New research details the evolutionary history of rhinos, exposing a surprising lack of genetic diversity throughout their long history. Since all rhino species are currently endangered and facing their own genetic bottlenecks, new research could improve conservation efforts.
At a scientific meeting held a few years ago in Copenhagen, paleogeneticist Esteem Dalén of the Swedish Museum of Natural History met with Tom Gilbert, an evolutionary biologist at the University of Copenhagen. Possible scientific collaboration was discussed, in which the subject of rhinos arose, as each one independently studied these mammals with horns. This set the ball in motion, which led to a project in which Dalén and Gilbert, along with experts from around the world, used both ancient and modern genomes to study the evolutionary history of the rhino family.
A meaningful collaboration, since scientists have struggled to reconstruct the family tree of rhinos. Biologist Charles Darwin even stabbed him, writing an essay on the subject 17 years before his primary work, On the origin of species, published in 1859.
Studying the history of rhinos has been a challenge because all the rhinos that exist today are in great danger and are the focus of conservation efforts. In addition, the vast majority of rhinos became extinct before the Pleistocene, which began about 2.58 million years ago. The rhino family emerged between 55 and 60 million years ago, after diverging from the tapirs. Rhinos would be hugely successful, spawning more than 100 different species and spreading to Africa, Eurasia and North and Central America.
Some rhinos grew very large, such as the woolly rhino (Coelodonta vell). These rhinos weighed more than 2,000 kg (4,500 pounds). a furry coat, a giant coat and a formidable 5 feetmeters) horn. When the Pleistocene ended about 11,500 years ago, however, only nine species of rhinos remained on Earth.
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To better understand rhinos in terms of their history and distant ancestors, the team traced the genetic relationships of five species of live rhino with three species of rhino. which became extinct just before the end of the last ice age: the siberian unicornElasmotherium sibiricum), The Merck Rhino (Stephanorhinus kirchbergensis), and the said woolly rhino. Black rhinos (Diceros bicornis), white rhinos (ravelobensis), Sumatran Rhinos (Dicerorhinus sumatrensis), larger single-horned rhinos (Rhinoceros unicornis), i Java rhinos (R. sondaicus), were the living species included in the study.
The following analysis showed that an ancestral division occurred 16 million years ago during the early Miocene, creating two different lineages of rhinos, one in Africa and one in Eurasia. This division was due to its geographical extent and not the result of an emerging physical difference, i.e., the appearance of rhinos with a single horn and two horns.
The other key finding is that rhinos have a long history of low genetic diversity. Lack of genetic diversity is a sign of small populations and can cause all kinds of genetic diseases as a result of harmful mutations. This happened to him woolly mammoths in his last days.
“All eight species typically show a continuous but slow decline in population size over the past 2 million years or small continuous size over extended periods of time,” said Mick Westbury, co-author of the study and researcher at the University of Copenhagen, explained in a press release sent by e-mail.
As this research suggests, ancient rhinos somehow managed to cope o adapt to continuously small populations. Scientists have a good term to describe the process that makes it possible: the debugging of the mutational load.
“Species do not adapt to low diversity, but somehow it could be said that they can adapt to having a small population,” Dalén wrote in an email. “What the theory predicts is that natural selection can eliminate harmful mutations in the population, even when the size of the population becomes small. This is not an adaptation to low diversity, but in my opinion it should be seen as an adaptation to inbreeding ”.
Therefore, low genetic diversity, despite being an indelible part of rhino history, did not cause a decline in health as a result of inbreeding and the notion mutations. Interestingly, rhinos are not alone in this regard. The family of cats (Felids) has an even lower genetic diversity, as Dalén explained. He said this is not too surprising, “since carnivores tend to have less diversity than herbivores, as the size of their general population is smaller.”
But while “low genetic diversity is a long-term feature” of the rhino family, “it has been especially aggravated recently,” probably because humans have pushed these creatures toward extinction., as biologists write in their paper to study, published today in Cell.
In fact, although the historical debugging of the mutational load may have prevented this the genetic problems that occur when crawling, are the extremely low population sizes of modern rhinos a different story. As the paper notes, the average genetic diversity observed in four modern rhino genomes was measured at about half of what was seen in ancient genomes (the Javan rhino was included as a historical species because its DNA came of an individual who lived 200 years ago, before human influences on rhino populations).
Rhinos, as the study suggests, successfully purged unhealthy mutations over the past 100 years, but current rhinos face lower levels of genetic variation and higher inbreeding rates compared to their ancestors. This is the result of excesshunting and destruction of habitats, and puts these species at risk of extinction.
Fortunately, the new document can report on current conservation efforts. Low genetic diversity, as research suggests, is not necessarily indicative that rhinos have problems. Rather, conservationists should focus on increasing the size of their population, rather than increasing their individual genetic diversity. In practice, “this means that the main focus of conservation should be to prevent illegal poaching and the destruction of the privileged habitat of rhinos,” Dalén said., and the approach should vary by species. African rhinos, for example, are threatened by poaching, while Sumatran rhinos are threatened by the destruction of their preferred habitat, he explained..
“That said, I also don’t think we can ignore the threat of low genetic diversity and inbreeding,” Dalén added. “All rhinos continue to have harmful mutations in their genome, although perhaps less than in ancient times. And given the small population sizes that most rhinos currently have, it is very likely that inbreeding will continue to increase in the future. If that happens, we will see an increase in genetic diseases. “
Dalén’s advice to conservation managers is to do everything possible to prevent poaching and protect the remaining habitat of rhinos, “if there is a chance that future generations will see these animals.”
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