Essay On Evolution

According to this view, even when lineages divide, the slight differences between the newly separate species will tend to grow steadily and gradually increase over time [Figure 1].Fossils tell a different story The fossil record indicates otherwise.And though it is difficult to measure small increments of time in the fossil record, my data seemed to suggest that speciation could be quite rapid—taking, perhaps, anywhere from five to 50,000 years.

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Interestingly, an early sample from upstate New York seemed to vary; some had 18 and some 17.

With one exception, all samples up and down the Appalachians had 17, while all the samples from the shallow limey seas that once covered Ohio, Michigan, and Iowa still had the ancestral 18 columns.

Having learned from another paleontologist that the lenses of the eyes of these kinds of trilobites are arranged into vertical columns holding anywhere from one to more than 15 large, bulging lenses, I counted the lenses on each specimen.

One day I suddenly realized that samples from different localities—different places in both time and space—differed mainly in the number of those vertical columns of lenses.

But what remained surprising were the long periods of time with little or no change at all.

The 17-column species, for example, seemed to have lasted unchanged for nearly 5 million years!

New species tend to appear in the fossil record already morphologically distinct from their closest relatives.

And once established, species tend not to change significantly or permanently, which in the case of marine invertebrates can amount to a 5-million-year or even 10-million-year history without change.

The birth of a new theory: speciation and punctuated equilibria In the late 1960s I was conducting my doctoral dissertation research on the evolution of a Middle Devonian trilobite called Phacops rana.

Along with many other species of trilobites, mollusks, brachiopods, corals, and other invertebrates, the abundant Phacops rana inhabited the shallow-water inland seas that ran from the present-day Appalachian mountains as far west as Iowa. rana, were derived from species that migrated into North America from Europe and Africa when those continents collided with North America about 380 million years ago.


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