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!
But what remained surprising were the long periods of time with little or no change at all.Tags: How To Write A Thesis Statement For A Persuasive Research PaperMilitary Communication EssayCreative Writing EssaysThe Basic Structure Of A Personal Essay Includes In OrderHow To Write A Written EssayExample Term Paper
Then it, too, spread westward, and the 17-column form became extinct. " I said to myself: This is a case of allopatric speciation.
Lineages are splitting into descendant species, with the ancestral species persisting alongside the new, daughter species.
Time In this example, a snail species, A, is isolated by a geographic barrier to form subspecies X and Y.
Eventually, this geographic separation leads to genetic isolation, and two distinct new species, B and C, form.
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.I had been trained to believe that evolution proceeds gradually, as depicted in Figure 1.Collecting all over the Midwest and up and down the Appalachian mountain chain over several summers, I was dismayed to detect no obvious evolutionary change across 6 million years and thousands of miles.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.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.Babies would add columns, then stop at the "mature" number: Some stopped at 18; many stopped at 17; and a few stopped at 15.Then I had an another idea: I simply plotted these numbers on a sequence of maps of the United States, moving from the earliest samples to progressively younger ones.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.