Graft a patch of skin from one wild cheetah onto another, unrelated cheetah, and the recipient’s body will not reject it. Researchers documented this in the 1980s, and the result told them something unsettling: the world’s cheetahs are so genetically alike that, to an individual’s immune system, every other cheetah reads as a twin.

We’ve written recently about the species that have disappeared, both the confirmed extinct and populations crashing toward genetic oblivion. There is a quieter loss happening inside animals that are still here. The North Atlantic right whale, the vaquita, and the cheetah are all still roaming the planet, albeit with less room to do so. The shocking truth is that none of them is genetically whole, and that distinction shapes what the science can do for them over the next century.

Survival and viability are not the same thing, and the gap between them is where this story lives.

The whale we can count to the individual

In October 2025, NOAA Fisheries and the New England Aquarium estimated that 384 North Atlantic right whales were alive at the start of 2024, a 2.1 percent rise from a recalculated 376 the year before, and a modest climb from the population’s all-time low of 358 in 2020. Researchers know these whales individually, by the callus patterns on their heads, in a photo-identification catalog built over decades. A species you can count one animal at a time is a species in trouble.

The arithmetic of recovery is harder than the headcount suggests. Fewer than 70 reproductive-age females remain, and the interval between calves has stretched from roughly three years to six or even ten. The 2025 calving season produced 11 calves; a genuinely productive year would top 20, and lasting recovery needs to be more than 50 births a year to raise the species from the brink of extinction. An Unusual Mortality Event declared in 2017, when 20 percent of the population died, is still ongoing, driven by vessel strikes and fishing-gear entanglement. Researchers estimate only about a third of right whale deaths are detected.

Underneath the visible threats sits an invisible one. The North Atlantic right whale carries among the lowest genetic diversity measured in any large mammal, a legacy of the whaling era that cut the population to a few dozen animals. There has been too much inbreeding, and low genetic diversity raises the odds that mating pairs share genetic profiles, which is linked to fetal loss and a reproductive rate roughly three times below the species’ biological potential.

There is one piece of better news: genomic work finds evidence of purging, a process that has lowered the frequency of the most harmful inherited genetic variants and leaves more room for recovery than the diversity numbers alone would predict.

Fewer than a classroom

The vaquita, a small porpoise found only in the northern Gulf of California, is the most endangered marine mammal on Earth. A joint visual and acoustic survey in 2025 put the count of distinct individuals observed at most likely seven to ten, in the same range as the six to eight seen in 2024. The entire species would not fill a school classroom.

The same survey surfaced the most surprising fact: the vaquitas it found were surviving and reproducing, with at least one or two calves observed, and there was no sign of the catastrophic single-year drops recorded earlier in the decade. The animals remain concentrated in and near the sanctuary at the heart of the Vaquita Refuge. Their one lethal threat is human: entanglement in illegal gillnets set for totoaba, a fish whose swim bladder commands high prices as a medicine and gourmet delicacy in an illicit trade.

Genetics, counterintuitively, is the part of the vaquita’s situation that offers hope. A 2022 study in Science sequenced 20 vaquita genomes and found that the species has been naturally rare for hundreds of thousands of years, which left it with a low burden of harmful genetic variation. In summary, they are not susceptible to the damage caused by inbreeding.

Simulations in that work concluded the vaquita is not doomed by inbreeding and is highly likely to recover — if gillnet deaths stop immediately. The vaquita is the clearest case of a species whose genetics are not the obstacle. The obstacle is fishing nets set for archaic and exploitative reasons.

The cheetah’s deep bottleneck

The cheetah’s genetic story is the oldest of the three and the hardest to undo. The global wild population is estimated at about 7,100 adults and adolescents across 33 fragmented populations, occupying just nine percent of the cat’s historical range; 91 percent of those populations include 200 animals or fewer. But the cheetah’s defining problem predates the modern range collapse by thousands of years.

That skin-graft result from the 1980s pointed to a species squeezed through an ancient bottleneck. A 2025 genetic analysis of southern African cheetahs supports a gradual decline over roughly the past 10,000 years — most likely driven by climate-era shifts in vegetation and prey — and estimates their present-day effective population size at just 700 to 1,600, which reflects a genetic measure of how many individuals are actually contributing their genes to the next generation; it typically runs far below the number of animals you could count in the field. A population can look reasonably stocked and still carry the “genetic thinness” of a far smaller one.  The costs of that thinness show up in cheetahs as poor sperm quality, heightened disease vulnerability, and diversity that keeps eroding even where total numbers hold steady.

The Asiatic cheetah, clinging to its existence in Iran, shows where this story ends. Its effective population size has been estimated at 11 to 17, and researchers argue its survival now depends on urgently protecting the habitat corridors that let isolated animals find one another to increase the chance they will mate.

Diversity itself. Genetic variation is the difference between individuals in a population, and it is not a luxury feature. A broad genetic menu is the raw material that natural selection runs on. A right whale or a cheetah population with little variation has fewer options stored away for whatever comes next.

The capacity to adapt. A new disease, a warming ocean, a shifting prey base — a diverse population contains individuals that happen to cope well with changes, and they become the next generation. A genetically narrow population may simply have no members equipped to adapt to change. This is why a species can be present and counted yet functionally unable to respond to change. Climate change is driving ecosystem changes that many species, even with large populations, struggle to cope with.

Time. Diversity is rebuilt by mutation over many generations, on timescales that dwarf any conservation budget. You can stop a ship or cut a net this year. You cannot restock a gene pool this century. That is the heart of the bottleneck problem.

These animals reached the same narrow path to survival by very different routes. The cheetah’s bottleneck is prehistoric, written into the species long before humans were a factor. The right whale’s was industrial, its world reshaped by centuries of commercial whaling and sea-going freight. The vaquita’s is unfolding inside a single human generation, the byproduct of an illegal fishery operating where it should not.

Yet the genetics also refuse a tidy moral. The vaquita and the right whale both show signs of purging, the quiet removal of the worst inherited variants, which means a small population is not automatically a doomed one.

The lesson is not that low diversity is a death sentence. It is that genetic poverty removes a species’ margin for error and then leaves the outcome to us. For the vaquita and the right whale, the limiting factor is human intervention in their ecosystems.

None of these are backyard problems. The levers are fishing gear, shipping rules, habitat policy, and international enforcement, which means individual action matters mostly as pressure on the institutions that hold those levers. With that said:

• Source seafood that doesn’t kill what it isn’t targeting. Bycatch in nets — the species the fishermen don’t want — is the proximate threat to both the vaquita and the right whale. Use a guide like Seafood Watch and avoid products tied to gillnet fisheries; support enforcement against the illegal totoaba trade driving vaquita deaths.
• Back ropeless gear and slow-speed zones. On-demand (“ropeless”) fishing gear and seasonal vessel speed limits are the concrete tools that reduce right whale entanglements and strikes. Support the regulations and the fishermen adopting them.
• Defend habitat connectivity for big cats. For cheetahs, corridors that reconnect fragmented populations are the only practical way to move genes between isolated groups. Support organizations working on rangeland coexistence and protected-area linkage.
• Fund the genetic insurance. Biobanks, genome sequencing, and breeding programs managed explicitly for diversity are how we preserve options we can’t yet name. They are unglamorous and chronically underfunded.
• Protect the legal scaffolding. The Endangered Species Act, the Marine Mammal Protection Act, and international agreements like CITES are what keep these species on anyone’s agenda. Engagement with that policy is the highest-leverage action on this list.

Editor’s Note: Our next installment of Environmental Losses moves from individual species to whole systems, beginning with the ocean’s most biodiverse ecosystem — the coral reef — and the cadence of bleaching that has collapsed the recovery time reefs need to survive.

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