--John
EVOLUTIONARY BIOLOGY
Microbes Hint at a Mechanism Behind Punctuated Evolution
A soldier's life is said to be filled with long periods of boredom =
punctuated by short periods of terror, and some researchers have long =
suspected that evolution proceeds the same way, with long periods of =
stasis and brief episodes of dramatic change. Results on page 1802 =
support that idea, showing that for thousands of generations, bacteria =
evolved in spurts toward larger sizes. Researchers suggest a simple =
explanation: that the lack of beneficial mutations kept the cells in =
stasis until such a rare mutation appeared. But paleontologists caution =
that these results from a single, asexual population may not apply to =
the sweeping forces that cause new species to arise.=20
Punctuated Evolution Caused by Selection of Rare Beneficial Mutations=20
Santiago F. Elena, * Vaughn S. Cooper, Richard E. Lenski=20
For more than two decades there has been intense debate over the =
hypothesis that most morphological evolution occurs during relatively =
brief episodes of rapid change that punctuate much longer periods of =
stasis. A clear and unambiguous case of punctuated evolution is =
presented for cell size in a population of Escherichia coli evolving for =
3000=A0generations in a constant environment. The punctuation is caused =
by natural selection as rare, beneficial mutations sweep successively =
through the population. This experiment shows that the most elementary =
processes in population genetics can give rise to punctuated =
evolutionary dynamics.=20
Center for Microbial Ecology, Michigan State University, East Lansing, =
MI 48824,=A0USA.=20
* To whom correspondence should be addressed. E-mail: =
selena@ant.css.msu.edu=20
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Volume 272, Number 5269, Issue of 21 June 1996, pp. 1802-1804
=A91996 by The American Association for the Advancement of Science.