Evolution Theories: From Fixism to Neo-Darwinism
Fixism and Creationism Before Evolution
Before the acceptance of the theory of evolution, the conviction was held that species remained invariable over time. The origin of species was attributed to a creator.
Lamarckism and the Inheritance of Acquired Traits
Lamarckism suggests that species transformed over the passage of time from one species to another through metamorphosis. The causes of these changes include:
- A trend towards complexity and adaptation to the environment.
- The appearance of organs due to use or disuse.
- The inheritance of acquired characters.
Lamarckism was eventually discarded because it does not explain how an acquired character can be inherited by descendants.
Darwinism and Natural Selection
Darwinism posits that species evolve according to natural selection. Darwin formed an expedition and collected data showing that despite the proximity of various islands, distinct species existed. He found it unreasonable that so many species originated from the same body but were different in such a small area through separate creative acts. He concluded that biodiversity and adaptation were the result of descendants inhabiting each island over generations. For evolution to occur, three factors are necessary:
- High reproductive capacity.
- Variability in descent.
- Natural selection.
Neo-Darwinism and Genetic Mechanisms
Neo-Darwinism explains descent through genetic recombination and mutations, which occur randomly (at azar). Hardy and Weinberg showed that mutations are the most important factors in individuals. Fisher, Haldane, and Wright demonstrated that several factors can alter gene frequencies in populations: natural selection, mutation, and genetic drift. For two populations to evolve differently, there should be no cross-breeding between them.
Neutral Theory and Punctuated Equilibrium
Neutral Theory (Neutralista): Motoo Kimura stated that mutations are neither favorable nor unfavorable but are neutral, and thus are not affected by natural selection.
Punctuated Equilibrium (Equilibrio): This theory accepts the selection point but suggests that a small (pequeña) population of an ancestral species evolved in isolation from the rest until a new species emerged. This new species returned to the initial area and grew when conditions were favorable. If environmental changes did not make them extinct, the natural selection pressure would have been higher in the zone where the small population settled, as they were better suited for survival through natural selection.
Key Differences in Evolutionary Processes
Evolutionary processes differ in how species form from an ancestral species: stages may alternate between very slow and very fast processes, and evolution often occurs in a small and isolated area.