Theories of Evolution: From Lamarck to Darwin and Beyond
1. Origins of Life
1.1 Creationism and Spontaneous Generation
Louis Pasteur Experiment:
Pasteur boiled a flask of meat broth to eliminate possible microorganisms. This flask was kept free of microorganisms, as they were trapped in the neck of the flask. After tilting the flask, the microorganisms came into contact with the broth and multiplied rapidly, decomposing it.
1.2 Prebiotic Synthesis and Panspermia
Stanley Miller and Harold Urey:
The gases that made up the primitive atmosphere were injected into a high-temperature water vapor circuit and subjected to electrical discharges simulating thunderstorms. By condensing the resulting molecules, organic compounds such as urea, organic acids, or certain amino acids were observed, which are the biochemical building blocks of all living things.
Although the experiment was successful, Miller and Urey were not able to reach the next level of complexity: obtaining living cells.
2. Evolution of Living Organisms and the Origin of Diversity
Evolution is a gradual process by which living beings undergo transformations over time and which explains how different species have formed from a common origin, resulting in the immense variety of living beings that inhabit our planet.
2.1 Gradualism and Saltationism
Gradualism: This school of thought argued that changes in organisms occurred little by little, slowly and gradually, over a period of years. The progressive accumulation of these changes was responsible for the evolutionary process and the formation of new species. This theory was supported by Charles Darwin and Alfred Wallace.
Saltationism: This is a current that defends that the changes that promote the evolution of living beings occur in an abrupt, irregular manner. According to the saltationists, many species would go through long periods without undergoing any changes and within a short time undergo highly accelerated evolution. Some of the saltationist scientists were Niles Eldredge and Stephen Jay Gould.
3. Theories of Evolution
3.1 Lamarck’s Theory of Evolution
By comparing similar fossils from different periods, Lamarck observed small modifications that accumulated over time and deduced that the newer species were derived from the older ones. Moreover, it appeared that the latter were clearly more complex than the former. This allowed him to base his theory on the following points:
- Tendency towards Perfectionism: Living beings innately tend to perfect their characteristics to adapt to their environment. This would be the case for the kangaroo rat, which has highly developed lower limbs due to frequent use.
- Inheritance of Acquired Traits: The modifications acquired by these individuals are passed on to their offspring, who can benefit from the advantage they provide.
- Law on the Use or Disuse of Organs and Formation of New Organs: Varying environmental conditions lead to changes in the use of organs in living beings.
3.2 Darwin-Wallace Theory of Evolution
The second major theory of evolution was developed by Charles Darwin and Alfred Wallace. The theory of evolution by natural selection is based on the following postulates:
- Within the different populations of a species, there is a certain heterogeneity, with small variations or differences between the individuals that compose it.
- Organisms tend to produce more offspring than the surroundings in which living beings live, often changeable.
- Individuals that show more favorable variations are better able to adapt to these environmental changes, to reach adulthood and to reproduce, passing these variations on to their offspring.
3.3 Comparison of Lamarck’s Theory and Darwin-Wallace Theory
Lamarck:
- All individuals of a species are similar.
- The need for self-improvement motivates changes in the individuals.
- Organs that are used are enhanced, and those that are not disappear.
- These organs are inherited.
Darwin-Wallace:
- There are variations between individuals of the same species.
- The environment around them selects the most favorable variations to adapt to it.
- Favored individuals are more likely to reproduce.
- These variations are inherited.
4. Evidence of Evolution
4.1 Paleontological Evidence
The study of fossils has allowed us to monitor the different changes that species have undergone over time.
4.2 Anatomical Evidence
- Homologous Organs: They have a common origin and a similar internal structure.
- Analogous Organs: They have a very different origin and internal structure.
- Vestigial Organs: They are atrophied structures that have become less useful as the evolutionary process has developed.
4.3 Geographical Evidence
As well as the associated parameters: temperature, humidity, proximity to the sea…
4.4 Biochemical Evidence
Advances in biochemistry and genetics have provided a major boost to the study of the evolutionary process.
5. Evolutionary Mechanisms
- Genetic Variability: Mutations modify the DNA of organisms and sometimes produce new types of alleles for a given gene.
- Natural Selection: Environmental pressure is responsible for increasing or decreasing the proportion of different alleles in a population.
- Migration: It is the movement of alleles between populations.
- Other Processes: Genetic drift, Bottleneck, and founder effect.
6. Consequences of Evolution
- Isolation: The starting point is a population that for some reason is divided into two isolated populations.
- Accumulation of Changes: The pressure exerted by natural selection causes different changes in the two populations that accumulate over time.
- Formation of New Species: The cumulative changes make reproduction between these populations impossible.