Evolutionary Theories: From Fixism to Cellular Evolution

Fixism

This theory proposes that species do not change; they remain unchanged from their creation. Linnaeus was a proponent of this thought.

Catastrophism

Georges Cuvier proposed that land disasters had destroyed existing species, leading to the creation of new species.

Neutral Theory

This theory posits that chance variations occur in populations, where a mutant gene can be dispersed without any selective advantage.

Punctuated Equilibrium

This theory suggests that the absence of intermediate steps in the fossil record is not due to incompleteness, but rather that evolution sometimes occurs in "leaps."

Evidence of Evolution

• Morphological: Studying comparative anatomy helps to understand the similarities and differences of the same organ in different species.
• Paleontological: Studying fossils allows us to know which life forms existed in the past and study their evolution. We can determine the age of fossils through geological dating.
• Embryological: Studying the early development of organisms.
• Taxonomic: The system of classification of living things is made by grouping organisms that have similar characteristics, which is the result of a common evolutionary process.

Darwinism

Darwinism creates a bond of kinship among all living beings. Present species are the result of the progressive divergence of earlier species. Key aspects include:

• Adaptive divergence
• High reproductive capacity of living beings
• Variability of populations
• Natural selection
• Species evolve

Lamarckism (Transformism)

This theory proposes that species gradually evolve into one another. Key tenets include:

• The function creates the organ.
• Acquired characteristics are inherited.

Neo-Darwinism

Neo-Darwinism states that evolution is due to two factors:

• The existence of genetic variability in the population is caused by mutations and genetic recombination.
• The action of natural selection eliminates less advantageous genotypes, allowing for better-adapted individuals.

Panspermia

Hermann Richter proposed that the Earth was fertilized by microorganisms from space. This theory suggests that life came to Earth in the form of bacterial spores from outer space, driven by the radiation pressure of stars. However, evidence-based thinking is elaborated.

Cellular Evolution

A biological membrane appeared, separating the internal and external environments, which led to the emergence of a metabolism to obtain energy and utilize it. Early cells were likely heterotrophic, fermentative bacteria able to obtain food and energy directly from their environment. When food reserves were exhausted, the first cells evolved. Cyanobacteria developed photosynthesis, and the release of oxygen transformed the primitive reducing atmosphere into one very similar to the current one. Many other cells managed to adapt and learned to use oxygen. Aerobic heterotrophic bacteria use oxygen to obtain energy through cellular respiration. Later, eukaryotic cells evolved.

Prebiotic Synthesis

Gases in the atmosphere reacted, giving rise to small organic molecules. Rain washed these chemicals from the atmosphere into the oceans, where they dissolved and formed the "primordial soup." This set of organic molecules reacted with water and formed biological building blocks, such as amino acids. These interacted, giving rise to the first giant molecules, such as proteins and nucleic acids. From the primordial soup emerged microstructures that grouped polymers, and in which autoreproductive processes developed, forming the first systems. These spread across the ocean, giving rise to biological evolution.