"Evolution" or "evolution"? The key lies in scale

Author's Note:

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

In theory, in a relatively short period of time when the environment is stable, it is OK to use "progress" to indicate better adaptation to the environment. However, the environment is a high-dimensional space, and it changes frequently over time in various dimensions, and the directions of change are different; therefore, the evolution of various species in response to environmental changes is also different over a considerable time scale, and it is impossible to make a simple comparison of advantages and disadvantages.

As the saying goes, there is no fixed position for soldiers, no fixed shape for water, and no fixed direction for evolution. I will start from the perspective of "the directionality of evolution" and introduce the concept of "evolution" which is quite complex in both time and space.

In fact, the replacement of species in nature is often not because the latecomers have improved their survival game skills, but because the rules of the game have changed - environmental changes have put forward new adaptation standards. Dinosaurs could not adapt to low temperature and low oxygen environments, so they gave way to more adaptable mammals. Since the environment is always changing - is it possible that the rules of the game will change again? At that time, where will humans "evolve" to?

Written by 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 frequent discussion in the Chinese scientific research and popular science circles for many years. The distinction between "evolution" and "evolution" often focuses on whether the evolutionary process has a direction - "evolution" means moving forward, implying that evolution has a direction and there is a distinction between the survival of the fittest and the low, while the word "evolution" is neutral and does not reflect any direction.

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

At the species level, biological evolution can be defined as "heritable changes in a species' genetic material and information," which may lead to changes in macroscopic morphology and function. The process of change is divided into two steps. The first step is the appearance of heritable changes in a species' biological population, that is, mutations in genetic material. This process is almost entirely caused by the random influence of physical and chemical factors and is generally recognized to have no directionality.

The second step is the "fixation" of the mutation in the entire population, that is, reaching a state where every individual organism in the population carries the mutation. There are two possible scenarios for this process: one is a neutral mutation that does not affect the organism's ability to survive and reproduce, that is, fitness. Its frequency of occurrence in the entire species population evolves randomly, and there is a certain probability that it happens to reach a fixed state. This scenario is called genetic drift, which is obviously not directional; the other is a mutation that affects fitness. If this type of mutation makes the organism more adaptable to the environment, it will tend to be "fixed" in the population due to natural selection. This adaptive evolution scenario driven by natural selection is also the source of what people usually think of as "evolutionary directionality."

Figure 1. Two steps in the biological evolution process (author’s drawing)

Figure 2. Two scenarios of mutation fixation. In these scenarios, natural selection is often considered to be "directional". (Author's illustration)

Leaving aside random mutations and neutral evolution, can the history of adaptive evolution of organisms be said to be directional? If we consider a scenario where a species enters a new environment and is not fully adapted to a certain stable environmental factor, then under the action of natural selection, mutations in the next generation that are more adapted to the environmental factor will indeed spread in the population, that is, "survival of the fittest", and this process is obviously directional. Therefore, compared to discussing whether evolution is directional, it may be more reasonable to consider to what extent and at what scale evolution is directional.

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

Environment and adaptation are high-dimensional

The so-called environment is a combination of many environmental factors such as temperature, oxygen content, humidity, sunshine, etc. If each factor is quantified into a dimension, 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 ecological niche (created by the author)

In an ecological niche, the characteristics of each dimension of the environmental space may exert selection pressure on species. So how do species adapt to a specific ecological niche under the action of natural selection? This requires the introduction of another spatial concept - trait space. Different organisms have many functional traits, such as body size, life span, diet, etc. The combination of different traits constitutes a trait space. The dimensions of the trait space are different traits of organisms, and the dimensions of the environmental space are different environmental factors. A species is a specific combination of values ​​of various traits, and has a specific fitness because of its adaptation to an ecological niche. This mapping from trait space to fitness is called a fitness landscape.

The process of species adapting to the environment (an ecological niche) can be seen as a process of changing traits on the fitness terrain and climbing to the "peak" of high fitness. Therefore, adaptive evolution is a specific heritable change in many trait dimensions of organisms under the action of natural selection in response to the selection pressure caused by certain environmental factors. When we expand our perspective from this single "climbing" process to more species and more ecological niches, we will find that the dimensions of adaptive evolution are very high and there are many directions.

First, for the same direction of the same environmental dimension, different species may undergo different functional trait changes to continuously adapt to the environmental factors. For example, in response to the low oxygen environment encountered during diving, the spleens of Southeast Asian fishermen are enlarged, suitable for storing more red blood cells for release during diving, thereby enhancing the blood's oxygen carrying capacity without suffering from high blood pressure; while cetaceans increase the myoglobin content in their muscles to enhance the muscles' oxygen storage capacity. These two different trait evolution directions can both adapt to the low oxygen direction in the environmental dimension of oxygen content.

Secondly, different organisms may adapt to different directions in the same environmental dimension, that is, to different ecological niches. For example, among birds, those that are good at long-distance flight tend to use fat metabolism for long-term continuous energy supply, while birds that need short-distance flight, such as pheasants, tend to use carbohydrate metabolism for rapid burst energy supply. This is a difference in adaptation direction caused by differences in ecological niches in the dimension of flight distance.

Finally, it is entirely possible that different organisms have evolved adaptively in different dimensions of the environment and adapted to different ecological niches. For example, among mammals, many plateau species have adapted to factors such as low oxygen, low temperature, and ultraviolet rays by increasing the oxygen-carrying capacity of hemoglobin, while many marine mammal species have adapted to factors such as diving and low light by increasing myoglobin and enhancing low-light visual perception. This is a difference in the direction of adaptation of different biological groups in different environmental dimensions.

Therefore, the dimensionality of adaptive evolution is very high, and species in different ecological niches have their own evolutionary directions, which makes it difficult to compare and discuss directions on a scale beyond species. Even sister groups are difficult to compare in multiple dimensions. For example, chimpanzees and humans are sister groups. Chimpanzees have far superior upper limb strength to humans because they are adapted to arboreal life, but it is obviously meaningless to say that humans are more "backward" than chimpanzees in a certain direction, because the two have different ecological niches.

Adaptation direction changes frequently as the environment changes

When we further expand our perspective in the time dimension, we will find that the earth's environment has undergone tremendous changes. Not only are there many dimensions, but the changes are also frequent. For example, the temperature changes between glacial periods and interglacial periods have repeatedly appeared in geological history. As the environment is constantly changing, the local directionality of the adaptation process in the corresponding dimension is also constantly changing. The "prosperity" of a biological group within a period of geological history may be due to the fact that it has evolved highly adaptive traits in one or several environmental dimensions, so it can move to a new ecological niche space, undergo radiation evolution, and increase diversity. However, even highly diverse groups can experience large-scale extinction due to dynamic changes in the environment, which makes it difficult to statically define the directionality of evolution.

Let's take the Mesozoic Era as an example. Compared with today, the environmental temperature in the Mesozoic Era was higher, the oxygen content in the atmosphere was higher, and reptiles were highly diverse, including dinosaurs, pterosaurs, ichthyosaurs, etc.; early mammals were relatively less diverse. However, at the end of the Mesozoic Era, events such as meteorite impacts caused environmental changes such as a drop in surface temperature and a drop in atmospheric oxygen content. The metabolic characteristics of early mammals and their further evolution were more adapted to this low-temperature, low-oxygen environment, and radiation evolution occurred, increasing diversity.

Figure 5: Considering the different values ​​of a single specific trait of different species, they correspond to different fitness at a certain historical point in time (environment), forming a fitness topography; this fitness topography will change with time (environment), and this change itself will affect the evolutionary trajectory of different species/biological groups.

In this process, mammals appear to be more adaptable to the environment, largely because the environment has changed - that is, the rules of the game have changed, not because mammals have improved their game-playing skills. This is like in a sports meeting, where athletes are competing in sprints and some perform better; but suddenly the event is changed to long-distance running, and the original winner will be eliminated. However, the new winner may not be better in sprinting, and it is meaningless to define the long-distance running victory as the "direction of progress", because there may not be no sprinting in the future. Similarly, environmental changes are oscillating and directionless, so it is difficult to define the direction of adaptive evolution.

Furthermore, organisms interact with each other. In the above example, the extinction of large reptiles caused by environmental changes "vacated" ecological niches, and the flourishing groups later could further radiate and evolve, increasing diversity. Even organisms and the environment interact with each other, and some ecological niches are simply created by the "pioneer" groups.

The example of cyanobacteria is very typical: a high degree of diversification of multicellular eukaryotes occurred in the Cambrian period, and the oxidative environment of the earth on which most eukaryotic cells thrive is the result of prokaryotic cyanobacteria producing oxygen through photosynthesis over nearly two billion years. Without cyanobacteria, there would be no prosperity of eukaryotes, let alone the "direction" of evolution such as the effective use of oxygen.

Therefore, in a limited part of history, evolution has a direction; but in different environments, the various directions cannot be compared, so there is no need to emphasize the directionality of evolution on a larger scale.

In the evolutionary tree constructed with existing species, the early groups are sparse due to extinction, and the prosperity of the later groups often gives people the impression of "evolutionary directionality" and "progress". This situation does exist, and there are many examples, but it is not a rule. Among bilaterally symmetrical animals, if you look at the prosperity of vertebrates, and find the sister group hemichordates, or find echinoderms, it may seem that there are more vertebrate species and more "progressive"; but if you expand outward and compare the deuterostomes composed of the above three groups with their sister group ecdysozoa, then the insect group among ecdysozoa accounts for more than half of the existing species in the Earth's biosphere, far exceeding vertebrates - it is difficult to say that vertebrates are more advanced or represent the directionality of evolution. For example, it seems that eukaryotes have evolved into complex and diverse multicellular groups, but in the Earth's biosphere, prokaryotes have always been higher than eukaryotes in terms of both abundance and group differentiation; and among eukaryotes, there are also cases where multicellular groups such as yeast have evolved into single-cell organisms. Therefore, the prosperity of taxa in the later stages of evolutionary history and the "directionality" of increasing complexity of biological traits are local patterns of the evolutionary tree, not stable laws.

In summary, the environment is high-dimensional and changes frequently, and the adaptive evolution of species is a multi-directional complex process, which is affected by the interaction between species and species, and between species and the environment. The so-called directionality of evolution refers to the adaptive evolution of a certain dimensional trait of a species under the selection pressure of a single stable environment in the local evolutionary history, and the resulting radiation evolution of diversity. A group that is more adaptable in one environment may not be adaptable in other environments. Therefore, the directionality of the evolutionary process is only reflected in the local space and time.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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