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A lot of animals have had severe bottlenecks like that in the past. Cheetahs, for example, have so very little genetic diversity that they're all essentially co

A lot of animals have had severe bottlenecks like that in the past. Cheetahs, for example, have so very little genetic diversity that they're all essentially cousins; cheetahs will accept skin grafts from any other cheetah with no fear of rejection. Humans have also had a pretty extreme bottleneck in our history, with estimates that there were no more than about a thousand alive at one point, which is why we don't have the diversity many other animals have. If we manage the remaining population carefully, even a small population can rebound with few fears of later genetic problems.

saalweachter 12y ago
Nitpick: a historical population bottleneck does not necessarily mean that only that many individuals were left alive at a single time; it only means that that many individuals from a single time successfully contributed to today's population. Simple example: imagine you have two groups of humans, A and B, living in two neighboring valleys. Group A consists of 1000 individuals, group B a million. Over the next several hundred or thousand years, group A thrives and slowly increases to a million people. Meanwhile, group B declines to extinction at an even pace. At no time were there fewer than a million people, but there was a population bottleneck of 1000 people.
- beachstartup 12y ago
  so then it would be more accurately described as a procreation or reproduction bottleneck, not a population bottleneck.
- Dylan16807 12y ago
  In that scenario you could make a pretty good argument that group B were not modern humans but instead a dead race. (Assuming they had all these unique genes that disappeared.) I'm not sure the nitpick makes things more accurate.
  - duaneb 12y ago
    > were not modern humans but instead a dead race. "Race" is not really a recognized concept outside of social interaction. I believe "sub-species" is preferred on the level that you're indicating—think neanderthals and denisovans. Even then, it's a measure of phylogenetic distance, not of any quality that directly "matters" (like fertility of offspring). Anyway, they're all humans in terms of having active sex with humans.
    - Dylan16807 12y ago
      But all the ones they mated with died out. Unless you mean they were capable of but never did?
      - duaneb 12y ago
        There are humans all over the world with neanderthal and denisovan ancestry. They may have gone extinct, but their relatives definitely live on.
        Dylan16807 12y ago
        I don't understand where you're going with this in relation to saalweachter's imaginary groups A and B. Neanderthal genes have nothing to do with population bottlenecks.
  - saalweachter 12y ago
    If you want to get more complex, you could assume that group A was a colony from group B and that they were a genetic subset of group B. It's also possible that group A and group B were still economically linked, with frequent trade between the two but no intermarriage. It's also possible that group A and group B were never at any time distinct groups, and the individuals who 'won' genetically were scattered through time and space among a larger population. Imagine a rare mutation that confers an immunity to a terrible disease which periodically ravages the population. It's not that the plague kills off all but 1000 people, it's that over centuries of large numbers of people being killed off, those descending from the few people with the mutation (and their mates) come to dominate the gene pool, even if there were always millions of survivors. The reason for the nitpick is that the idea of a human population bottleneck is almost-always interpreted as "humans almost went extinct!". Maybe that happens to be the case, but it isn't necessarily so.
- wkimeria 12y ago
  If I remember correctly, at one point there were only 1000 human breeding pairs on the planet (due to the Toba eruption)
j-b 12y ago
Do you have a reference for that estimate? Would love to read more.
- Sanddancer 12y ago
  The Toba Catastrophe Theory [1] is one of the frontrunners as to why humans have such little genetic diversity in modern humans. [1] http://en.wikipedia.org/wiki/Toba_catastrophe_theory#Genetic_bottlenecks_in_humans http://en.wikipedia.org/wiki/Toba_catastrophe_theory#Genetic...

saalweachter 12y ago

Nitpick: a historical population bottleneck does not necessarily mean that only that many individuals were left alive at a single time; it only means that that many individuals from a single time successfully contributed to today's population. Simple example: imagine you have two groups of humans, A and B, living in two neighboring valleys. Group A consists of 1000 individuals, group B a million. Over the next several hundred or thousand years, group A thrives and slowly increases to a million people. Meanwhile, group B declines to extinction at an even pace. At no time were there fewer than a million people, but there was a population bottleneck of 1000 people.

beachstartup 12y ago
so then it would be more accurately described as a procreation or reproduction bottleneck, not a population bottleneck.
Dylan16807 12y ago
In that scenario you could make a pretty good argument that group B were not modern humans but instead a dead race. (Assuming they had all these unique genes that disappeared.) I'm not sure the nitpick makes things more accurate.
- duaneb 12y ago
  > were not modern humans but instead a dead race. "Race" is not really a recognized concept outside of social interaction. I believe "sub-species" is preferred on the level that you're indicating—think neanderthals and denisovans. Even then, it's a measure of phylogenetic distance, not of any quality that directly "matters" (like fertility of offspring). Anyway, they're all humans in terms of having active sex with humans.
  - Dylan16807 12y ago
    But all the ones they mated with died out. Unless you mean they were capable of but never did?
    - duaneb 12y ago
      There are humans all over the world with neanderthal and denisovan ancestry. They may have gone extinct, but their relatives definitely live on.
      - Dylan16807 12y ago
        I don't understand where you're going with this in relation to saalweachter's imaginary groups A and B. Neanderthal genes have nothing to do with population bottlenecks.
- saalweachter 12y ago
  If you want to get more complex, you could assume that group A was a colony from group B and that they were a genetic subset of group B. It's also possible that group A and group B were still economically linked, with frequent trade between the two but no intermarriage. It's also possible that group A and group B were never at any time distinct groups, and the individuals who 'won' genetically were scattered through time and space among a larger population. Imagine a rare mutation that confers an immunity to a terrible disease which periodically ravages the population. It's not that the plague kills off all but 1000 people, it's that over centuries of large numbers of people being killed off, those descending from the few people with the mutation (and their mates) come to dominate the gene pool, even if there were always millions of survivors. The reason for the nitpick is that the idea of a human population bottleneck is almost-always interpreted as "humans almost went extinct!". Maybe that happens to be the case, but it isn't necessarily so.
wkimeria 12y ago
If I remember correctly, at one point there were only 1000 human breeding pairs on the planet (due to the Toba eruption)

j-b 12y ago

Do you have a reference for that estimate? Would love to read more.

Sanddancer 12y ago
The Toba Catastrophe Theory [1] is one of the frontrunners as to why humans have such little genetic diversity in modern humans. [1] http://en.wikipedia.org/wiki/Toba_catastrophe_theory#Genetic_bottlenecks_in_humans http://en.wikipedia.org/wiki/Toba_catastrophe_theory#Genetic...