Lions/tigers gene pool
October 20, 2011 4:33 PM Subscribe
What's the genetic diversity situation like with lions and tigers?
The Ohio animals story got me thinking about this. Still trying to wrap my head around 18 lions and 18 bengal tigers being killed. I guess the most obvious threat to lions and tigers is outright extinction, but as the numbers dwindle, is there a critical point at which the species is as good as extinct due to the limited gene pool, and how close are we to that point?
I assume there aren't any ways to turn back the genetic clock as it were, unless someone had the foresight a long time ago to starting spermbanks when there were more of these guys around.
The Ohio animals story got me thinking about this. Still trying to wrap my head around 18 lions and 18 bengal tigers being killed. I guess the most obvious threat to lions and tigers is outright extinction, but as the numbers dwindle, is there a critical point at which the species is as good as extinct due to the limited gene pool, and how close are we to that point?
I assume there aren't any ways to turn back the genetic clock as it were, unless someone had the foresight a long time ago to starting spermbanks when there were more of these guys around.
There are significantly more tigers in private hands in the US than there are in the wild, and one of the issues with the tigers that end up in those kinds of amateur 'sanctuaries' in states with lax regulation on those kinds of animals is that most are genetic mutts. (Related SF Gate story.)
All signs point to Texas Backyard Tiger outlasting the wild subspecies.
posted by holgate at 6:29 PM on October 20, 2011
All signs point to Texas Backyard Tiger outlasting the wild subspecies.
posted by holgate at 6:29 PM on October 20, 2011
The whooping crane's total extant population reached a low of 23 (21 wild) on two occasions before breeding and reintroduction programs began to swell their numbers, now around 500 (still fewer than 400 in the wild).
Wikipedia has a bit on the challenges faced by captive breeding programs.
posted by dhartung at 11:12 PM on October 20, 2011
Wikipedia has a bit on the challenges faced by captive breeding programs.
posted by dhartung at 11:12 PM on October 20, 2011
There are two issues to be concerned about when you have a small population.
Actually, first let me note that a 'population' is a group of individuals that can interbreed to produce viable offspring. The important part of this phrase is actually 'can interbreed' because if there is significant genetic diversity over the population but they can't breed with each other (e.g. they're on separate islands), your effective genetic diversity is very low (and really you have two separate populations each with their own genetic diversity. So, relating that back to the lions and tigers in Ohio, they were never (probably) going to get to breed with other lions and tigers in a healthy manner. I don't know what private breeders do to select breeding pairs but zoos put together extremely detailed studbooks with genetic information so that they can breed the most diverse individuals and reduce inbreeding. The classic example of this is the Przewalski Horse stud book which was used to bring this species back from the brink of extinction and minimize inbreeding. The starting population was 9 individuals; although there were 31 individuals (population size), they only bred 9 for whatever reason (effective population size which indicates the number of individuals of breeding age, condition, etc.). Now there are over 4000 including over 500 in the wild.
I assume this gentleman did not participate in any zoo-related breeding schemes so his animals were effectively contributing no genetic diversity to their world wide populations. But you say, in an emergency, couldn't they act as refuges of genetic diversity being potentially individuals not represented in the wild or zoo populations? Yes. So that's sad. Although I'm willing to bet that he got the animals from breeders, rather than from the wild (I don't think many lion or tiger cubs are smuggled into the US) so they are probably inbred themselves.
But back to the two problems with small populations: genetic variation and inbreeding depression. Genetic variation enables species to survive if conditions change. For example, a gene that allows individuals to survive with less water could allow the population to survive a drought, even if the proportion of the population without the gene doesn't survive. The classic example of this problem is with cheetah populations which have incredibly low genetic diversity. But they seem to be puttering along so far (fingers crossed).
The other issue is inbreeding depression. Because animals have two copies of genes the impact of deleterious genes is often muted because there is a copy of the 'good' gene. This is the case for recessive deleterious genes - Gg where the dominant gene (G) would prevent the bad parts (g) from being expressed. Parents pass on their genes to their offspring and if they are heterozygous for the deleterious gene, they have a 50% chance of passing on the deleterious gene. That's okay in normal, non-inbred populations because the other parent probably has another form of the gene entirely (HH) or is homozygous for the gene (Gg). But with inbred populations, the odds of offspring having these deleterious genes increase because there isn't enough variation. I'm not so familiar with examples of this problem (but see this adder) because you only need one individual per generation to prevent inbreeding depression. Yep, that's all.
Back to the lions and tigers in Ohio. You can kind of guess by now that, kind of surprisingly, they weren't that important to the genetic diversity of their species - because their breeding pedigree probably wasn't healthy/good and because we're not quite at the point where their bad genes are better than none/inbreeding. I'm having issues with my internet at the moment so I can't access actual journal articles about the genetic diversity of lions and tigers. Once things are working for me again, I'll link to some scholarly articles.
posted by hydrobatidae at 9:02 AM on October 21, 2011
Actually, first let me note that a 'population' is a group of individuals that can interbreed to produce viable offspring. The important part of this phrase is actually 'can interbreed' because if there is significant genetic diversity over the population but they can't breed with each other (e.g. they're on separate islands), your effective genetic diversity is very low (and really you have two separate populations each with their own genetic diversity. So, relating that back to the lions and tigers in Ohio, they were never (probably) going to get to breed with other lions and tigers in a healthy manner. I don't know what private breeders do to select breeding pairs but zoos put together extremely detailed studbooks with genetic information so that they can breed the most diverse individuals and reduce inbreeding. The classic example of this is the Przewalski Horse stud book which was used to bring this species back from the brink of extinction and minimize inbreeding. The starting population was 9 individuals; although there were 31 individuals (population size), they only bred 9 for whatever reason (effective population size which indicates the number of individuals of breeding age, condition, etc.). Now there are over 4000 including over 500 in the wild.
I assume this gentleman did not participate in any zoo-related breeding schemes so his animals were effectively contributing no genetic diversity to their world wide populations. But you say, in an emergency, couldn't they act as refuges of genetic diversity being potentially individuals not represented in the wild or zoo populations? Yes. So that's sad. Although I'm willing to bet that he got the animals from breeders, rather than from the wild (I don't think many lion or tiger cubs are smuggled into the US) so they are probably inbred themselves.
But back to the two problems with small populations: genetic variation and inbreeding depression. Genetic variation enables species to survive if conditions change. For example, a gene that allows individuals to survive with less water could allow the population to survive a drought, even if the proportion of the population without the gene doesn't survive. The classic example of this problem is with cheetah populations which have incredibly low genetic diversity. But they seem to be puttering along so far (fingers crossed).
The other issue is inbreeding depression. Because animals have two copies of genes the impact of deleterious genes is often muted because there is a copy of the 'good' gene. This is the case for recessive deleterious genes - Gg where the dominant gene (G) would prevent the bad parts (g) from being expressed. Parents pass on their genes to their offspring and if they are heterozygous for the deleterious gene, they have a 50% chance of passing on the deleterious gene. That's okay in normal, non-inbred populations because the other parent probably has another form of the gene entirely (HH) or is homozygous for the gene (Gg). But with inbred populations, the odds of offspring having these deleterious genes increase because there isn't enough variation. I'm not so familiar with examples of this problem (but see this adder) because you only need one individual per generation to prevent inbreeding depression. Yep, that's all.
Back to the lions and tigers in Ohio. You can kind of guess by now that, kind of surprisingly, they weren't that important to the genetic diversity of their species - because their breeding pedigree probably wasn't healthy/good and because we're not quite at the point where their bad genes are better than none/inbreeding. I'm having issues with my internet at the moment so I can't access actual journal articles about the genetic diversity of lions and tigers. Once things are working for me again, I'll link to some scholarly articles.
posted by hydrobatidae at 9:02 AM on October 21, 2011
This thread is closed to new comments.
This old askmf thread includes a lot of relevant information about MVP.
Something to check out would be the Species Survival Plan for the tiger..
posted by buttercup at 5:04 PM on October 20, 2011 [1 favorite]