How is the direction of biological evolution determined?

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There are no completely identical organisms on Earth, nor are there completely identical individuals. Even identical twins have some differences. However, scientists have concluded that all the diverse organisms living on Earth have only one ancestor. The different organisms are like different branches on the same tree, they have different shapes and sizes, but they have a common root. So what is the basis for the conclusion that all organisms on Earth have a common origin?

Whether it is a bacterium with a very small size and a simple structure, or a large elephant and whale, their bodies are composed of cells without exception, their life activities are carried out in cells, and their reproduction is also completed by cells.

In addition, all biological life activities are inseparable from proteins, and their trait transmission is inseparable from DNA. It is these similarities between organisms that led biologists to conclude that all organisms have a common ancestor. The research of comparative anatomy and comparative embryology also supports the conclusion that all organisms have a common root.

However, how did a common ancestor produce offspring that are completely different and diverse? This question has puzzled humans for a long time. Although there are different answers, only one is the most convincing, that is, the diverse organisms are formed by the continuous change and development of a common ancestor. The change and development of organisms is also called the evolution of organisms, which is what we call "biological evolution".

Geologists’ research on fossils has proved the correctness of the "biological evolution" point of view. The earth was formed 4.6 billion years ago. Geologists and paleontologists found bacterial fossils in strata 3.6 billion years ago, and found fossils of different species in strata after 3.6 billion years.

Scientists have found that the earliest animal fossils are invertebrates living in water, followed by fish, amphibians, reptiles, birds and mammals. The earliest plant fossils are aquatic algae, followed by ferns, gymnosperms and angiosperms. Combining the age and type of fossils, scientists saw the trend of biological development and change, that is, the organisms on the earth changed and developed in the direction from simple to complex, from aquatic to terrestrial, and from low to high.

So what drives the evolution of organisms? The answer of British scientist Darwin is the most convincing. He believes that the reason for biological evolution is "natural selection" or "survival of the fittest". Darwin saw Malthus’s argument that population growth is faster than the growth of living materials in his book "On Population", and agreed with this view.

He believes that as long as any organism leaves two offspring for each individual, as their offspring increase in number, the growth of the total number of organisms must be faster than the growth of living materials. The only way to resolve this contradiction is to compete for living resources among each other. The competition for living resources for survival is called "survival competition" or "survival struggle".

In the "survival struggle", it must be the "brave" who wins. Just like the relationship between wolves and rabbits, wolves that run fast can easily catch rabbits and survive, while rabbits that run fast can save their lives.

Wolves and wolves, rabbits and rabbits, wolves and rabbits are all competing for survival, and the winners of the competition are preserved by nature. It can be seen that nature selects the winners of the survival competition, which is called "natural selection". Of course, it can also be said that the winners in the "survival competition" are suitable for nature, so "natural selection" is also called "survival of the fittest".

From the example of wolves and rabbits, we can see that although they are all wolves, there are also differences between fast and slow runners. The different individuals in the same species are mutant individuals.

Darwin had a five-year voyage around the world. He saw that there was a wide range of variation among organisms. Although he did not know the specific cause of variation, he clearly knew that it was because there was genetic variation among various organisms that caused individual differences, which led to survival competition. It was also because of survival competition that natural selection was caused. It was because the mutant individuals who were suitable for nature survived that organisms developed and "evolved".

Darwin believed that when the same species lived under the same conditions, natural selection could proceed in different directions, so that the same species would produce different offspring. About 2.5 billion years ago in the primitive ocean, the number of primitive single-celled organisms increased continuously, and the consumption of ready-made organic matter became larger and larger, and food was in short supply. Death was approaching primitive single-celled organisms. At the same time, the carbon dioxide released by primitive single-celled organisms in the process of digesting and decomposing food also accumulated more and more, and the increase of carbon dioxide created conditions for photosynthesis.

At this time, at least two types of mutations occurred in primitive single-celled organisms. One type is single-celled organisms with thin cell membranes. These cells can roll with the flow of cytoplasm, and some can even extend "pseudopodia", as if they have feet. These moving cells can change places to find food after consuming the ready-made food around them, while those that cannot move can only wait for death in place.

Another type of mutant single-celled organism cannot move, but has chloroplasts in its body and a thick wall outside its membrane. Because they can make their own organic matter to feed themselves, they do not have to rely on ready-made organic matter, and truly achieve "self-reliance". These two types of mutants have won in the survival competition, and their offspring have been able to reproduce and grow.

Scientists speculate that it is through natural selection that one kind of primitive single-celled organism produces primitive single-celled animals and single-celled plants. The ancestors of today’s amoeba, paramecium and chlorella may have come from the same kind of primitive single-cell.

In addition to natural evolution, artificial evolution caused by human factors also plays an important role in the formation of species. Humans use natural variation and induced variation of organisms to cause changes in organisms through gene recombination and chromosome aberration. Goldfish are gold-colored variants of Asian carp that were selected and raised separately as red carp. Then they appeared bubble eyes, three-tailed leaves, four-tailed leaves and other mutations, and then bred a variety of goldfish varieties through hybridization.

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