Essay On Evolution

Essay On Evolution-35
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.

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.

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North America lay astride the Equator in those days, and its shallow tropical seas literally teemed with life that has left a rich and dense fossil record.

The fauna persisted for some 6 million to 8 million years.

The white circles indicate some of the more important localities where the trilobite, Phacops rana, were found by Dr. © AMNH Two million years later, those Midwestern shallow seas dried up.

When marine conditions were once again restored, the 18-column species of Phacops was gone.

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.

Later, the daughter species spread, and in the case of Phacops, at least, eventually replaced the ancestral species.I was delighted to see that a clear pattern emerged, one that reflected NOT gradual change through time, but geographic speciation.Like their European/African ancestors, the earliest samples, from the eastern part of the range, had 18 columns of lenses in the eye.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.I wrote these results up in a paper in 1971 called "The allopatric model and phylogeny in Paleozoic invertebrates," concluding that speciation has played a critically important role in evolution since the dawn of time.It was beginning to look like evolution is mostly correlated with speciation events. In the following year (1972) I published a longer paper with Stephen Jay Gould, with whom I had worked in graduate school.© AMNH Is evolutionary change mostly dependent upon speciation, which occurs when one or more descendant species split from an ancestral one?Do most anatomical changes occur during these events? And how long does it take the process of speciation to work in geological time?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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