Key Theories and Evidence of Evolution
Evolutionary Theories: Lamarck, Darwin, and Beyond
Evolutionary theories propose that species undergo evolution over time, transforming one into another new species. Among these, the evolutionary theories of Lamarck and Darwin are particularly emphasized.
Lamarck’s Theory and Lamarckism
Jean-Baptiste de Monet, Comte de Lamarck (1809), attempted the first systematic explanation of how species evolve. He considered evolution a slow and gradual process in which changes in the environment caused changes in species. Lamarck believed that acquired characteristics were heritable. However, he could not provide any data to prove his theory. Today, we know that acquired traits are not inheritable.
Darwin’s Theory and Natural Selection
Charles Darwin (1859) explained evolution through a mechanism called natural selection. Darwin also believed that species evolve slowly and gradually. This explanation was supported by numerous tests and observations.
Principles of Natural Selection
This theory, proposed by Darwin, explains the evolution of species and consists of the following statements:
- Within a species, there are differences or variations among individuals that are heritable.
- More individuals are born within a species than can survive.
- Some heritable variations provide individuals with benefits for survival compared to those without them.
- Individuals who have these advantages leave more offspring.
- Slowly, and over successive generations, advantageous heritable variations are selected, and the rest are eliminated. As a result, the population evolves into a new species.
Evidence for Evolution
Comparative Anatomy
Comparative anatomy investigates structural homologies, or similarities inherited by organisms, both in their skeleton and in any other organ. Species with a recent common ancestor have more in common than those whose anatomy indicates a more distant common ancestor. Homologies reflect the process of evolution, and phylogeny can be used to build the evolutionary history of organisms.
The Synthetic Theory of Evolution
The Synthetic Theory of Evolution integrated Mendel’s laws, the chromosome theory of inheritance, and a new approach to genetic studies (population genetics) into the framework of Darwinian theory. It is based on the following principles:
- The evolutionary unit is not the individual, but the population.
- Differential reproduction is the mechanism through which the joint action of genetic variation and environmental stress causes selection.
- Evolution is caused by a gradual change in the genetic makeup of species.
Neutral Theory of Evolution
Most mutations are neutral to natural selection. This has a double meaning: that if there is change, it does not affect the adaptation of the species (e.g., the ostrich), and that there can be protein polymorphism (various proteins, or mutations, with the same function). Therefore, the maintenance of these changes is a matter of chance.
Study of the Fossil Record
The presence of fossils of extinct species and their temporal distribution reveal a process of change over time. Although only a small part of organisms that die become preserved as fossils, it is enough to prove that species appear, die, and are replaced by others throughout the fossil record. However, only in some cases do fossils allow the reconstruction of body forms. A fossil with characteristics intermediate between two independent groups is called a “missing link.” These intermediate forms provide evidence of kinship between them.