Population bottleneck
A sharp crash in a population's numbers that leaves the survivors with only a fraction of the original genetic variety.
A population bottleneck is a sudden, severe drop in the number of individuals in a population, caused by events such as disease, drought, hunting or loss of habitat. Even if numbers later recover, the descendants come from only the few survivors, so they carry only part of the original range of genes.
That loss matters. With fewer gene variants a population has less raw material to adapt to new diseases or changing conditions. In small groups, chance alone, called genetic drift, can wipe out more variants, inbreeding becomes more likely and harmful mutations can build up. The northern elephant seal was hunted down to about 30 animals in the 1890s; it now numbers in the hundreds of thousands, but with very little genetic variety. American bison fell from tens of millions to about 750 in 1890.
Very small populations can be caught in an extinction vortex, in which low numbers lead to inbreeding and bad luck, which drive numbers lower still, until the species disappears. A plant or animal with a narrow range, harvested faster than it can replace itself, is especially at risk.
A bottleneck can also hide behind apparent abundance. A species may depend, at hard times, on a small refugium, a pocket of suitable habitat where it survives until conditions improve. If that refuge is destroyed, even a species that once seemed endlessly plentiful can vanish surprisingly fast.
Coming up in future stories
- The 3.5 Trillion Locusts That Vanished 1875 · Great Plains, USA
- The Plant Rome Ate to Extinction 1st century AD · Cyrene, Libya