I heard that the word "evolution" can no longer be used?

The word "Evolution" has two translations: "进化" and "进化". ​​Careful friends may find that the word "进化" has been rarely seen in recent years. Many people are only willing to use "进化". ​​Even "进化论" has been deliberately changed to "进化论". Some people even explicitly point out that the word "进化" is wrong.

For example, some typical remarks:

"The word evolution has been harmful to people for a long time. High school teachers even go to great lengths to prevent students from thinking it has a direction. It is better to use evolution, especially in popular science and mainstream media."

"The higher organisms we are talking about account for a very small part of all living things... The total number of all bacteria on this planet is far greater than the total number of so-called higher organisms... If they all evolved, why do most of life today still remain in a simple state?"

So what’s wrong with evolution? Is it really wrong?

This is what the evolution process looks like (Image source: 2008,2017 Leonard Eisenberg. All rights reserved. evogeneao.com)

From the literal meaning, "evolution" implies "progress" such as "upward, improvement, perfection, and development". "Evolution" does not show any trend, similar to "change". Therefore, the fundamental reason why many people oppose the use of "evolution" is that they do not recognize that life will show "progress" such as "upward, improvement, perfection, and development" in the process of evolution.

So does "progress" exist in the evolution of life on Earth?

Part 1

How to understand the "progress" in evolution?

The answer to this question seems to be found in biology textbooks: the jawed vertebrates developed mandibles to enhance their bite force; the emergence of amniotic eggs freed embryonic development from dependence on water; the formation of seeds freed the fertilization process of plants from the constraints of water; the appearance of wings in the winged subclass of the class Insecta improved movement efficiency... these are all "progress".

Why is the emergence of the above traits considered "progress"?

Because these traits bring benefits to the adaptation of organisms, making them more adaptable to the environment. In other words, the "progress" in evolution is meaningful only when it is implemented in the adaptation to the environment.

However, dialectically speaking, all benefits come at a price: for example, although the flying ability of insects improves the efficiency of movement, it also increases energy consumption; the formation of seeds also requires the mother plant to invest additional time and nutrients.

In addition, whether a trait is beneficial to an organism depends on the environment: for example, amniotic eggs allow embryonic development to break free from the constraints of water, which is beneficial to the terrestrial environment, but not necessarily if it returns to the water.

The amniotic egg creates a water environment for the embryo. Image source: Wikipedia

Careful readers may have noticed that if we only subjectively believe that "certain traits are excellent in adaptation, which leads to organisms with these traits being more adaptable to the environment than those without them", it is ultimately just a theoretical deduction and lacks evidence. In theory, we can also reversely deduce the disadvantages of these excellent traits.

Is there any objective evidence that these organisms are indeed better adapted to their environment? Yes!

Let us take jawed vertebrates as an example to see how the improvement of adaptability is manifested.

Before the emergence of jawed animals, vertebrates did not have mandibles (jawless animals). Jawless animals were widely distributed in the oceans from the Silurian to the Devonian. Later, a branch of jawless animals produced mandibles, and jawed animals were born. Since then, jawed animals and jawless animals have competed for living space on the earth.

Based on the lower jaw, jawed fish developed hard bones, while jawless fish still retained cartilage; then jawed fish developed swim bladders based on the hard bones, which allowed them to land on land, while jawless fish remained in the sea; later, jawed fish developed amniotic eggs, which allowed them to completely adapt to the terrestrial environment, while jawless fish were still just "fish" in the water.

The jawed animals that produced amniotic eggs were not only more adaptable to the terrestrial environment, but they were also able to return to the ocean several times, while the terrestrial jawed animals (i.e. amphibians) that did not produce amniotic eggs never returned once. What’s more amazing is that they flew into the sky several times (only two families of life have managed to crawl from the ocean to the land and then fly from the land to the sky: insects and amniotes. Among them, insect wings were only produced once, but amniotes produced them three times: pterosaurs, birds, and bats).

All in all, there are 50,000 species of jawed fish today, spread across the ocean, land, and sky, and even into space; while there is only one class of jawless fish left, the Cyclostomata, with about 70 species, and they are still "fish" that swim all day long.

Gnathodes were originally a small branch of jawless animals, but today they far outnumber jawless animals. Image source: Author: Zeng Gang

The jawless species appeared earlier than the jawed species, and should have had more time to expand their living space and develop their family size. Moreover, the distribution and species of jawed species at the beginning were far smaller than those of jawless species - they were not on the same starting line. But after hundreds of millions of years of competition, whether jawed species are better in adaptation, nature has given the answer.

This example shows that the phenomenon of "progress" in which "adaptability is improved" can be reflected by comparing the adaptation results of two families: that is, comparing the sizes of living taxa.

The size of living taxa is the result of the adaptation competition. Image source: Author Zeng Gang

There is another point that needs attention: compared with comparing adaptation outcomes, it is not feasible to discuss the strength of adaptation based on specific traits.

First of all, each organism has its own advantages in adapting to the environment: although jawed organisms have developed lower jaws and possess the ability to bite, jawless organisms have also developed large suction cup-shaped mouths and have professional blood-sucking abilities. How do we judge which is better?

In addition, the adaptation of an organism to the environment is the comprehensive result of the adaptation of all its traits, and one cannot single out a single trait to generalize. The level of adaptation of jawed and jawless organisms cannot be determined simply by whether or not they have mandibles, but also by the comprehensive adaptation between other traits and the environment.

In the end, each organism has infinite adaptive traits. If you talk about the advantages of jawed organisms and I talk about the advantages of jawless organisms, we will fall into a deadlock of the exhaustive method and will not be able to come to any conclusion.

So don't measure adaptation the same way you evaluate traits. Nature gives you the most objective results.

Image source of living jawless animals: Wikipedia

Part 2

Why can't you see the existence of "progress"?

Maybe there is something wrong with the comparison method.

The evolution from jawless to jawed animals has long been a well-known "common sense" in the biological community. If "progress" is so obvious as in this example, why do some people still think that "progress" does not exist? This has to mention one person: Stephen Jay Gould, who is an important figure against "progress".

He once wrote in his book "Full House. The spread of excellence from plato to darwin": "50% of vertebrates are bony fish. They fill the oceans, lakes and rivers. There are almost 100 times more species than primates. How can we say that they (bony fish) are not the backbone of evolution?"

Stephen Jay Gould Image source: Wikipedia

His views influenced many people.

For example, the author of the article “Is Evolution Progress?” believes that: “…According to the views expressed by Lamarckism and Social Darwinism, cartilaginous fish are considered inferior and bony fish are considered superior. However, whether from the perspective of adaptation to the environment or the perspective of position in the food chain, it is difficult to believe that sharks, as cartilaginous fish, are inferior to cod, as bony fish.”

Screenshot of the article "Is Evolution Progress?"

There is another point of view: "Bacteria have experienced billions of years of vicissitudes and are still huge in number and ubiquitous, while humans have only existed for a few hundred thousand years. How can humans' adaptability be compared with that of bacteria..."

Image source: Gould's The Magnificence of Life: From Plato to Darwin

In short, the three views are: primates have not improved compared to bony fish, bony fish have not improved compared to cartilaginous fish, and humans have not improved compared to bacteria. These three views do not directly evaluate traits, but compare the results of biological adaptation. Why do they reach conclusions that do not support the existence of "progress"?

This is because they all made a serious logical error: instead of linking life together according to phylogenetic relationships, they viewed it as a pile of loose sand and treated it in isolation. Comparing primates with bony fish, sharks with cod, and humans with bacteria are all examples of a pile of loose sand.

A scattered perspective will view these 10 objects in parallel. However, in systematics, the status of bacteria is equivalent to the sum of the other 9 (which is not enough), so no matter which one is selected to compare with bacteria, it is incorrect. Image source: Author Zeng Gang

You may ask: comparing one type with another, another family with another, another class with another... does that belong to the perspective of phylogeny?

Neither! Because whether a taxonomic group is called a "domain", a "kingdom", a "phylum", a "class", an "order", a "family"... is purely a man-made definition, and there is no comparability between them. Only "species" has a certain objectivity.

For example, although the two families of angiosperms, Ampelopsaceae and Asteraceae, are both families, if we treat these two families on an equal level, we still lack phylogenetic thinking and regard life as "a plate of scattered families". In fact, the family on an equal level with Ampelopsaceae is the sum of all other families of living angiosperms. In other words, angiosperms are divided into two branches, one of which only Ampelopsaceae survives to this day, and the other branch survives to this day, including all angiosperms except Ampelopsaceae.

Perhaps you will ask: Since "species" has a certain objectivity, why is it wrong to compare that kind with species?

Let's take the example of Camphora: Since Camphora only has one species in the Camphoraceae family, the species that is on the same level as Camphora is still the sum of all the remaining nearly 300,000 angiosperm species. One species vs. the remaining 300,000 species... It can be seen how unbalanced their development is after they parted ways. It is this unbalanced development that reflects the existence of "progress".

In cladistics, the relationship between Ampelopsis and all other angiosperms, cyclostomes, and gnathostomes is called a sister group relationship. A sister group is two families that arise from the same ancestor, or two branches that arise from the same branching point on the tree of life. The sister group relationship is the only equal relationship.

Part 3

What is the correct comparison?

The answer is: compare among sister groups.

This comparison is correct for three reasons:

First, the size of the sister group is fixed and not artificially selected. If the left hand chooses bony fish and the right hand chooses primates, the result of the comparison depends entirely on the size of the group each chooses, which is not objective at all.

Second, the sister groups have the same development time. Bony fish originated in the Late Silurian period 400 million years ago, while primates originated in the Late Cretaceous period less than 100 million years ago. Bony fish have more than 300 million years of development time than primates. Is this fair?

Third, there is a direct genetic relationship between sister groups. Bony fish and primates, sharks and cod, humans and bacteria are all "not related at all", and the results of comparison cannot explain anything. Only by comparing sister groups with direct genetic relationships (such as Cyclostomes and Gnathodes) can we explain whether evolution has "progress".

Correct and incorrect comparison methods (the two sides of the comparison are represented by orange and blue) Image source: Author Zeng Gang

If we re-examine the above viewpoints using the sister group comparison method, we will find obvious "progress".

Let’s first look at the first view (that primates have not made any progress compared to bony fish): Primates belong to mammals, and mammals are just one branch of the amniotes shown in the figure below. In addition, amniotes also include birds and reptiles, a total of more than 21,000 species. Bony fish include ray-finned fish (24,000 species), coelacanths (2 species) and lungfish (6 species).

If we follow the sister group comparison principle, teleosts must be separated: compare ray-finned fishes with “coelacanths + lungfishes + amphibians (4,000 species) + amniotes” (orange and blue in the image below);

Or compare the coelacanth to “lungfish + amphibians + amniotes” (orange and blue in the image below);

Or compare lungfish to “Amphibians + Amniotes” (orange and blue in the image below).

In these three comparison methods, the orange side is only distributed in the water, while the blue side is widely distributed in the water, land and sky. In terms of variety, the blue side is always the same as the orange side. Therefore, the blue side can win in all three comparison methods, showing obvious progress.

Let's look at the second view (that there is no progress between bony fish and cartilaginous fish): If we compare the entire bony fish (ray-finned fish + coelacanth + lungfish) with cartilaginous fish, cartilaginous fish will be completely defeated. But this still does not conform to the sister group principle, because the sister group of cartilaginous fish is not bony fish, but the broad bony fish (nearly 50,000 species) that includes amphibians and amniotes.

If we artificially select bony fish (24,000 species) from the broad category of bony fish for comparison, then cartilaginous fish have an advantage. However, there are less than 1,000 species of cartilaginous fish, and very few of them are freshwater species, so even if they have an advantage, they still cannot be compared.

Let's look at the third view (there is no progress between humans and bacteria): humans belong to a species of eukaryotes, and bacteria belong to a group of prokaryotes, which includes many species. A direct comparison between the two obviously does not conform to the sister group principle.

If bacteria refers to "true bacteria", then its sister group should be the sum of the blue parts in the figure below, which is the main body of today's ecosystem and is much larger than true bacteria. If we must compare it with humans, then the object of comparison should be the sister group of humans - chimpanzees. It is obvious which of these two families is more widespread and more populous.

Image source: Author Zeng Gang

If we compare them using the sister group method, we will find many examples of "progress".

We know that evolution includes two steps: the generation of mutations and the retention of mutations. The generation of mutations is completely random and directionless, but the retention of mutations is not necessarily the case: although genetic drift, bottleneck effect, founder effect, etc. are directionless random factors, natural selection is not random. In addition, the environment and resources are limited, and the selection pressure caused by competition will also favor "stronger and more perfect" organisms.

Due to the role of natural selection, evolution is destined not to be random and directionless like mutation. Although this direction cannot be unified and specific, it can be abstract: that is, it develops in the direction of "upward, improvement, and perfection" of adaptability.

Part 4

The relationship between adaptability and environment

Some people say: Fish and quadrupeds (amphibians + amniotes) live in water and land respectively, and their living environments are completely different. How can we compare their adaptability?

This view has a rigid understanding of adaptability.

Fish are not restricted to water, and quadrupeds are not restricted to land. On a large scale of life evolution, the environment is fair and open, and it all depends on whether the organisms have the ability to adapt.

After the four-legged animals came to land, they went down to the sea several times to invade the fish's territory (how did they "go down to the sea"), but the fish did not go down to the sea to invade the four-legged animals' territory again. This shows that there is a clear difference in their ability to adapt to the earth's environment.

Some mammals living in the ocean. Image credit: NOAA

There is also a view that: although eukaryotes appear to be "evolved" today, if a major disaster such as a meteorite impact occurs, only bacteria will survive, so bacteria are more adaptable.

This view is nothing more than an unrealistic fantasy.

After the birth of eukaryotes, there have been more than one meteorite impact event, but they still became the absolute main body of today's ecosystem. At the same time, meteorite impacts did not allow prokaryotes to seize even one opportunity to turn things around and surpass the diversity of eukaryotes again (the reason why I say "surpass again" is that when eukaryotes first appeared, the diversity of prokaryotes must have far exceeded that of eukaryotes). Even if eukaryotes really became extinct, we cannot ignore the "progress" they have shown over billions of years just because of occasional disasters.

Meteorite impacts have occurred more than once on Earth, but the final result is that eukaryotic organisms have become dominant on Earth. Image source: Pixabay

Conclusion

As discussed above, the reason why there is "progress" in the evolution of life is because natural selection is at work. Only in an environment where natural selection does not work at all will there be no "progress". But natural selection is probably everywhere.

Of course, even if natural selection is everywhere, it does not mean that every branch on the tree of life has obvious "progress". In cases where "progress" is not obvious, we can certainly use "evolution". But "progress" is often very obvious in many large branches and cannot be ignored.

Why can't we ignore it? Because in a world without "progress", the older groups would be more prosperous, and the appearance and pattern of life would not be what it is today: jawed animals would not have an absolute advantage over vertebrates; angiosperms would not account for more than half of all plants; winged insects would not become the vast majority of the class Insecta; and even eukaryotes would not become the main body of today's ecosystem... But the real world is just the opposite.

In such an objective world where "progress" cannot be ignored, should we really avoid using "evolution" and only use "evolution"?

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Author: Zeng Gang

Author unit: Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences

Source: Science Institute

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