Re: Alex's gibbon-like CA

Alex Duncan (
6 Nov 1995 15:01:58 GMT

In article <> Paul Crowley, writes:

>I'm sure this is the "evolution forces" error. A species faced with
>a shrinking habitat does not change its diet. It declines or dies.

Well, when do taxa change their diet then? (And BTW, I didn't suggest a
change in diet, but a change in the proportions of the dietary items
eaten. After all, human and chimp diets aren't that different, especially
in areas where ranges overlap.)

>We could look at squirrels as an analogy. They are nearly everywhere.
>They spend a lot of time on the ground and must sometimes, reluctantly,
>travel some distances. But nothing could ever "force" them to forget
>their life in the trees. If all trees became 200 metres apart, they
>would not lose their bushy tails, diurnal habits and climbing
>adaptions. They'd just go extinct.

"reluctantly"? Where'd you get that? I'd say some squirrel taxa are
cautious about going to ground, but I've also seen populations that don't
even think twice about it.

There is a lot of variation between squirrel taxa in how much time they
spend in the trees. I've observed at least one species that spends at
least as much time on the ground as in the trees (a lot of the trees
where it lives are separated by substantial distances).

I think you make way too much of your cute little "evolution forces" idea.

As far as squirrels go -- yes, maybe they'd go extinct if all the trees
were suddenly 200 meters apart from each other, but what if the trees
were suddenly just 1 meter further apart than they are now? And what if
a few years down the road they were another meter apart? It sounds to me
like you're telling me there is no way a squirrel could ever evolve into
a strictly terrestrial critter. Much of mammalian evolution seems to
have involved a transition from the trees to the ground, however.
Somehow mammals are still here.

I would also point out that squirrels are not a good analogy for this
particular point, as they are so small. A thinning forest might be a
pretty dramatic change for a squirrel, but not necessarily for a larger

As far as your thing about "shrinking habitat" (or changing habitat) and
extinction goes -- as far as I can see you've ruled out most of the
important evolutionary changes in the history of the earth.

>Can you give any example of any
>species adopting a form of behaviour for which it was apparently
>quite ill-adapted? A solid example, please - not a "maybe".

How about goats climbing trees? (I'm already anticipating your response
"oh yeah, well how about another one?")

How about fish climbing trees? Does that count?

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086