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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), by Lu Ping

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Should Evolution be translated as "evolution" or "evolution"? Some researchers take a clear-cut stand in support of the use of "evolution", believing that the "progress" in evolution can be clearly defined and observed by comparing with the size of living groups at the sister group level, so the translation of "evolution" should not be abandoned.

In theory, it is not impossible to use "progress" to show better adaptation to the environment in a relatively short period of time when the environment is stable. However, the environment is a high-dimensional space, and changes frequently with time in various dimensions, and the direction of change is different; so the evolution of different species adapting to environmental changes is also different on a considerable time scale. it is impossible to make a simple comparison of the advantages and disadvantages.

The so-called soldiers have no constant potential, water has no normal shape, and the evolution is also impermanent. From the perspective of "the direction of evolution", the author will introduce to you the concept of "evolution", which is quite complex in time and space.

In fact, the change of species that takes place in nature is often not due to the improvement in the level at which newcomers play the game of survival, but because the rules of the game have changed-environmental changes have proposed new standards of adaptation. Because dinosaurs could not adapt to low temperature and low oxygen environment, they gave way to more adaptable mammals. Since the environment is changing all the time-is it possible to change the rules of the game again? At that time, where will mankind "evolve"?

Article | Lu Ping (Institute of Zoology, Chinese Academy of Sciences)

Whether the word "Evolution" should be translated as "evolution" or "evolution" has been a topic of discussion in Chinese scientific research and popular science circles for many years. The distinction between "evolution" and "evolution" often depends on whether the evolution process is directional-"evolution" has the meaning of moving forward, implying that evolution has a direction and a distinction between survival of the fittest and survival of the fittest, while the word "evolution" is neutral and does not reflect any direction.

So, can the actual process of biological evolution be said to be directional?

At the species level, biological evolution can be defined as "heritable changes in the genetic material and information of a species", which may lead to changes in macroscopic morphology and function. The process of change is divided into two steps. The first step is that heritable changes occur in the biological population (population) of a species, that is, mutations in genetic material (mutation). This process is almost entirely caused by random physical and chemical factors and is generally recognized to be undirectional.

The second step is to "fixation" the mutation in the whole population, that is, to reach the state that every individual in the population carries the mutation. There are two possible scenarios in this process: one is the neutral mutation that does not affect the survival and reproduction of the organism, that is, the neutral mutation of fitness (fitness), whose occurrence frequency in the whole species population evolves randomly and happens to reach a fixed state with a certain probability. This scenario is called genetic drift (genetic drift) and is obviously not directional. The other is the mutation that affects fitness, which tends to be "fixed" in the population because of natural selection if it makes the organism more adaptable to the environment. This natural selection-driven adaptive evolution (adaptation) scenario is also the source of what is commonly known as "evolutionary directionality".

Figure 1. Two steps in the process of biological evolution (drawn by the author)

Figure 2. The mutation is fixed in two scenes. Among them, natural selection is often regarded as "directional". Random mutation and neutral evolution aside, is the history of biological adaptive evolution directional? If we consider a scenario in which a species enters a new environment and does not fully adapt to a stable environmental factor, then under the action of natural selection, mutations that are more adapted to this environmental factor in the next generation will indeed spread in the population, that is, "survival of the fittest", a process that is obviously directional. Therefore, compared with discussing whether the evolution is directional or not, it is more reasonable to consider the degree and scale of evolution.

When we remove the assumptions of a single species and a single stable environmental factor in the above example, we will find that it is difficult to define the direction of evolution, because adaptive evolution has two characteristics, one is that the dimension is very high, and the other is that it changes with the environment.

Environment and adaptation is a high-dimensional so-called environment, which is a combination of many environmental factors, such as temperature, oxygen content, humidity, sunshine and so on. Each factor is quantified into a dimension, and the environment is a high-dimensional space; the "point" (actually a small subspace) that a species can occupy in this space is the so-called "niche".

Figure 3. Environmental space and niche (plotted by the author) in a niche, the characteristics of each dimension of the environmental space may exert selection pressure on species, so how do species adapt to a particular niche under the action of natural selection? This requires the introduction of another concept of space-character space. There are many functional traits (trait) in different organisms, such as body size, life span, eating habits and so on. The combination of different characters forms a character space. The dimensions of character space are different characters of organisms, while the dimensions of environmental space are different environmental factors. A species is a specific value combination of various traits, which has a specific fitness because of the adaptation to a niche. This mapping from character space to fitness is called fitness topographic map (fitness landscape).

The adaptation process of species to the environment (a niche) can be regarded as the process of climbing to the "mountain" with high fitness, which can be seen as the change of characteristics on the fitness terrain. Therefore, adaptive evolution is a specific heritable change under the action of natural selection in response to the selection pressure caused by some environmental factors in many character dimensions. When we expand the perspective from this single "mountain climbing" process to more species and more niches, we will find that the dimensions of adaptive evolution are very high and there are many directions.

Figure 4. For the fitness terrain in a particular environment, when only two characters are considered, the character value space is a two-dimensional plane, and the fitness terrain is like a real topographic map. The white dots in the picture represent a biological population, and it can be seen that its fitness increases gradually with the change of generations. First of all, different species may have different functional traits changes in the same environmental dimension in the same direction, so as to adapt to environmental factors continuously. (dynamic map) By Randy Olson and Bj ø rn «stman-Own work, CC BY-SA 3.0, https://commons.wikimedia.org/ w / index.php?curid=32273838. For example, also in response to the hypoxic environment encountered during diving, fishermen in Southeast Asia have enlarged spleen, which is suitable for storing more red blood cells to be released during diving, thus enhancing the oxygen-carrying capacity of blood without suffering from high blood pressure; cetaceans, on the other hand, increase the content of myoglobin in muscle and enhance muscle oxygen storage capacity. These two different evolution directions of traits can adapt to the hypoxia direction in the environmental dimension of oxygen content.

Secondly, different organisms may adapt to different directions of the same environmental dimension, that is, to different niches. Among birds, for example, birds that are good at long-distance flight are more likely to use fat metabolism for long-term continuous energy supply, while birds such as pheasants that need short-range flight are more likely to use carbohydrate metabolism for rapid explosive energy supply. This is the difference of adaptation direction caused by the difference of niche in the dimension of flight distance.

Finally, different organisms may have adaptive evolution in different dimensions of the environment and adapt to different niches. For example, in mammals, many plateau species adapt to hypoxia, low temperature, ultraviolet light and other factors, while many marine mammal species adapt to diving, low light and other factors, such as myoglobin increase, low light visual perception enhancement and so on, which is the adaptation direction of different biological groups in different environmental dimensions.

Therefore, the dimension of adaptive evolution is very high, and species with different niches have their own evolution directions, so it is difficult to compare and discuss directions on the scale beyond the species. Even sister groups are difficult to compare in multiple dimensions. For example, chimpanzees and humans are sister groups. Because chimpanzees adapt to arboreal life, their upper limbs are much stronger than humans, but obviously it doesn't make much sense to say that humans are more "backward" than chimpanzees in a certain direction, because of their different niches.

When we further expand the perspective in the time dimension, we will find that the earth's environment is changing not only in many dimensions, but also frequently, such as temperature changes in glacial and interglacial periods, which are repeated in geological history. As the environment is constantly changing, the local direction of the adaptation process in the corresponding dimensions is also constantly changing. The "prosperity" of a biological group in a period of geological history may be due to the fact that it has evolved highly adaptive characters under one or more environmental dimensions, so it can move towards a new niche space and appear radiation evolution. diversity increases, but even groups with a high degree of diversity will become mass extinction because of the dynamic changes of the environment, so it is difficult to define the direction of evolution statically.

Let's take the Mesozoic as an example. Compared with today, the Mesozoic era had higher ambient temperatures, higher oxygen content in the atmosphere, and high diversity of reptiles, including dinosaurs, pterosaurs, ichthyosaurs, etc., while early mammals were relatively less diverse. However, at the end of the Mesozoic era, events such as meteorite impact led to environmental changes such as the decrease of surface temperature and atmospheric oxygen content, while the metabolic characteristics and further evolution of early mammals were more adapted to this low-temperature and hypoxia environment, resulting in radiation evolution and increased diversity.

Figure 5: considering the different values of a single specific character of different species, they correspond to different fitness at a certain historical point in time (environment) to form a fitness topographic map; the fitness terrain will change with time (environment), and this change itself will affect the evolution trajectory of different species / biological groups. In the process, mammals seem to be more adaptable to the environment, in large part because the environment has changed-that is, the rules of the game have changed, not the skills that mammals use to play games. It's like in a sports meeting, when the athletes are competing in the sprint, some of them perform better, but suddenly change the event to a long-distance race, and the original winner will be eliminated. But the new winner may not be better in the sprint, and it is pointless to define the long-distance race as "the way forward", because it may not stop running sprints in the future. Similarly, environmental change is shaky and directionless, so adaptive evolution is difficult to define direction.

Furthermore, biology and biology interact with each other. As in the above example, the extinction of large reptiles caused by environmental changes "vacated" the niche, and the later thriving groups were able to further radiate evolution and increase diversity. Even biology and the environment interact with each other, and some niches are created by the forerunner group at all.

The example of cyanobacteria is very typical: a high degree of diversity of multicellular eukaryotes occurred in the Cambrian, and the earth's oxidation environment on which most eukaryotes flourished, it is the result of the photosynthesis of prokaryotes cyanobacteria to produce oxygen in the past 2 billion years. Without cyanobacteria, eukaryotes would be impossible to thrive, let alone the "direction" of evolution such as the effective use of oxygen.

Therefore, in the limited part of history, evolution has a direction, but in different environments, different directions can not be compared, so it is not necessary to emphasize the direction of evolution on a larger scale.

In the evolutionary tree based on existing species, the early groups are sparse due to extinction, while the later prosperity often gives people the impression of "evolutionary direction" and "progress". This situation does exist, and there are many examples, but it is not regular. Among bilaterally symmetrical animals, if you look at the prosperity of vertebrates and find sister groups of hemispheres, or echinoderms, it may seem that there are more vertebrate species and more "progress". But if we look at the comparison of the metatodonts composed of the above three groups and their sister groups of molting animals, one group of insects in the molting animals accounts for more than half of the existing species in the biosphere of the earth. Far more than vertebrates-it's hard to say that vertebrates are more progressive or represent the direction of evolution. For example, it seems that eukaryotes have evolved complex and diverse multicellular groups, but in the earth's biosphere, prokaryotes have always been higher than eukaryotes in terms of quantity abundance and group differentiation; and in eukaryotes, there are also cases in which multicellular groups such as yeast evolve into single-celled organisms. Therefore, the "direction" of the prosperity of groups and the increasing complexity of biological characters in the later stage of evolution history is the local pattern of the evolution tree, not a stable law.

To sum up, the environment is high-dimensional and changes frequently, and the adaptive evolution of species is a complex process in many directions, which is affected by the interaction between species and species, species and environment. The so-called direction of evolution is the adaptive evolution of a dimensional character change of a species and the resulting diversity radiation evolution under the selection pressure of a single stable environment in the local evolution history. A group that is more adaptable in one environment may not adapt in other environments. Therefore, the direction of the evolution process is only reflected in the local part of space and time.

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