I’ve been wondering this for a while. I will occasionally read an article about endangered species recovering from very low numbers thanks to conservation efforts.
For example, California Condors, “From the 1980s, when the last 22 wild birds were captured and placed in a breeding program, to 2016 when more animals were born in the wild than died there, the population had only increased to 276 wild individuals.” ( https://www.goodnewsnetwork.org/endangered-california-condor-flies-into-oregon-for-the-first-time-in-122-years/ )
With such low numbers, how do they regain enough genetic diversity to not suffer from inbreeding, and the deformities or sterilization that can cause? I know there are tools to map their genes to help pair the best match, but the mix of genes during reproduction is pretty random.
I’m sure the answers will go way over my head, but I still think it’s interesting.
Until/unless we develop the tools to tinker with condor DNA, there is no quick fix for this dilemma. Regular mutation rates and the process of natural selection will hopeful contribute to the continuing adaptation and improved viability of this small population, but that will take many, many generations of birds.
So I looked it up. Someone who understands genetics better can feel free to correct me, but it seems that the California condor has a heterozygosity of 1.7 per kilobase, which is roughly equivalent to turkey vultures and far exceeds Andean vultures at 0.8. Meanwhile, if I am reading this article right, cheetahs are at 0.40 and they’re clearly…well, maybe thriving isn’t the right word, but they’re doing alright. So we certainly expect some effects, but it’s not necessarily catastrophic for the species.
Out of curiosity, I looked up two other species that are extinct in the wild: Hawaiian crows are at 0.46 het/kb, while milu/Pere David’s deer are at 0.51. This preprint indicates the Devils Hole pupfish is at 0.014.
The Island fox (unbelievably adorable btw) has six subspecies. It seems the San Nicholas subspecies has a heterozygosity of only 0.2-0.014/kb.
I’ve seen both Pere David’s deer and Island foxes in the wild in such numbers that I feel like with moderately competent management, neither is in danger of extinction, and if they do go extinct, it would be because of anthropogenic reasons and not because of inbreeding depression, disease, or other issues stemming from genetics. Likewise, the primary extinction threat to the Devils hole pupfish (and I expect other pupfish!) is from changes to its very limited environment, not from genetic issues. Meanwhile, the California condor is well established in both California (Pinnacles NP/Julia A Burns) and further east in Arizona/Utah.
One interesting thing about inbreeding depression is that the degree to which inbreeding is deleterious varies among species and populations and can change rapidly as a result of natural selection. Imagine, for example, that you have a very small (and therefore inbred) population of some animal. Everyone’s parents are closely related, leading to low heterozygosity (they have the same genetic material from mother and father because the parents are close relatives) which leads to the expression of whatever harmful recessive traits they have. This leads to low mean fitness and high mortality, also known as inbreeding depression. But any individuals who have the dominant and nonharmful versions of those genes will tend to have higher fitness, and the frequency of the recessive alleles will decrease each generation. Over several generations, this can effectively remove deleterious recessive alleles from an inbred population, a process called genetic purging. Many laboratory populations (think white lab mice) have been intentionally inbred for genetic uniformity, and part of the process of doing this is to put the population through genetic purging. Establishing a new inbred breed of dog/horse/etc. is harder than maintaining existing breeds, because the existing breeds have already gone through genetic purging. This does not mean they have no inbreeding depression, but that it is lessened by previous selection. Genetic purging can also come into play in conservation biology, with captive breeding populations or small isolated wild populations for example.
The most extreme example I’ve heard of is Black Robin from New Zealand which had only one female and 4 males in 1980. The Wiki article has some discussion about minimum viable populations; it sounds like maybe repeated bottleneck events in species living on small islands may have already caused the genetic purging that @dlevitis mentioned, making them more resilient to future events.
The Mauritius Kestrel was down to an estimated effective population size of 5. Inbreeding depression due to population bottlenecks seems to be worse in previously outbred species (due to lack of the abovementioned genetic purging) and after prolonged bottlenecks. Each successive generation in a bottleneck loses genetic diversity and fixes deleterious traits through genetic drift.
There is a widespread belief (hard to prove for island endemics, but testable for recent introductions) that island colonisations are often by small populations, down to a little as a pregnant female, or even a facultatively parthenogenetic female. At first sight that appears to be in contradiction to the other widespread belief that species passing through a severe bottleneck are destined for near term extinction. My attempt to reconcile these beliefs is that after colonisation/introduction, in the absence of competition and predators populations increase rapidly, suppressing the worse effects of the bottleneck.