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Re: AAT reply from Elaine Morgan
Phillip Bigelow (n8010095@henson.cc.wwu.edu)
Fri, 6 Jan 1995 01:55:57 GMT
patdooley@aol.com (Pat Dooley) writes:
>You may have read the recent book about finch evolution in the Galapogos
>Islands. I think it is a safe bet to say that different finch species on
>different
>islands would have evolved similar bills in response to similar
>environmental
>pressures. Convergence at that level, took decades, not 2 millon years.
I know of no confirmed case of convergence between species taking only
decades to occur. I would be interested in the full reference to this
research. (I'm not claiming you're wrong; I just want the reference).
<pb>
snip...
>When different species occupy the same
>ecological niche, you get convergence. Both occur in similar time scales -
>ranging from decades to millions of years.
It's not quite as simple as that. A lot depends on your starting-point;
in other words, the basal stock that evolved into the species you are
comparing. For instance, the ecological niche for most birds and bats is
the same; yet, the two groups of animals converged only _slightly_. They
have an airfoil in common. How they _achieved_ the airfoil is vastly
different, and the airfoil structures are not homologous between the two
taxa. Otariids (sea lions) and cetaceans (whales and dolphins) share
similar ecological niches (the toothed whales, particularly), yet there is
only partial convergence between the two groups. In particular, the skulls
of the two groups appear to be in stasis, with few signs of convergence. On
modern otariids, the skull is short and the snout squared-off; in whales,
the snout is telescoped anteriorly. In otariids, the naris (the nose
opening in the skull) is positioned roughly in the terrestrial carnivore
fashion: at the front of the snout. In whales, the naris is positioned on
the top of the head.
The time it took for the whale-ancestor to modify the skull to vaguely resemble
modern-day whale skulls was on the order of 5-10 million years. Sea lions, on
the other hand, have aquatic ancestors that go back roughly 25 million
years. Yet, that still wasn't enough time for this group of animals to
develop skulls similar to whales. The _main_ convergences between sea lions
and whales is in the appendicular skeleton; yet, otariids stubbornly refuse
to loose their back limbs in 25 million years. So, the convergence was only
partial. Convergence has limitations. It happens intermitently in the
fossil record, and doesn't always occur when it is supposed to occur. It's
not a sure-thing.
<pb>
D
D
D
D
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and whales is in the well-developed flippers onvergence has limitations.
<pb>
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