How a population bottleneck erases genetic variation
A population can rebound in numbers without regaining the genetic variety it lost. The key is that genes are passed on by survivors, not by the population that existed before the crash. If only a small group reproduces, its members carry just a chance-made sample of the earlier population’s gene variants, or alleles. Variants absent from that group cannot be passed on in later generations.
This loss happens through genetic drift: random changes in how common variants are from one generation to the next. During an ordinary generation in a large population, chance is less likely to wipe out a rare variant entirely because many individuals may carry it. After a sharp decline, the same variant may have only one or a few chances to be passed on. Whether it survives can hinge on which individuals reproduce and which offspring inherit it.
A numerical rebound cannot restore what vanished
As the population grows, surviving variants can become common, but growth alone does not recreate missing ones. New variation can arise through mutation, and later gene flow—the arrival of individuals from another population—can introduce additional variants. Both processes take time or depend on outside populations; neither automatically restores the original mix.
This is why a bottleneck can leave a lasting mark even when a species or population appears to recover. A larger head count may mean more individuals, but not the same range of inherited options. The lost variants could have been neutral, harmful, or useful under conditions that had not yet arrived. The bottleneck does not sort them by future value: if none of the survivors carried one, that possibility vanished with them.
