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[[Microevolution]] refers to small-scale changes in gene-frequencies in a population over a few generations ([[population genetics]] is the branch of biology that provides the mathematical structure for the study of the process of microevolution). These changes may be due to a number of processes: [[mutation]], [[gene flow]], [[genetic drift]], as well as [[natural selection]]. [[Macroevolution]] refers to large-scale changes in gene-frequencies in a population over a long period of time (and may culminate in the evolution of new [[species]]). The difference between the two is hard to distinguish because, over time, successive tiny mutations like those evidenced in microevolution could build up in isolated populations and eventually create entirely new species, which is known as macroevolution. While microevolution has been demostrated in the laboratory to the satisfaction of most observers, macroevolution has to be inferred from the [[fossil record]], and its precise mechanisms are an active topic of discussion amongst scientists. Some critics hold that while microevolution may occur with an existing gene pool, macroevolution requires the introduction of newly-evolved genes. These newly-evolved genes would represent beneficial mutations; it has been argued that such mutations are observed to be almost always detrimental. The recent discovery of extensive genome wide [[gene duplication]] in many organisms, however, allows for the preservation of an existing functional gene copy leaving other gene copies free to accumulate mutations, some of which may be beneficial.
* Why did the major groups of animals suddenly appear in the [[fossil record]] (known as the [[Cambrian]] Explosion)?
* Similarly, why are there [[missing link]]s in the fossil record, and a scarcity of fossils for [[transitional species]]?
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