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Is it possible, at least in theory, for a species to evolve into another species and then evolve back into the first species?

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It's possible, but very unlikely. Usually it takes a substantial number of genetic differences to qualify two organisms as different species. One might think that reversing the direction of selection pressure could reverse a genetic change, but genetic change begins with random mutations which are not determined by selection pressure. It's very unlikely for one random process to undo another random process (imagine mixing black and white sand in a jar by shaking it, then trying to unmix by unshaking!).

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While it doesn't go nearly to the extent of speciation, the classic example is the British peppered moth: https://en.wikipedia.org/wiki/Peppered_moth_evolution Originally it was mostly white with black spots, which made a good camoflage on tree trunks &c. During the industrial revolution, the "dark satanic mills" put out a lot of soot, turning the tree trunks dark. The whiter moths were easy to spot, and got eaten before reproducing, so within a few decades most of the moths in industrial areas were black.

Later on, emissions control laws drastically reduced the amount of soot, the tree trunks became light again, and the moths evolved back to white.

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