Darwin, Natural Selection, and Competing Evolutionary Theories
Darwin's Origin of Species and Natural Selection
In On the Origin of Species by Means of Natural Selection, Darwin sets out evidence that supports the theory of the evolution of species based on the role played by the mechanism of natural selection.
Natural Selection Summarized
Evolution by natural selection can be summarized in four points:
- High reproductive capacity. Most species have a large reproductive capacity (alta capacidad reproductiva).
- Struggle for existence. Population growth is limited by available resources, so individuals enter into a struggle for existence (lucha por la existencia).
- Individual variability. There is considerable individual variation; some individuals have characteristics that differentiate them from the rest.
- Survival of the fittest. Some individual characteristics give the bearer a greater capacity to survive and to transmit those traits to their offspring (supervivencia del más apto).
After many years of natural selection, individuals of a population may differ from their predecessors. If these differences are significant, they may form a new species. We now know the mechanisms of inheritance of traits and that the individual variation that characterizes the individuals of a species is due to mutations.
Alternatives to Neo-Darwinism
Theory by Motoo Kimura — Neutral Theory
The proposal is called the neutral theory of molecular evolution. It postulates that most mutations that occur in DNA neither favor nor harm organisms. This theory implies the existence of a molecular clock of evolution to quantify the time elapsed since two species diverged from a common ancestor, according to the number of differences that exist in the nucleotide sequence of a gene.
Punctuated Equilibrium
Punctuated equilibrium considers that the fossil record faithfully shows what happened in evolution: one species changes over long periods of stasis followed by short periods in which it undergoes rapid speciation. This theory contrasts with the neo-Darwinist view, which considers that change occurs gradually.
Gradualism
Gradualism supports the Darwinian point of view, which posits a gradual and more or less constant evolutionary model.
Gene-Centric (Selfish Gene) Theory
This theory does not place the population or the individual as the basic unit of evolution, but rather a part of the individual — genes — which, according to this theory, are the basic units of evolution. Competition is established between genes to increase their number in the gene pool of a population at the expense of other alleles.