Evolutionary Theories: From Fixism to Darwin’s Natural Selection
The Long Road to Evolutionary Thought
Early ideas about life on Earth often diverged significantly from modern evolutionary theory. Key among these was Fixism, the belief that all species were created as they appear today and have remained unchanged since their creation. This view was supported by several prevailing notions:
- The Anthropocentric View of the World: Despite Copernicus removing Earth from the center of the universe, the anthropocentric view persisted, suggesting that everything surrounding humanity was created for its benefit.
- The Idea of an Earth Just 6,000 Years Old: Based on interpretations of Genesis, this belief provided insufficient time for significant evolutionary change to occur.
- False Evidence from Common Sense: Observations that seemed obvious were often presented as irrefutable, yet incorrect, evidence against change.
Georges Cuvier, a pioneering scientist, studied fossils in depth and discovered that many extinct species were significantly different from those of today. He was the first to articulate the concept of species extinction. To explain the presence of extinct life forms, Cuvier proposed a history of Earth marked by catastrophic events that eliminated portions of living organisms.
Lamarckism: Transformation of Species
Lamarckism, or transformism, posited that biological species are not immutable but undergo change over time. Jean-Baptiste Lamarck believed that living beings possess a natural tendency towards complexity and progress, leading to the transformation of species. His basic ideas included:
- Organisms necessarily change over time.
- Changes in environmental conditions cause species to alter their habits and needs.
- The habits of organisms determine the changes that occur within them; if an organ is used extensively, it develops. Necessity may even produce a new organ.
Lamarck’s theory, known as the theory of acquired characteristics, suggested that changes acquired during an individual’s lifetime are inherited by offspring. The accumulation of these changes, he believed, would lead to new species. However, his biggest mistake was the premise that modifications acquired by an individual are transmitted to offspring, a concept later disproven.
The Darwinian Revolution
The scientific understanding of evolution underwent a profound shift with Charles Darwin’s work. As a naturalist aboard the HMS Beagle for five years, Darwin circumnavigated the world, meticulously observing and collecting data. His basic ideas, forming the foundation of modern evolutionary theory, include:
- Overproduction: More individuals of any species are born than can survive, as environmental resources are limited.
- Variation: Among the individuals of any species, there exist heritable variations or differences.
- Natural Selection: If more individuals are born than can survive, and they are not identical, those who survive possess some advantage in their environment.
- Population Change: The individuals who survive are those that reproduce and pass on their advantageous variations to their offspring, leading to gradual changes in the population over generations.
Natural Selection and Adaptation
Natural selection shares similarities with, yet differs from, the artificial selection performed by breeders who choose individuals with suitable characteristics for reproduction. For Darwin, natural selection is a continuous process carried out by nature itself. While it was evident that all organisms are adapted to their environment, the mechanism of adaptation was understood differently by Lamarck and Darwin:
- For Lamarck: The environment induced the most appropriate changes in organisms. Development was seen as finalistic or teleological, implying a particular goal, and once initiated, could determine the outcome.
- For Darwin: The environment can only select the most viable options from the variability already present among individuals. Adaptation, therefore, is a result of this environmental selection.
Dynamic Nature of Selection Criteria
The criteria for natural selection are not static. The environment is constantly modified by factors such as climate change, the introduction of a predator, parasite, or competitor organism that did not exist previously, or by other causes. These changes modify the criteria for natural selection. Consequently, a very homogeneous population is more vulnerable to environmental changes than one that offers greater diversity, as diversity provides more raw material for selection to act upon.