>> NILES ELDREDGE: I thought maybe I’d have a better chance at spotting evolutionary change
if I started working on trilobites.
People don’t seem to realize that, particularly in the east coast of the United States—but
basically all over the world—the ancient seaways used to flood the interiors, leaving
a huge, dense, fossil record with millions of specimens.
Not single specimens.
And they preserve rather well.
The trilobite that was the most common, easily- collected species was a thing called
Phacops rana.
Rana means frog in Greek.
And it’s because the big eyes on the heads of these things
give it a sort of frog-like appearance.
I found out very quickly that they occur all the way across New York State, all the way
down the Appalachians and through southern Ontario, into Michigan, into Ohio,
even into Iowa.
The thing that started bothering me—I was finding that I couldn’t tell the difference
between the specimens that I was getting in New York from the ones in Michigan.
I remember once in Alpena, Michigan we were in a laundromat washing our dirty field clothes,
and I took out a gorgeous specimen that was rolled up in my pocket.
And I just said, “[sighs] This thing looks exactly like the stuff in New York.”
Turns out it was a different species entirely.
It wasn’t even Phacops rana. It was Phacops iowensis.
I did get this sinking feeling that there was not a lot of change going on, either geographically
or even through time.
A guy—his name was Euan Clarkson—was studying related kinds of trilobites
from the Silurian of England.
And he was looking at those eyes, and the lenses are very easily seen on these things,
even to our naked eye.
They’re not like a fly’s eyes where the lenses are all tiny.
These things were bulging separate lenses.
They’re actually considered to be separate eyes like an aggregate eye.
Not a compound eye.
And he noticed that the lenses were arranged in vertical rows.
And so he counted them row by row.
So, I thought I would tabulate that data to the extent that it was present on each of
my specimens.
Then I had another kind of a eureka, eventually.
I saw that specimens from some localities typically had 17 columns of lenses.
But some other ones like,
for instance the famous ones from the Silica Shale in Ohio, had 18.
So I started plotting the numbers on maps.
And it soon became stunningly obvious that the 17 column of lens Phacops rana—was in
New York and all the way down the Appalachians.
But in the Midwest, there was something different.
Very similar, but it had 18 columns of lenses.
And then I found out what was really going on.
The 18-dorsal-ventral-file form, the one that ended up in the Midwest and surviving, was
also present in New York for a very brief time, and in geographic isolation
it evolved quickly into the descendant species with 17 columns of lenses.
So, for two or three million years you had, in the Midwest slightly different species,
with 18 columns of lenses in it.
Whereas, in the eastern muddier environments, you had the 17 one.
What happened was, the seas disappeared for a period of time in the American Midwest,
evidently driving to extinction that earlier form.
What really was going on is that 17 column lens, when it showed up after the 18 column
lens ceased to exist —it was not an evolutionary change.
It was a migration when the seas came back.
That’s what punctuated equilibria is.
It’s a combination of two things.
It’s the combination of two things.
It’s a combination of the notion of geographic isolation as being important in evolution,
and the realization that once a new species appears, if it survives,
it tends to do solargely unchanged.
There might be some variation within populations and between populations geographically,
but that species really tends to hang on for—in the case of marine invertebrates—
five to ten million years.