The differences between different species have increased with the passage of time, and some endemic species have gradually formed around the world. However, a study published in the academic journal Nature Communication shows that between two crows that formed reproductive isolation 6.5438+0.4 million years ago, the situation that was previously considered irreversible seems to have reversed.
Why is there reproductive isolation? Reproductive isolation is a gene protection mechanism between different species on the earth, and genes between different species cannot be exchanged naturally. For example, horses and donkeys, although these two animals can mate and give birth to mules, mules can't breed the next generation, so they can't be regarded as an animal, just like tigon. That's Darwin's basic view in evolution, that is, the origin of evolutionary tree.
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One of the most important factors is the separation of geographical location. Darwin discovered this in the year he traveled around the world: two different creatures were born on two adjacent islands. Now we know that with the drift of continents, some isolated continents have evolved some unique species, such as polar bears in the Arctic, kangaroos in Australia, penguins in the Antarctic and so on.
Of course, there are also a few red pandas living in Sichuan, China, which is unique in the world. The phylogenetic tree may be wrong, because hybridization is much richer than we thought. Life on earth is not like a big tree, but more like a complex network. There are two kinds of crows in North America, which vividly show us what is called "anti-speciation". These two long-divided species have actually begun to cross and merge again!
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Geographical isolation is not the only reason, there are other reasons for living habits, such as different day and night activities, different seasons, different living environments, different physiological structures, etc ... But the subsequent appearance of these factors will eventually be traced back to geographical isolation, because the differences in gene variation among individuals caused by gene exchange in the same area will soon disappear.
Can reproductive isolation be reversed? Genetic variation is random and generally will not be reversed, but in the case of near species and artificial intervention, reproductive isolation can be broken to some extent. But occasionally, the offspring of distant species are often infertile, which leads to the failure of new varieties to produce the next generation, thus truly breaking through the reproductive isolation between the two species. For example, the most familiar hybrid mule of horses and donkeys is the representative of infertility. Although there are occasional special circumstances, the characteristics of mules cannot be inherited.
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For example, a female mule mated with a donkey and gave birth to a new generation of ponies. The appearance of a pony is the same as that of a mule, but the characteristics of infertility are the same as those of a mule. The female mule mates with the horse and gives birth to the next generation of foals, which have the same appearance as the horse and have fertility ... but as a result, no new species can be produced. However, a study published in the journal Nature Communication entitled "Genomic Evidence for the Formation Reversion of Raven Species" found that two raven populations were mixed together to produce a new species.
Crows subvert evolution? Reverse the phenomenon of reproductive isolation. Raven is a large crow, which is the most widely distributed species at present. None of them is this kind of crow, which is distributed in a vast area of the northern hemisphere, so you can call it crow directly (but for the sake of distinction, it is called crow here). According to the study of DNA genetics, ravens can be divided into two main branches according to their evolutionary history: one is the California branch, which lives in the southwestern United States, and the other is the whole northern part of other parts of the northern hemisphere.
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The research shows that the two species formed reproductive isolation about 6.5438 0.5 million years ago, forming two different branch species, which is obviously caused by geographical isolation. According to scientists' analysis, during the Ice Age, glaciers in the north covered a large area, and Alaska, which connects Eurasia and North America, was blocked by glaciers all the year round, making it impossible for crows in the two places to migrate, while ravens in the north and ravens in California, North America, could not achieve gene exchange, eventually forming reproductive isolation.
However, due to the melting of glaciers, the whole northern crow originally living in Eurasia began to migrate to the North American continent through the Bering Strait and Alaska. Scientists speculate that the whole northern raven may enter America at the same time as Homo sapiens. Although the entire northern branch and the California branch of North America have been isolated, and the isolation time is relatively long, and the genetic differences are relatively large, they still have similar shapes and habits, and gradually form an intersection near California in North America.
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Through the analysis of mitochondrial DNA, it is found that the California branch of North America is genetically mixed with the whole northern branch, that is, in the past tens of thousands of years, two different distant relatives have lived together in the same area and naturally hybridized, and for unknown reasons, they have bred the next generation. This fusion led to the extinction of the original purebred California raven, which became a mixed new species-the common raven.
A powerful race without assimilation-Chihuahua Raven As shown in the picture, there is a raven that has never been seen before, called Chihuahua Raven. According to the genome analysis, Chihuahua ravens, all independently evolved in southwest North America, are more closely related to the whole northern ravens. According to the analysis, they separated a new species about 6.5438+0 million years ago, which was obviously isolated from the whole northern raven and did not genetically mix with the California raven.
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Crossbred with the existing all-northern California-the common raven also has obvious reproductive isolation, maintaining a noble and pure bloodline. It's like a group of crows who are superior to others, just like Jews among crows ... These two crows have been fused for nearly 400,000 years. Crows, which have become two independent species after the melting of glaciers, seem to disagree, so they used to cross frequently.
Although for a long time before, their hybrid offspring could not produce the next generation, but this did not make crows yield. They spent 400,000 years rewriting their population history. The reproductive isolation in nature seems so indestructible, but the evolutionary history of North American ravens seems to have broken through this limitation, even now, from entering the American continent with humans to the present.
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The genetic difference between the common ravens that have been mixed for tens of thousands of years and the ravens in the whole northern region is still as high as 4%, far exceeding the difference between ordinary birds. However, genome analysis clearly shows that this cross-species mixing phenomenon did happen, and in the natural environment, millions of years of intense reproductive isolation was broken!
In the distance of 900 kilometers between Washington State and Canada, you don't know which is the northwest crow and which is the American crow. Although the American ornithological society still thinks that they are two different animals, it is believed that crows will overcome human prejudice one day.