Re inbreeding. Hypothetically, if you started with an initial breeding pair population of a brother and a sister and contained the population in a defined area (say a good sized island), what would happen to that population genetically over time, assuming the breeding population were all effectively cousins of each other after the first generation. Would the population eventually become genetically defective and die out or not?
I read one, that once a population goes below around 200 the species is pretty much doomed, because there’s not enough genetic diversity left.
No idea if it’s true or not, and I can’t remember where I read it, sorry.
Isn’t that what happened to the cheetahs, sort of? I know they got sick and a lot of them died so there’s very little genetic variety among them…
They probably wouldn’t go extinct, as long as the population on the island didn’t remain small for too long. The most deleterious effects of inbreeding happen when a population remains small for a long period of time, since much of whatever genetic diversity the initial population had is eliminated through random processes.
Most populations of plants and animals on remote oceanic islands were generated from very small founder populations that often must have consisted of a single pair or even just a pregnant female. Since even remote islands have a fair number of species, obviously being descended from only a few individuals initially doesn’t automatically doom a population.
Cheetahs must have gone through an extreme population bottleneck thousands of years ago, possibly during the late Pleistocene. They have extremely low genetic diversity, and can even accept skin grafts between unrelated individuals. Despite this, up until the recent past it was a highly successful species, with a vast range extending over most of Africa and southern Asia.
If the founders have some deleterious recessive genes, those genes will end up being paired in a quarter of the individuals in the generations past the 2nd. Those individuals will die or not breed well. However, if the healthy individuals manage to survive and thrive the population can nevertheless increase. The percentage of affect individuals will decrease over time as the bad genes are culled.
The population will still have a low genetic diversity for many generations. If a nasty disease arises there may be no individuals around that have natural resistance. But then again, the population may dodge those bullets and do fine.
Nobody knows for sure how “undiverse” some populations were, but consider the polynesian expansion… The group that reached Easter Island I bet did not number 200. My wild-ass guess is 1 or 2 canoeloads? They crashed ecologically but no indication of genetic problems.
The biggest problems with inbreeding I think are with animal husbandry, where problems that natural selection would normally cull are supported by human intervention since they come with other desirable traits.
For example, I read that corn is no longer able to spread naturally; if the Indians had not kept planting it, it would have died off very quickly. Modern pigs are bred to the point of size and meat production but are actually mental retards with no parenting skills; a wild pig is as smart as a dog (non-purebreds are smart!) but a modern farm sow is so stupid she often won’t avoid rolling over onto her piglets…
Humans tend to deselect obvious problem genes from the pool. The problem with a sibling pair is that about 50% of their genes are identical, so a defective recessive gene has a good chance of “pairing up” with itself. Do the math - the first generation, 25% will be defective offspring, 50% carriers. Assume the defectives will not reproduce. That generation’s offspring - about 1/6 defective and not part of the next pool, 1/3 carriers. Etc. Natural selection at work; however if the defect is subtle, or there are too many, then the population may be doomed.
Another argument is that genetic diversity is useful in fighting disease. Modern monoculture farms, for example, mean that if a particular genetic makeup is more susceptible to an infection or parasite, then that problem will spread like wild-fire across a popultion where everyone is vulnerable. (This is my objection to genetically modified food - how many “parent” plants were modified, or did the modification essentially come from one tiny batch of parent plants - and now Monsanto or whoever wants to plant that one parent across all of North America?)
Note the Vadoma tribe of Zimbabwe, or the “Ostrich People,” who have lived and bred in isolation and exhibit an unusually high occurrence of ectrodactyly, but are otherwise healthy. They certainly did not begin with only a pair of siblings, but are a demonstration that a small genetic pool is not necessarily a recipe for doom.
If they aggressively removed their culls from the gene pool, they’d probably do okay in the long run. This would re-inforce the beneficial effects of inbreeding (promoting the expression of neutral or positive genes) while reducing the bad (decreasing the expression of the bad genes.)
If they’re really hard-core, they’ll eat their culls.
Yeah, I read Time Enough For Love, too… 
Remove their culls? I’m a little confused. I thought to cull meant to reduce a group’s numbers. So what is a cull?
“Cull(s)” is often used to identify the thing being culled as well as the process itself.
Right. Although deleterious recessive genes have a greater chance of becoming paired and thus expressed in a small population, by the same token they also have a greater chance of being completely eliminated from the population. In large populations, rare deleterious recessives only infrequently become exposed to selection by being expressed, so that it will take a very long time (if ever) to eliminate them.
People have frequently made inbred lines of other animals, often through repeated brother-sister mating. My understanding is that some attempts fail, but clearly many succeed.
How much would random mutations balance out the trend of defective genes pairing up and add more diversity again?
Thanks.
It’s a much slower process. Beneficial mutations, that would be preserved by selection, are few and far between. Most evolution takes place as the result of changes in the frequency of existing gene variants (alleles), rather than by adding entirely new ones.
So… re “culling” if you started with the brother + sister pair, but really, really agressively weeded out (ie killed or neutered) any genomes that were detectably defective and kept the closer relatives from breeding, would you be able to go in the other direction, and breed a genetically superior, physically robust and healthy population even if you started with a brother and sister?
That would be the best way to maintain the genetic diversity that was present in the original pair, at least the beneficial alleles. Of course, the population is never going to have the same genetic diversity of one that started out large (or it will take a very long time to develop it by means of mutations). This is exactly the approach that is taken with captive-bred populations of endangered species in zoos. A stud book is carefully maintained, and zoos swap individuals from different lineages in order maintain as much diversity as possible.
As I’ve already mentioned, species on remote oceanic islands often have robust populations despite being descended from only a few individuals, and the cheetah has been a successful species even though it has low genetic diversity.
Maybe a dumb question: Could cloning and/or gene splicing be used to remove bad genes and to artificially genetically “diversify” a population?