Throughout history, different theories have been postulated trying to explain the diversity that exists between species. Let's go through them.
Fixism and Creationism
At the beginning of the 19th century, the diversity that exists between species was justified as creations of God that remained immutable over time. The outcome of this idea was what is known as creationism (creation of God) and fixism (species do not vary over time). The ideas of both concepts arose from the literal interpretation of Genesis and other sacred books. We want to highlight one of its great defenders, Carl von Linnaeus, to whom we owe the current binomial classification of species, and one of its great critics, Richard Dawkins who was an important neodarwinism contributor (see below).
A very important fact that discredited fixism was the discovery of fossils (evidence of living beings that no longer existed) giving way to the catastrophism theory that justified the existence of fossils, but defended fixism.
This theory was proposed by the naturalist Georges Cuvier, father of paleontology. This theory infers that the disappearance of living beings was the consequence of some natural catastrophe, but that those living beings that lived in the past were created and remained unchanged. As a consequence of the disappearance of species due to natural catastrophes, this creation process was repeated from time to time.
It is not currently accepted, but it was the first theory that spoke about the concept of evolution. It is known as transformation, Lamarckism or theory of the inheritance of acquired characters and was proposed by Jean Baptiste de Lamarck and argued that one species was transformed into another over time. He defended that:
- Changes produced in the environment force the species to adapt to the environment by modifying certain organs depending on their use or disuse. As a result, the original characters are slowly being replaced by a series of acquired characters.
- As acquired characters are inheritable, they remain from generation to generation.
- Organisms evolve from simple to complex forms because they have a tendency toward complexity.
Charles Darwin is considered as the father of evolution. Lamarck was the first to talk about evolution, but current knowledge of genetics demolished his theory since acquired characters such as muscle development are not inheritable. Only those characters whose information resides in the genes are inherited.
The theory of evolution or Darwinism was not developed solely by Darwin since Alfred Russel Wallace, in parallel, reached the same conclusions as Darwin and his support was key for the publication of Darwin's book. Darwin made a 5-year trip around the world in which he made numerous observations and collected a lot of information with which he was able to elaborate his theory. All this information was compiled in his book The Origin of Species.
Darwinism defends that evolution does not depend on environment changes but on genetic randomness. Although the word mutation usually has a negative connotation, mutations are what makes evolution possible.
Within this great genetic diversity that exists among living beings, the environment plays a very important role known as natural selection (the environment selects the best adapted organisms based on the random variation). Within a population, those individuals with advantageous variations will survive longer, reproduce more and transmit changes to their offspring as it provides them a better adaptation to the environment. Conversely, individuals with disadvantageous variations will be less likely to survive and thus reproduce. In this way, the species are in continuously and gradually change, with a struggle for survival.
This theory is current theory of evolution and unifies knowledge from different areas of biology such as genetics, paleontology, biochemistry, ecology and population genetics. Fisher, Haldane, Wright, Dobzhansky, Mayr, Huxley, Simpson, Stebbins, Richard Dawkins, Futuyma, Gould and Smocovitis were major contributors to this theory.
The points on which neodarwinism or synthetic theory of evolution is based are the following:
- Does not accept the inheritance of acquired characters (Lamarck theory) - Mendelian genetics contribution.
- In asexual individuals the only source of genetic variability are mutations. In individuals with sexual reproduction, mutations, genetic recombination and natural selection are responsible for genetic variability – Contribution of Mendelian genetics.
- Natural selection leads to changes in the set of alleles in a population. As a result, the alleles that confer an advantageous phenotype to the individuals will increase their allelic frequency in the population – Contribution of population genetics.
- The population evolves, not the individuals, since the result of reproduction is what leads to evolution - Contribution of population genetics.
- Evolution occurs gradually as Darwin argued. Evolution is the result of small changes in the allelic frequencies (population frequencies), for that reason having a new species is a very long process – Contribution of population genetics.
- The speciation occurs when they arise reproductive isolating mechanisms between the populations of a species since the exchange of genes between them is interrupted- Contribution of biogeography.
To summarize the current theory of evolution, we could say that today it is known that genetic variability is due to mutations and the process of genetic recombination and that it is the environment that acts on this genetic variability, triggering evolutionary change in a process known as natural selection.
Within Neo-Darwinism we want to highlight a work that had a great impact, The Selfish Gene (1976) by Richard Dawkins, which popularized evolution focused on genes (selfish gene theory). This theory defends that genes are the units of natural selection (not creatures) where evolution operates, a truly revolutionary theory where you can go depper here.
Evidence of evolution
There are plenty of evidence that supports the theory of evolution. They show that all the species have a common origin.
- Anatomical tests: compare the body structures in order to establish possible relationships of kinship.
- Paleontological tests: they are based on the study of fossils and demonstrate evolution since many fossils have certain similarities with current species or present intermediate forms of two current species such as the Archeopteryx fossil (reptile and bird traits).
- Embryological tests: they compare the embryonic development of different animals, making evident the great similarity that many animals present when observing the first stages of embryonic development.
- Biogeographic tests: they are based on the study of the geographical distribution of the species. The theory of evolution states that the organisms that live together in a certain area evolve in a similar way, but that when certain populations are isolated tend to evolve into different forms, as happened with the monkeys of Africa, South America and Asia.
- Biochemical tests: they compare different species at the molecular level, establishing an evolutionary relationship (the more similar they are at the molecular level, the greater). The most widely used methods are based on DNA and amino acid sequences (components of proteins). Thanks to this methodology, phylogenetic trees have been developed that represent the kinship relationships between living beings.