Coevolution and the Formation of Biodiversity

Section 4: Coevolution and the Formation of Biodiversity

In nature, it is not uncommon for a specific plant species to be pollinated by a specific insect species. Consider the significance of this specialization for the reproduction of plants. What is the relationship between the evolution of insect-pollinated flowers and the evolution of pollinating insects?

Coevolution

You may have seen on television the scenes of a cheetah chasing a gazelle (Figure 6-12). Natural selection favors individuals in the gazelle population that are muscular and agile, just as it favors individuals in the cheetah population that can run fast. The evolutionary processes of these two species resemble a long "arms race."

Have you ever wondered if the presence of predators is only harmful to the prey? In reality, predators often target the older, weaker, or younger individuals in prey populations, which can objectively promote the development of the prey population.

Regarding the role of predators in evolution, American ecologist S.M. Stanley proposed the "harvesting theory": predators tend to prey on species with large populations, preventing any one or a few species from dominating an ecosystem. This creates space for the formation of other species, and the presence of predators contributes to increased species diversity.

Not only are different species closely related in their evolution, but the evolution of organisms is also influenced by changes in the inorganic environment. For example, Earth's original atmosphere lacked oxygen, so the earliest organisms were anaerobic (carrying out anaerobic respiration). The appearance of the first photosynthetic organisms introduced oxygen into the atmosphere, creating the conditions for the emergence of aerobic organisms.

This mutual influence between different species and between organisms and the inorganic environment, which drives continuous evolution and development, is known as coevolution. Through a long process of coevolution, Earth has seen the emergence of diverse species, a rich gene pool, and various ecosystems.

Formation of Biodiversity

Biodiversity primarily includes three levels: genetic diversity, species diversity, and ecosystem diversity. The formation of biodiversity has been a long evolutionary process (Figure 6-13).

Based on current evidence, the earliest biological fossils are from ancient bacteria dating back 3.5 billion years (Figure 6-14). For the next approximately 2 billion years, life on Earth mainly consisted of a small number of cyanobacteria and bacteria in the oceans, all of which were prokaryotes. The ecosystem during this period was a two-level system with only producers and decomposers. With the emergence of eukaryotes, sexual reproduction also appeared as a new mode of reproduction. Through sexual reproduction, organisms achieved genetic recombination, increasing the diversity of variations and accelerating the rate of evolution. The number of multicellular plants and animals began to increase rapidly. During the Cambrian period, from 540 million to 500 million years ago, there was an explosive formation of a large number of invertebrate species in the oceans, known as the Cambrian explosion. This abundance of animals created the third level of the ecosystem—consumers—resulting in more complex ecosystem structures (Figure 6-15).

In contrast to the bustling marine world, land was almost devoid of life at that time. About 400 million years ago, due to orogeny (mountain-building processes), the oceans shrank, and the land expanded, allowing some marine plants to adapt to terrestrial life and form primitive land plants, mainly ferns (Figure 6-16). Primitive amphibians, the first animals to adapt to land, appeared afterward. The colonization of land by organisms altered the terrestrial environment, and the complex land environment provided a vast stage for further biological evolution. Gymnosperms and angiosperms successively became the dominant producers, while birds, mammals, and other groups became the dominant animals, leading to the gradual formation of diverse terrestrial ecosystems.

Throughout evolution, many species went extinct because they could not adapt to environmental changes. For example, dinosaurs, which "ruled" Earth for about 100 million years during the Mesozoic era, went extinct at the end of the Cretaceous period for reasons that are still not fully understood. The extinction of dinosaurs opened up space for the rise of mammals, turning a new page in biological evolution.

Development of the Theory of Evolution

The modern theory of biological evolution, centered around the concept of natural selection, provides a scientific explanation for the history of life on Earth: adaptation is the result of natural selection; populations are the basic units of biological evolution; mutations and genetic recombination provide the raw materials for evolution; natural selection leads to directional changes in population gene frequencies, which, combined with isolation, results in the formation of new species; the process of biological evolution is essentially a coevolutionary process involving both organisms and the inorganic environment; biodiversity is the result of coevolution.

The process of biological evolution is complex and lengthy. Researchers cannot witness past events, and future developments are difficult to predict. Thus, while the key points of the modern theory of biological evolution summarized above are widely accepted in academia, this does not mean that all the mysteries of evolution have been uncovered—there are still debates and uncertainties.

Some researchers suggest that the effects of gene mutations on biological adaptability are not simply beneficial or harmful, with many mutations being neutral. Natural selection does not act on these neutral mutations, which can accumulate over time, leading to significant genetic differences between populations. As a result, some argue that the gradual accumulation of neutral mutations, rather than natural selection, determines the direction of biological evolution. However, many researchers believe that not all gene mutations are neutral; some mutations affect individual traits and vary in their degree of environmental adaptation. Therefore, the role of natural selection cannot be denied.

Based on observations of species formation occurring rapidly over short periods, some propose that species formation is not always a gradual process but rather a process in which long periods of stability alternate with the rapid formation of new species, a phenomenon still under investigation.

In summary, biological evolution is so complex that there are more unanswered questions than those explained by existing evolutionary theories. Among these theories, the theory of evolution centered on natural selection has had the most widespread and profound impact and remains the foundation for future research. Like other scientific theories, the theory of biological evolution will not remain stagnant but will continue to develop.