Evolution: From Fossils to Modern Synthetic Theory
Fossils
Fossils are the remains of organisms or their activity that lived on Earth in times past and became preserved. The fossil record shows that many species that existed in the past no longer exist today, i.e., they are extinct. Moreover, fossils of modern organisms are not found in ancient rocks. The study of fossils reveals a process of change in living beings over time.
Comparative Anatomy
Structural similarities inherited by organisms are called homologies, and the affected organs are called homologous organs.
Molecular Genetics
Molecular genetics is a recent discipline that was developed after the discovery of the DNA double helix in the mid-19th century. This discipline provides the most convincing evidence for biological evolution.
Source of Variation
Mutations are heritable material changes. Most mutations are neutral, meaning they do not cause an advantage or disadvantage to their possessor. However, there are some cases in which a mutation is beneficial. A mutation is neutral if it does not provide any advantage or disadvantage to the individual in the environment where it lives. But if there is a change in the environment, it may become beneficial or harmful. If mutations do not kill the individual, they remain in the genetic material and are transmitted from generation to generation, leading to differences between individuals.
- Sexual Reproduction: Although sexual reproduction does not produce new genes, it increases variation by originating new combinations of genes.
Natural Selection and Adaptation
With the passage of time, the population changes, and the result of this change will be a population more adapted to its environment. The process followed by adaptation is called adaptation.
Biodiversity
Biodiversity includes three concepts:
- Genetic Diversity: The variability that exists in the genetic information between individuals of the same species.
- Species Diversity: The variety of species existing in different parts of the planet.
- Ecological Diversity: The variety of forests, deserts, and other biological communities that interact among themselves and with their nonliving environment.
What Happens if the Environment Changes?
If individuals are not adapted to the environment, natural selection will begin to act. If there are no individuals with advantageous characteristics, the species will eventually become extinct.
Synthetic Theory of Evolution
Variations are caused by mutation and are inherited by genes. New discoveries in the field of genetics led to a review of some points of Darwin’s theory and their interpretation from a different viewpoint:
- The Lamarckian principle of the inheritance of acquired characteristics was rejected.
- The idea of inheritance as a phenomenon of blending was definitively discarded.
- A new theory of evolution was proposed: the synthetic theory of evolution.
The synthetic theory is summarized as follows:
- The evolutionary unit is not the individual but the population.
- Individuals of any population are carriers of different alleles that have originated by mutation.
- Certain phenotypes give individuals the ability to leave more offspring.
Gradualism
According to Darwin’s theory and the synthetic theory of evolution, evolution is a slow and gradual process. This way of thinking is known as gradualism.
Saltationism vs. Gradualism
Some scientists believe that intermediate forms are not fossilized because they never existed or that changes occurred too fast for fossils to be found. This way of thinking is known as saltationism. It states that the speed of evolution is irregular. In 1972, very few scientists advocated a form of saltationism in which a species went through periods of no change, interrupted by periods of very intense evolution. This theory is called punctuated equilibrium.
How a Species Emerges in Nature
It occurs in the following stages:
- Isolation of Populations: A population can be isolated by a barrier that prevents it from reproducing with the original population.
- Gradual Differentiation: The isolated population has the same genes as the original species, and as mutations arise randomly, differences between both populations are generated. Furthermore, if environmental conditions change, natural selection will act.
- Two Different Species: Prolonged isolation can produce a population so different from the original that it loses the ability to interbreed with it.
Speciation and Extinction
Speciation is the process by which two species arise from one as a result of natural selection acting on variability. Extinction is the process by which a species becomes extinct because it cannot adapt and reproduce in new environmental conditions.