Meet the Inbreeders
Image Credit: Mike Baird, CC BY 2.0; Chuck Szmurlo, CC BY 3.0; Ryan Hagerty, USFWS, Public Domain)
Nowadays we know to avoid mating with close relatives so that your children are healthy. As close relatives accumulate similar genetic diversity, mating between relatives of most species can lead to genetic diseases in their offspring. Humans have a lot of options; there are 7 billion of us worldwide. But how about an endangered species with less than 10 individuals in the wild?
For many populations, especially small, fragmented or endangered ones, if there are not many of your kind left in the world, avoiding mating with close relatives is hard. Some species that became extinct shortly after the emergence of the first human hunters were documented to have their population bottlenecked to several individuals before finally meeting their demise by being sampled as museum specimens, killed by locals, or just dying off by chance without leaving any offspring. Elizabeth Kolbert summarises these kinds of stories nicely in her book, “The Sixth Extinction: An Unnatural History”, in which she tells the story of the last auk and other all-time favourite megafaunas of the last Ice Age.
As far as I know, conservation of endangered species was not a widespread concern until around the 1980s. There were of course earlier efforts, but the majority of literature I found on the first attempts to breed endangered species were mostly published around this time. At that time, captive breeding efforts started whenever we found any species that was thought to be sufficiently close to extinct. The development of molecular techniques and population genetics in the 1980s also established the need for high genetic diversity in maintaining the existence of endangered species.
Nonetheless, many captive breeding attempts fail to maintain a healthy population, as sourcing extra genetic diversity from outsiders is easier said than done. For some heartbreaking examples, read about the Asian lions or black-lion-tamarins. The results of heavy inbreeding can be truly horrifying, even leading people to questioning if captivity breeding was even worth it.
Yet some species actually manage with such limited options. A quite legendary example in my opinion is the Mauritius kestrels (Falco punctatus) that were reduced to only four individuals in 1974 but bounced back to around 30 in the late 1980s, thanks to a variety of conservation techniques applied to this species. Similar stories can be found in:
- Californian condors (Gymnogyps californianus), which bounced from dozens to hundreds in a couple of decades.
- Northern elephant seals (Mirounga angustirostris), one of the many species that were heavily exploited during the mid 1800s were found to be having very low genetic diversity in 1974, but also bounced back to dozens of thousands in the early 1980s.
- Black-footed ferrets (Mustela nigripes) managed to reproduce healthy little ferrets with quite small populations despite low genetic diversity after their population declined around the 1980s and has been retaining their genetic diversity since their reintroduction.
- Hawaiian crows (Corvus hawaiiensis) were declared extinct in the wild in 2003 and their captive population was really inbred but re-wilding efforts were successful just last year.
- Chatham island black robins (Petroica traversi) that were thought to be extirpated in late 1893 and recovered to 250 individuals since 2013 thanks to the last breeding pairs and a foster species.
Those seemingly resilient species were able to persist with human intervention despite a tiny genetic pool. Their habitats were protected, their hunting was regulated, their reproductive efforts were intensively assisted and managed, the list goes on.
I found no evidence of any species that can persist in a really low population without human intervention (though please leave examples in the comments if you have them) except for the Channel Island fox (Urocyon littoralis). This island fox is a dwarf species that inhabit six of the California’s Channel Islands. They have been persisting in a very small population with very low genetic diversity for quite a long time, have accumulated dangerous mutations, but show no diseases or deformities that we can discern. No human intervention has been known to have assisted their population growth.
Nonetheless, if genetic diversity is really that important, how could species in such a small population persist for a very long time? Would they not get inbred and diseased?
Inbreeding Does Not Always Mean Depression
All individuals that we are seeing now from any species inherited their DNA from their ancestors, coding all the things that make up an individual. There are two copies of DNAs, arranged in pairs called homologous chromosomes. If one of the chromosomes mutates in certain parts, the other pair usually do not, and our protein-coding machinery stays safe. When closely related individuals mate, however, similar mutations tend to meet in the same homologous chromosome and the protein-coding can often be disrupted. This is where genetic disease shows up and why we want to avoid inbreeding. A great shout out for this Quora answer linked below for doing such a great job in explaining how inbreeding depression happens.
Related: Why Do Asexual Organisms Produce Healthy Offspring, While Incest Among Humans Will Lead to Abnormalities in the Baby?
When a very inbred species seems to be persisting for quite a long time with their horrible genomic makeup, there are several things that could be happening. The Channel Island fox has been thought to purge their dangerous mutations in the past. This is possible when the mutations are so dangerous, they get eliminated by natural selection as soon as they appear. Some species could also be adapted to inbreeding, as is the case for many invasive species. Invasive species have been thought to be able to pass their ability to adapt to new environments to their offspring, allowing the following generations to prevail even better in the new environment.
Conservation Gone Too Far?
These invincible beasts are yet to face the trials of climate change. The northern elephant seals may be impacted by the sea level rising in the future, pushing them to inbreed once again. Lack of genetic pool was also the lurking problem for the recently recovered hawaiian crows and black-footed ferrets. As if their super-low genetic diversity is not enough, the black-footed ferrets are currently threatened by the sylvatic plague and cannot persist in the wild without continuous captive breeding. This encourages a plan to clone new ferrets with the help of domestic ferrets and cryopreserved cell lines.
The term “cryopreserved” definitely got me thinking, would that not cost a lot of money? Should that money be used for other species with more potential to survive? Who decides such things?
One thing that is certain is that we humans have the power to prevent extinction, as the several success stories of reintroduction have told. Whether we should do it or not, especially until the extent of cloning near-extinct and extinct species, that is another question I leave for the readers.
Sabhrina Gita Aninta is a conservation geneticist currently pursuing her PhD at Queen Mary University of London to understand how genome-wide variation of the endemic pigs and buffalos from Southeast Asia could assist their conservation. Follow her Twitter here for an update of her work, along with a mix of conservation, biodiversity, evolution, but mostly various rants and random stuffs in Indonesian and English. You can find more of her work at Ecology for the Masses at her profile.