In March 2018 at the Ol Pejeta Conservancy in Kenya, surrounded by his devoted keepers, Sudan the northern white rhino breathed his last. He wasn’t the only remaining northern white rhino because three females in protective captivity survived him. But Sudan’s death ended any hope of those females breeding naturally and rendered the northern white rhino effectively extinct. The moment made headlines, and the world lamented the high-profile extinction.

But it wasn’t a true species extinction event. Northern white rhinos are just a distinct subset, or subspecies, of white rhino. More than 10,000 white rhinos are still left in the southern subspecies. White rhinos as a species aren’t even endangered.

Nevertheless, ever since Sudan’s death, Cynthia Steiner, an associate director in conservation genetics for the San Diego Zoo Wildlife Alliance, and her colleagues have gone to great lengths to try to reboot the northern subspecies. They’re working to turn stem cells collected from the remaining females into embryos for in vitro fertilization. They want more northern white rhinos, and mixing in genes from southern white rhino males just won’t do.

“Before, people would say, we need to save the species,” Steiner explained. “But that’s not enough if you think about it. When you talk about species, you’re not considering the whole evolutionary potential of all the different groups that make up that species. That’s why the new notion is: We really want to save the genetic structure of the species, these different populations — subspecies — that have unique characteristics at the genomic level.”

In recent years, biologists have developed a deeper understanding of how the relationship between genetic diversity and population structure can influence the fate of a species. They’ve long understood how geography and ecological variations often partition species into subspecies or other small distinct populations of individuals who are more closely related to one another than to outsiders. Now, with the help of better tools and techniques for studying the genomes of creatures in the wild, researchers are discovering the full extent of how much the genetic dynamics within and among those populations can affect how resiliently a species can evolve and adapt to changing conditions over time. Those distinct populations can sometimes act as refuges or reservoirs for uncommon genes, and they can become the salvation of a species if new threats suddenly make those genes more valuable. On the other hand, if the smaller populations become too isolated, they can die out and make chunks of a species’ genetic diversity vanish forever.

It’s an important insight into how species naturally adapt and evolve. Luckily for the many species of rhinos, whales, panthers, amphibians and other endangered organisms around the world, this richer understanding of genetic diversity and population dynamics can also help conservation practitioners make better choices about how to save imperiled animals.

But the choices involved sometimes mean probing more deeply into a crucial question: When we try to save a species, what exactly are we trying to save?

Not Just Species

Biologists typically use “species” as a label for “reproductively isolated” populations of organisms that generally breed among themselves and not with outsiders. In effect, a species is a breeding pool that holds all the variant forms (or alleles) of genes that one might find in a type of organism. The species concept is a linchpin of modern biology and evolutionary theory, although it has been intensely criticized for failing to capture the reality of how some organisms actually behave and breed.

Yet although species are pools of genes, those pools are not evenly mixed. Subgroups of lineages within them may tend to breed with one another more often, and they may develop sets of distinctive traits. Naturalists sometimes recognize such groups as subspecies. But even without a subspecies label, there can be populations of individuals within a species (or even within a subspecies) that have a recognizable identity over time.

The U.S. Endangered Species Act of 1973 was ahead of its time on this: It has always allowed the listing of “any distinct population segment,” not just fully distinct species. A quarter of U.S.-listed endangered “species” are actually subspecies, including well-known examples like Florida panthers, northern spotted owls and Mexican gray wolves.

The idea is to consider not just the current survival of a species but its potential evolutionary ability to adapt when faced with environmental changes or emerging diseases in the future. The source of this resilience is its genetic diversity. If a new pest or disease strikes a genetically diverse population, any individuals that happen to be naturally resistant can survive it and reproduce. If all the individuals are genetically alike and lack resistance, the population will die out.

This is currently happening in North American forests, where tens of millions of ash trees are succumbing to a beetle, the emerald ash borer. Some individual ash trees, however, are genetically resistant to the beetles, and they are the last hope for the species.

Population structure bears on genetic diversity because sometimes rare variants of genes are only able to survive by getting sequestered in subpopulations more conducive to their survival. Moreover, researchers have discovered that the importance of genetic diversity goes beyond a population’s specific inventory of genetic traits. Recent studies have helped to confirm a proposal by Michael Lynch, an evolutionary biologist at Arizona State University, that the efficiency of natural selection depends on the “effective population size,” which describes the amount of genetic diversity; this dictates how quickly or slowly species (and populations within them) may evolve, which in turn affects how well species may adapt or whether they will splinter into new species.

That’s why conservationists didn’t simply release the last lonely northern white rhino females into the southern population and call it a day, and why Steiner’s group has been trying to create northern white rhino embryos. Keeping both the northern and southern subspecies in existence separately will better preserve the adaptive potential of the white rhino species as a whole by preserving the unique characteristics and the full genetic diversity of each.

An Inbreeding Bottleneck

When a species or population gets too small, it can lose more than its adaptive potential for surviving future threats. It can lose its ability to sustain itself, as was seen in a study of killer whales, or orcas, that was published in March in Nature Ecology & Evolution.