Biological Evolution: Core Theories and Evidentiary Support

Posted on May 9, 2025 in Biology

Unit 5: Understanding Evolution

Evolution explains the origin of diverse life forms as a result of changes in their genetic makeup. It posits that all species descended from other, distinct, pre-existing species. Natural processes, including mechanisms like natural selection, have shaped life on Earth over vast periods, long before Darwin’s pivotal contributions.

1. Fixist Theories

These theories propose that living beings were created by God and have experienced no changes or modifications (remaining unchanged). A significant proponent was Georges Cuvier, with his theory of catastrophism. Due to the existence of organic fossils (remains of organisms that lived in the past), which the fixist theory could not adequately explain, Cuvier proposed that God had performed several successive creations. These creations were supposedly destroyed by great catastrophes, the last being the universal deluge. After each catastrophe, a new generation followed, an enduring creation from the previous one. According to this view, the Earth would be between 4,000 and 6,000 million years old.

2. Evolutionary Theories

These theories propose that living beings are not immutable but have changed, emerging from one another through evolution.

2.1. Lamarck’s Theory

Lamarck proposed that living beings evolve because the environment causes adaptations in them, which are then inherited by their offspring. The bases of Lamarckism are:

  1. The Law of Use and Disuse of Organs: Organs that are used more develop further, while those not used atrophy.
  2. The Inheritance of Acquired Characteristics: Acquired traits are transmitted to offspring.

2.3. Synthetic Theory or Neo-Darwinism

Natural selection chooses the best genotypes, and evolution occurs within a population through a gradual change in the genetic constitution of species. Key aspects include:

  1. The evolutionary unit is the population.
  2. Variations between populations are due to variations in the genetic endowment of their individuals, mutations, and sexual reproduction, which increases variability.

2.4. Neutrality Theory

Proposed by Motoo Kimura, this theory suggests that most DNA changes are neutral. Therefore, whether these changes remain in populations depends largely on chance (genetic drift).

2.5. Punctuated Equilibrium Theory

Proposed by Niles Eldredge and Stephen Jay Gould, this theory states that evolution occurs very irregularly, with long periods of high stability (stasis). Species remain very stable for a long time and then disappear or change abruptly over relatively short periods.

3. Evidence for Evolution

These are the facts and evidence upon which biological evolution is based.

3.1. Paleontological Evidence

Based on the study of fossils:

  1. Fossils show us the transformation of living beings over time, making it possible to establish similarities between extinct and present species.
  2. They allow for the creation of phylogenetic series: reconstructions of the evolutionary process where living beings show kinship with others.
  3. They reveal an increasing complexity of living beings over time.
  4. They reveal a great diversity of living beings that have existed.

3.2. Morphological Evidence

Based on the comparative study of the anatomy and function of different organs in different groups of living beings to establish or rule out evolutionary relationships. Two types of organs are distinguished:

  1. Homologous Organs: These have the same internal structure even though they may look different and perform different functions (e.g., the forelimb of a human, bat, whale, and dog). They indicate that species possessing these organs share a common ancestor from which these organs originated and subsequently adapted to different environments. This leads to Evolutionary Divergence and Adaptive Radiation.
  2. Analogous Organs: These have different internal structures but similar functions (e.g., the wing of a bird and the wing of an insect). They indicate that species possessing these organs have come from different ancestors but have adapted to similar environments. This leads to Convergent Evolution.

3.3. Embryological Evidence

This consists of a comparative study of the embryonic development of different groups of animals in order to:

  1. Establish similarities in early developmental stages.
  2. Establish evolutionary relationships based on these similarities.

3.4. Biogeographical Evidence

The geographical distribution of living beings allows us to appreciate that:

  1. The more remote and isolated an area is from another, generally the greater the difference between the living beings of both regions.
  2. Similarities between geographically isolated species are explained by a common ancestor; however, as landmasses (like continents) separated, these populations evolved differently.

3.5. Biochemical Evidence

The biochemical composition of living beings shows remarkable universal similarities:

  1. Four primary atoms (Carbon, Hydrogen, Oxygen, Nitrogen – CHON) represent approximately 98% of the composition of any living being.
  2. Four main types of organic molecules (carbohydrates, proteins, nucleic acids, lipids) are the most important molecules in all organisms.
  3. Hereditary information is encoded in nucleic acids (DNA or RNA) in all living beings.

Furthermore, the proteins of all living beings are formed from the same set of approximately 20 amino acids. When analyzing the similarity in the amino acid sequence of equivalent proteins in different species, it is noted that the greater the similarity, the closer the evolutionary relationship. This allows for the construction of phylogenies, which depict the evolutionary history and relationships of species.

Conclusion

Life on Earth shares a common origin; all living beings are variations stemming from a unique and continuous biological history.