Brian Ross wrote on Nov 13th, 2019 at 2:14pm:
rhino wrote on Nov 12th, 2019 at 10:22pm:
Has it? Can't read your article 'cause it insists that I turn off my ad blocker - something I refuse to do.
Until you post the full article I suspect I'll never read it, Rhino. ::) |
|
Here it is, Bwian.
A Long-Busted Myth: It's Not True That Animals Belonging To Different Species Can Never Interbreed
Michael Marshall Contributor Science I write about evolutionary biology, earth science and the environment
On 22 August, an astonishing discovery was reported. A sliver of bone from a cave in Russia turned out to belong to a hominin, one that was utterly unprecedented. Denny, as she is known to the scientists who are studying her, was a first-generation hybrid. Her mother was a Neanderthal and her father a Denisovan. She was a child of two species. The findings were reported in Nature.
I covered this discovery, as did many others, and within hours the question arrived: "I thought the definition of a species was that they couldn't interbreed?"
Many people seem to believe that animals belonging to different species cannot breed together, and that this is what defines a species. I suspect many of us acquire the idea in childhood when we learn about mules. The offspring of a horse and a donkey, a mule is a useful working animal but is entirely sterile and incapable of breeding. We all seem to generalise from this and assume that no interspecies pairings can produce fertile offspring.
This is not just a piece of folk science. The biologist Ernst Mayr proposed in 1942 that a species is a population of organisms that can all interbreed with each other, and which either cannot or do not interbreed with anything else. This idea became known as the Biological Species Concept, and evidently many of us learn it as fact.
The thing is, Mayr's idea is not accepted as the be-all-and-end-all by other biologists. Instead, the problem of how to define a species is still being argued about today, 76 years after Mayr published his definition.
Let's come back to mules. They are not a terribly good example of what happens when two species interbreed. Horses have 64 chromosomes and donkeys 62, so when the two breed their mule offspring ends up with 63. Because this is an odd number, it's impossible for them to divide evenly into two. That means the mule cannot produce sperm and egg cells that carry exactly half the animal's chromosomes, as should happen. When these defective sex cells are fused with those of another mule, the resulting embryo is likely to have crucial chunks of its DNA missing, and will not be viable.
However, many distinct species have the same numbers of chromosomes. For instance, all great apes (apart from humans) have a total of 48 chromosomes, arranged in 24 pairs. All else being equal, that means it ought to be easier for them to interbreed than it is for horses and donkeys.
So it has proved. Chimpanzees and bonobos have interbred several times since their populations split a few million years ago, and the bonobo genome also carries DNA that seems to have come from a third, unidentified species. Other ape pairings don't seem to have happened, but that might be partly because they live in separate habitats and don't meet: orangutans are confined to Borneo and Sumatra, and are unlikely to encounter gorillas and chimpanzees from Africa. But the idea captivates people: there are long-standing (unsubstantiated) rumours of a chimpanzee-gorilla hybrid called the koolakamba or kooloo-kamba.
Similarly, human evolution was rife with interspecies sex. Modern humans have interbred with both Neanderthals and Denisovans, Neanderthals and Denisovans interbred, and Denisovans interbred with an unidentified hominin. There is reason to suspect that the first-generation hybrids had some health issues, such as reduced fertility, but they were evidently able to get by well enough to leave descendants. Today many people carry some Neanderthal and/or Denisovan DNA.
This illustrates the problem with Mayr's species concept: where do you draw the line? If two animals can produce offspring, but that offspring's fertility is reduced by 10 per cent, are the parents members of different species? What about a 20 per cent drop in fertility - or a 10 per cent drop in fertility combined with a 20 per cent reduction in average lifespan? We could insist that the offspring be 100 per cent infertile, but that would mean collapsing a lot of species that we currently think of as distinct, beginning with chimpanzees and bonobos. Insisting that no offspring are produced at all would destroy even more distinctions.
Species are often separated, not by reproductive anatomy or courtship habit, but by geography - and those separations are reversible. In the lakes of the European Alps, pollution has caused oxygen levels to crash in the deeper waters, forcing the species that once lived there to move closer to the surface. There they have begun hybridising with longstanding surface-dwellers. These species had been separated for millions of years, but they weren't distinct enough to be unable to breed.
In fact it has been estimated that 88 per cent of all fish species could hybridise with at least one other, given the opportunity. The same may be true of 55 per cent of all mammals.
