Evolutionary Biology: From Fixism to Neo-Darwinism

Historical Theories of Biological Evolution

As the old theories explain the evolution of species, Fixism is a theory that proposes that species do not change, but rather remain essentially unchanged over time since their creation. This materialized in the 18th century with the work of Linnaeus.

Catastrophism is a theory that proposes that each geological cataclysm destroys existing species, so that subsequently, a creation of new species is produced. In the 18th century, it showed a very fragile state due to new input against it by Leclerc and other scientists.

Lamarck's Evolutionary Hypothesis

Lamarck's evolutionary hypothesis is called Transformism and assumed that species evolve to be gradually transformed into one another. It is based on two points:

• "The function creates the organ."
• "The acquired characteristics are inherited."

Principles of the Darwin-Wallace Theory

The principles underlying the theory of evolution by Darwin and Wallace are:

• High reproductive capacity: Living beings are born in more offspring than needed to replace the dead. Over time, resource limitations establish a struggle for existence in which individuals must compete for food and reproduction.
• Variability of populations: There is great variability among individuals in a population, even among the descendants of the same couple, who have certain heritable characteristics.
• Natural selection: Caused by the struggle for survival among individuals in a population in which individuals are best suited and have advantageous features; these are the ones that reproduce and transmit their characters to the offspring.
• Species evolve: Individuals who survive are the next generation, and thus advantageous changes are transmitted from generation to generation.

Neo-Darwinism and the Synthetic Theory

Also known as the Synthetic Theory of Evolution, Neo-Darwinism is a theory proposed by Dobzhansky. It is called the synthetic theory because it carries out a synthesis of Darwinian selection theory and Mendelian genetics. Since Darwin did not know how hereditary traits are transmitted, this theory explains the cause of the variability of the populations on which natural selection acts, completing it with Mendel's laws.

According to this theory, evolution is due to the genetic variability of the population caused by mutations in the genes and genetic recombination that occurs during sexual reproduction, alongside the action of natural selection.

Stephen Jay Gould and Punctuated Equilibrium

Stephen Jay Gould's theory of Punctuated Equilibrium does not question evolution itself. Gould proposes that evolutionary change does not work gradually through a slow accumulation of variability, but in many instances, it progresses rapidly, followed by large periods of stability among species.