▎ WuXi AppTec content team editor
What are the possible causes when two species have similar trait traits? The answer may be that the two are closely related, or it may be the result of convergent evolution – species with more distant relatives can evolve somewhat similar traits in order to adapt to the same environment. However, a new study tells us that in addition to these familiar answers, some random events can also cause this phenomenon.
In the latest study, published online in the journal Cell, Professor Zhang's team revealed that during rapid species differentiation, a class of random events can also lead to similar phenotypes in distant species. This discovery provides important clues for accurately understanding the evolution of life.
From the most primitive single-celled organisms to complex humans, many species in nature have collectively formed a "tree of life" – a tree that reflects the continuous differentiation and evolution of species from a common ancestor to the present. Finding their place on the Tree of Life is crucial for all species, but it can also be challenging.
How do you infer the evolution of a species? Early scientists took advantage of trait traits from different species: the more similar traits they had, the closer they were to each other. Based on such a principle, scientists can depict morphological trees of different species.
And with the discovery of genetic material DNA, scientists have also been given new tools to map the tree of life. Differences in DNA sequences between species can reflect the differentiation process of species, that is, the more similar the DNA sequences, the closer the species are to the tree of life.
Since the phenotype is determined by DNA, the closer the evolutionary relationship of species carrying the same phenotype in theory, so both morphological and DNA data should be used to construct species trees. In practice, however, molecular and morphological trees constructed from DNA often have contradictory results, especially in taxa that have experienced rapid species differentiation.
What causes such a contradiction? Which means are used to trace the origin of the species, and the results obtained are more reliable?
Scientists have found that incomplete lineage sorting is a factor that can lead to the above contradictions. Simply put, when multiple species rapidly differentiate from a common ancestor, they may randomly acquire different genotypes of certain genes of their ancestors. At this time, these specific genes are more similar in which species, which is a random event.
Schematic diagram of incomplete lineage shunts during early speciation of marsupials. The latest study uses marsupials to reveal the effects of incomplete lineage shunts on evolution (Source: Feng Shaohong et al.)
Let's take people, chimpanzees, and gorillas as examples to understand this phenomenon. We know that humans are more closely related to chimpanzees than gorillas. As shown in the figure below, starting from the common ancestor of the three, the first species differentiated to form gorillas, and they eventually fixed the blue genotype. At the same time, the common ancestors of humans and chimpanzees inherited orange and blue genotypes, and at the second time the species differentiated, such a random phenomenon may occur: humans fixed the blue genotype, while chimpanzees randomly fixed the orange genotype.
▲ Incomplete genealogical shunt diagram: each group of two dots represents an individual, and each dot represents a gene (image source: Feng Shaohong et al. painting)
At this point, we will observe that some of the genome sequences of humans are more similar to gorillas and more different from chimpanzees. This is also reflected in the genome sequencing data: more than 15% of the genome regions in the human genome are more similar to gorillas.
In fact, scientists have observed incomplete lineage shunts in a variety of animal populations that have experienced large explosions of species, but it is unclear whether this phenomenon has an effect on the evolution of traits.
To answer this question, in the latest study, Professor Zhang Guojie's team used marsupials that have experienced species outbreaks to conduct research. For a long time, there has been a controversy over the evolutionary relationship of marsupials, and the source of the controversy lies in the evolutionary status of the South American order Microbiotheria.
As the only extant species of microdermazoa, the small mountain monkey in South America is more similar to the marsupials in Oceania (especially the kangaroos, koalas and other dipterodons) in terms of bones, reproductive organs and brain structure. Therefore, early research speculated that the small mountain monkey should be more closely related to the kangaroo and koala, and the small mountain monkey in South America should originate in Oceania.
▲ A small mountain monkey eating (Image: Lida M. Franco)
However, the latest research has completely overturned this conjecture through genome-wide studies. The findings suggest that the little mountain monkey shares a common ancestor with the marsupials of Oceania, but does not belong to the latter. Interestingly, in the marsupial genome, more than 50% of the regions construct molecular trees that are inconsistent with the real process of species differentiation. An intuitive manifestation is that the similarity between the small mountain monkey and some Oceanian marsupials is higher than the similarity between Oceania marsupials.
Subsequently, the research team used specimens from the museum collection to confirm that many of the skeletal morphological characteristics of the small mountain monkey are more similar to those of kangaroos and koalas. After further screening out the candidate genes, the authors verified in a mouse model that genotype replacement affected by incomplete lineage shunts did produce phenotypic results that matched expectations.
▲ The little mountain monkey is a group of marsupial sisters in Oceania, not inside the marsupial species in Oceania (Image source: Feng Shaohong et al.)
Therefore, in the case of marsupials, it is likely that it is the incomplete lineage shunt that causes the trait characteristics between species to be inconsistent with the real evolutionary process. Some different taxa that look more alike, or even some of the DNA regions are closer together, may in fact be more distantly related.
Obviously, the randomness of incomplete lineage shunts complicates the mapping of the Tree of Life. It also reminds us that if we directly rely on some genes or some similar traits to trace the evolutionary path of species, it is likely to be misleading and produce wrong results. For the study of biological evolution, this study tells an important criterion: genome-wide data is the gold standard for reconstructing the course of species.
The results have been published online in the international top academic journal "Cell" magazine, by Professor Zhang Guojie's team, Shenzhen Huada Institute of Life Sciences, Kunming Institute of Zoology, Chinese Academy of Sciences, Institute of Zoology, University of Copenhagen and other research groups to complete. Professor Zhang Guojie, the corresponding author of the paper, recently joined the School of Medicine of Zhejiang University as a chair professor to establish the Life Evolution Research Center. The co-first authors of the paper are Dr. Feng Shaohong, who is currently a researcher at Zhejiang University, and Bai Ming, a researcher at the Institute of Zoology, Chinese Academy of Sciences.
Resources:
[1] Shaohong Feng, Ming Bai et al. Incomplete lineage sorting and phenotypic evolution in marsupials. Cell(2022). DOI: https://doi.org/10.1016/j.cell.2022.03.034
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