to the conclusion that it is as arbitrary as any other
definition. There are problems with any definition of a species. Any
definition of "species" is just an attempt to choose a specific point on a
continuum.

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I both agree and don't agree, Willi. Species are the principal units of
evolution. While there's been a long history of debate about the definition
(the "species problem" in biology), few if any serious biologists would
deny that species are objective, concrete phenomena. This greatly
constrains the possible definitions of "species."

There is a spectrum of opinion. On the far right, so to speak, are people
like Jon who insist on the most rigid and absolute definiton -- if two
organisms can produce fertile offspring then they belong to one species.
This leads to absurdities, like the lions and tigers example.

On the left are the nominalists who argue that "species" is an arbitrary,
man-made concept. That is, IMO, the looney deconstructionist wing, who
argue from a political agenda. I don't take them seriously.

The truth is in the middle. Species are "real," but pinning them down with
a simple definition is hard.

(Somewhere off in Cloud Cuckoo Land are those who claim that species are
the immutable creations of God and that evolution doesn't exist.)

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