Evolutionary Theories and Evidence

Evolutionary Theories

The fossil record suggests that organisms appeared on Earth 3,000 million years ago and have been changing gradually, leading to the organisms present today.

Fixism

Considers that species have always been the same. It lasted until the eighteenth century. Cuvier studied fossils and explained the changes in specimens using a catastrophic theory that coincided with the Bible.

Lamarckism

Proposed in the book Philosophie Zoologique, it states that organisms change and adapt to the environment:

• Organisms have an innate tendency towards perfection, aiming for 'more perfect' species (e.g., humans).
• Believes in spontaneous generation.
• Proposes the Law of Use and Disuse and the Inheritance of Acquired Characters.

Darwinism

Published On the Origin of Species (1859), explaining that evolution occurs by natural selection:

• Reproductive potential of species.
• Wide variability exists within populations.
• Selection of organisms best adapted to the environment (survival of the fittest).

Neodarwinism or Synthetic Theory

Genes are the elements acting on evolution. Mutations are the source of variability. Evolutionary units are populations. Natural selection, among other factors, is the engine of evolution. Reproductive isolation plays a key role in the emergence of new species.

Evidence of Evolution

Various fields provide evidence for evolution:

• Anatomy: Study of organisms, including comparative anatomy (e.g., analog, homologous, and vestigial structures).
• Embryology: Study of embryonic development.
• Taxonomy: A hierarchical system based on similarities between organisms. Animals of the same group often have a common origin.
• Paleontology: Study of fossils, providing evolutionary proof (e.g., the transition from horses with four toes to one).
• Biogeography: Study of the geographical distribution of animals and plants. Similar species in similar territories often show differences (e.g., dromedaries and camels).

Population Genetics and Evolutionary Agents

Hardy-Weinberg Law

A Population is all subjects of the same species living in the same territory. Within each species, there are differences between individuals, each having different genetic alleles. The Hardy-Weinberg Law describes a population in equilibrium (panmictic, large number of individuals, no selection, no mutation, no gene flow), where allele and genotype frequencies remain constant, meaning the population is not evolving.

Evolutionary Agents and Sources of Variability

Evolution occurs when these conditions are not met. Agents causing evolution and originating variability include:

• Mutations: Changes in gene frequencies.
• Lack of Panmixia: Non-random mating.
• Gene Flow: Occurs when migration mixes characters and gene frequencies between populations.
• Genetic Drift: Significant in small populations. A small number of individuals may separate from the main population due to changed conditions, and their limited variability can lead to the formation of a new species (e.g., founder effect in island formation).
• Bottlenecks: A drastic reduction in population size, leaving very few individuals who may not represent the total variability that existed in the original population.