Évolution de l'achigan



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Scientists used a molecular clock to determine all 8 species of black bass share a common ancestor that lived 11 million years ago.  The molecular clock is a method scientists have of estimating the age of a species.  It’s especially helpful when studying the evolutionary history of organisms that are absent or rare in the fossil record.  It is based on the assumption that DNA  protein sequences evolve at a relatively constant rate.  Scientists assume a substitution rate of appearance of new mutations in each member of the population.  Scientists eventually learned the substitution rate varies among different kinds of organisms, so they calibrate the molecular clock with the fossil record and known geological events that may have caused species divergence.  This involves a lot of computer modeling and statistics, but they are confident that using a molecular clock is a reliable method of estimating evolutionary ages.

The oldest bass fossil ever found was unearthed in Texas and dated to 23 million BP.  Scientists estimate the Micropterus (black bass) genus originated about 26 million years ago, based on molecular clock data.  This suggests their findings are consistent with the fossil record.  Today, there are 8 species of bass that began to diverge from a common ancestor 11 million years ago when a marine transgression flooded most of the coastal plain in the southeast.  This dramatic rise in sea level isolated many populations of bass, resulting in allopatric speciation.  Scientists believe this is the most common type of speciation, and it occurs when founding populations become geographically isolated.


 

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