From: Jonathan B. Marder (jonathan.marder@newmail.net)
Date: Thu May 01 2003 - 16:47:20 BST
Hi Johnny, Platt, Phyllis, Paul, Steve and all,
JONATHAN (before)
>Biological evolution proceeds in small increments. It is humans who decide
>when the accumulated increments add up to a species difference.
JOHNNY
>I thought it had something to do with not being able to create fertile
>offspring. If they can create fertile offspring, they are the same
species,
>if they can't, they are different species. Are there exceptions to this
>rule?
JONATHAN replies.
This rule of thumb is partly right, but (as you will see soon), completely
backwards.
Individuals within the same species tend to show the high breeding
compatibility (though certain pairs may be incompatible). However,
interspecific fertility is not at all unusual, e.g. in the plant kingdom,
particularly among closely-related species. To put it generally, the ability
to produce fertile offspring declines as the genetic difference between male
and female increases. This is not a black-white issue, but a sliding scale
breeding compatibility.
The fact that two population groups are compatible means that biologists are
more likely to consider them as the same species. This is almost what Johnny
said, except "cause" and "effect" are reversed.
The fact that interspecific breeding is more common among plants than
animals says more about biologists than about either plants or animals.
BTW, this has profound implications for ALL classifications, including those
within the MoQ!
Jonathan
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