Point of no return for a species?

[Grimlock]

It has happened at least once in human history and its theme in many a movie, Humanity getting this close to permanent extinction.

But exactly how many individuals is required for a species to have enough genetic differences for a species to survive.

I think i heard somewhere that it is 10.000 though other says that 5000 is more than enough, but what is the exact number?

At what point does a species get so few, that its only a matter of time before Inbreeding is the only way to reproduce?

 

[scalyblue]

http://en.wikipedia.org/wiki/Minimum_viable_population

 

[Aught3]

But exactly how many individuals is required for a species to have enough genetic differences for a species to survive.

Two individuals minimum (see for example black robins) but it does depend on the species.

 

[Spase]

I'm pretty much with Aught on this. In a little more detail.... My understanding of the dangers of inbreeding is largely that you can get very exaggerated traits and you get a lot of recessive disorders manifesting because you end up with a concentration of them in the population. However, even with that increased incidence assuming the population you start with doesn't have terrible genetics you should be able to go pretty low.

And of course there's the fact that a population of 100 people who are are close relatives as compared to a very genetically diverse population will result in very different gene pools. If the human population was suddenly 2 individuals though you'd have the interesting artifact that whatever alleles they carried would have been selected for in the most dramatic way possible meaning there would be a pretty limited set of possible individual genotypes until mutation kicked in some. And even when mutation kicked in it would still mostly be a shuffling of the traits of those individuals, not novel traits... i.e. someone might have a different combination of one of four noses heights hair-colors etc but you wouldn't likely suddenly see someone with dark skin unless it was in the original population since mutations that are more than just swapping aren't quick to pop up.

I believe the 10,000 number you remember actually is from the fact that (if I remember right..) the human population actually hit a bottle-neck of ~10K individuals at one point before we left Africa. Can't remember the precise method they used to determine this but I can think of a few approaches you could use to try to measure this.

 

[Eidolon]

There was an old star trek episode that kind of explained how to re-diversify a genetic population. In the show, they were on a planet comprised completely of clones, since for whatever reason, they didn't reproduce naturally but instead cloned themselves resulting in X number of copies of each individual. Their problem arose when their genes began to deteriorate with each cloning cycle and diseases and disorders began to threaten their existence since with each new clone, some genetic material was corrupted. What eventually happened in the end, was (And I can't remember if it was the original or TNG) but the capt and medical officers explained to the culture, how to reproduce sexually, and explained that in order to re-diversify their gene pool and help diminish the genetic disorders they were incurring, that they would need a minimum of 30 people (they didn't specify how many males to how many females), each one of them interbreeding with the others.

Its been a long time since I saw that episode, but Im pretty sure thats how it went.

 

[Nogre]

There was an old star trek episode that kind of explained how to re-diversify a genetic population. In the show, they were on a planet comprised completely of clones, since for whatever reason, they didn't reproduce naturally but instead cloned themselves resulting in X number of copies of each individual. Their problem arose when their genes began to deteriorate with each cloning cycle and diseases and disorders began to threaten their existence since with each new clone, some genetic material was corrupted. What eventually happened in the end, was (And I can't remember if it was the original or TNG) but the capt and medical officers explained to the culture, how to reproduce sexually, and explained that in order to re-diversify their gene pool and help diminish the genetic disorders they were incurring, that they would need a minimum of 30 people (they didn't specify how many males to how many females), each one of them interbreeding with the others.

Its been a long time since I saw that episode, but Im pretty sure thats how it went.

It was TNG, and the actual number was about 300. The group that was cloning was only made up of 4 or 5 people, but the Star Trek gang found a group of people from the same initial colonization mission (which had gotten separated generations beforehand) who had lost technology, but had retained a viable population, and brought them together in order to save both groups. They also had to do some special stuff with each woman having to have children with something like 7 husbands or something like that. That's grossly oversimplified, and I'm unsure whether they were realistic with the numbers or not, but it would be interesting to know how many human beings we would always have to insure survived something in order to insure the continuation of our species.

It could technically be 2, but I'm pretty sure with sexual reproduction, if interbreeding carries on for too long, you get a large ammount of infertility. This happened repeatedly in the Egyptian Pharoahs because the men ruled, yet property was passed down through women, so all the men married their sisters in order to be the next Pharoah. The inbreeding was the reason why there were so many different dynasties compared to other cultures of the time; the lines just died out eventually (along with lots of "divinely inspired" mutations).

 

[Eidolon]

Ah I was thinking it was TNG. I remember the medical officer was girl but I couldn't remember if it was Crusher or just some female doctor from the original. I saw that episode when I was about 9 or so, so I remember very little of it, just the fact that they were on a planet of clones and they needed to reproduce naturally or they would die out.

 

[xman]

Although I can't find anything on it for you now, I recall seeing a documentary that explained the cheetah as having been driven almost to extinction around 10,00 years ago. Their breeding pool could have been as low as a few dozen then. This is believed because the genetic variability between cheetahs is so narrow. As a species it is very specialised and as such doesn't stand a great chance of evading extinction for much longer. Exactly how long will be determined by environmental stresses,

Homo Sapiens are a very crafty animal, however and we could rebound from an equally small population or perhaps even smaller.

 

[Squawk]

This relates pretty well to something I heard reference to as the evolution of evolveability.

If we are to consider genetic diversity as a barrior to population growth (ie, not enough leads to inbreeding etc) then we must take into account the robustness of the genome in question.

Something I often batter creotards with is the idea of the typical human getting 100-200 mutations in their genome. Consider though if this is optimal for species survival and propogation. 100-200 mutations and a 20 year generation time for a population of 7 billion is extremely unlikely to make the human species adaptable to a big upheaval in conditions (except for technology). However, for a much smaller population (go back through our evolutionary history) it is likely that this is actually an optimum, or very close to it, for a certain population size.

Consider a population of size 10, but with no mutations in the genome and a genome with no genetic weakness. This population could in theory live forever, self sustaining, if the environment never changes.

The MVP is going to be a balance of a number of factors and will be difference in each species, but the influencing factors will always be the same. Genetic diversity in the population, generation time, mutation rate and population size (to make things seem extremely simple).

I seem to recall reading somewhere that teh rabbit population in australia is the result of either 1 pair or a very small number of pairs of breeding rabbits.

 

[c0nc0rdance]

There is a whole field of science (preservation population genetics) surrounding this question. The phrase you need to look for is "Minimum Viable Population". Whether the group is outbreeding or inbreeding is actually somewhat irrelevant. Genetic diversity can be generated by larger population sizes, high diversity, or high mutability. Sometimes switching a group from outbreeding to inbreeding will help to preserve the rare traits that should be part of geographically diverse population.

This is what is being done with the Giant Panda. They no longer have the option of maximum outbreeding distances, so they are going to start inbreeding close relations. With any luck, this will actually increase fertility somewhat (eventually, if they make it that long and start to show a bit of interest in the act of procreation).

If you are curious about the study of population genetics as it results in preserving endangered species, check out the Wikipedia articles:
http://en.wikipedia.org/wiki/Effective_population_size
http://en.wikipedia.org/wiki/Minimum_viable_population

 

[Mapp]

I think at one point during humanity's early history, the species was down to no more than 600 viable, breeding individuals, that's probably the lowest the population got. Of course, if Homo Sapien had not succeeded, it's plausible that another hominid might have.

 

[Durakken]

If you killed off 99.9999999% of all humans on the planet we'd as long as there isn't some other after effect that kills them off as well the species would still go on and if it was all individuals that could understand the circumstances and how personal sacrifices would benefit themselves... and there was roughly 1:1 ratio of male to female...600 starting pop... 300 children a year * 14 = 4200... breading production goes up 150 each year for 14 year...

1-14: 300 (4800)
11: 450 (5250)
12: 600 (5850)
13: 750 (6600)
14: 900 (7500)
15: 1050 (8550)
16: 1200 (9750)
17: 1350 (11100)
18: 1500 (12600)
19: 1650 (14250)
20: 1800 (16050)
21: 1950 (18100)
22: 2100 (20200)
23: 2250 (22450)
24: 2400 (24850)
25: 2550 (27400)
26: 2700 (30100)
27: 2850 (32950)
28: 3000 (35950)

That's 1 Generation... Assuming the original people started in their 20s and they only breed for those 28 years you have 150 kids the 29th year and then the 30th->43rd year there would be +300 kids to the breeding stock. 44-58 you'd have +600 and so on... Assuming the rules are the originals start at 20 and have a hard time having viable offspring after 50. However... you could push those number back either way...you could push it back to 12, and in the near future you can push the viability age up to 60s-80s... So considering that... within a single generation it is possible to repopulate a small city. Within 2 generations, 4 more small cities...3 generations you have well over 800,000 people.

When we aren't trying to breed ourselves we have shown an exponential curve where we double our numbers if half the time it took to double our number the previous doubling... 1billion went to 6 billion in 100 years ... that means 25 years ago there were 3 billion people and 50 year before that there were 1.5 and 100 years before that 750,000,000 and 200 years before that 375,000,000... and 400 before that 175,000,000... that's half the US population in the entire world 775 years ago... 1300CE. 800 years before that there was 87,500,000. That was the fall of the roman empire. 1600 years before that...1200 BCE we're nearly at the beginning of recorded history with only 43,750,000 people. That's 4 large cities today. 3200 years before ... 21,875,000. And 6400 years before that in 10800 BCE when if Atlantis existed it might have just been starting...10,937,500 people on earth. The size of 1 large city.

And that only accounts for 1/10 of our species time span so... more or less we are what we kill cockroaches are like