Theories of Organic Evolution (PreDarwinian, Darwinian and Post-Darwinian)

Introduction

The concept of evolution is fundamental to understanding the development and diversity of life on Earth. Theories of organic evolution have evolved over time, from the early speculative ideas of pre-Darwinian thinkers to the well-formulated theory of natural selection proposed by Charles Darwin, and further refined in post-Darwinian times. Each of these stages in the development of evolutionary theory contributed to our current understanding of how species change over time and how new species arise. This article explores these theories, highlighting their historical significance and scientific relevance.

Definition of Evolution

The term “evolution” was first used by the English philosopher Herbert Spencer to describe the historical development of life. Charles Darwin later defined evolution as “descent with modification,” emphasizing that closely related species resemble each other due to shared inheritance but differ due to hereditary changes that occurred during the separation of their ancestors. Other definitions, such as those by Dodson and Dobzhansky, emphasize the divergence of related populations and the development of dissimilarities between ancestral and descendant populations, respectively.

Pre-Darwinian Theories of Evolution

Before Darwin, several thinkers proposed ideas about how life evolved, though these ideas were often speculative and lacked empirical support.

• Aristotle believed that new forms of life arose from old ones, driven by an organism’s inner instinct to adapt and transform.
• Herodotus and Empedocles suggested that plants evolved before animals and that there was a gradual evolution of higher organisms, culminating in the emergence of humans. They also introduced early concepts of adaptation.

These early theories, although not scientifically rigorous, laid the groundwork for later evolutionary thought by introducing ideas about change and adaptation in living organisms.

Darwinian Theory of Evolution by Natural Selection

Charles Darwin’s theory of evolution by natural selection, published in his 1859 book “The Origin of Species,” revolutionized the understanding of biological evolution.

Key Principles of Darwinism

1. Over-production: More individuals are born each generation than can survive and reproduce.
2. Variation and Heredity: Natural variation exists among individuals of the same species, and favorable adaptations are hereditary, passed on to the next generation.
3. Struggle for Existence: Organisms compete for limited resources, leading to struggles within and between species, as well as with environmental conditions.
   • Intraspecific Struggle: Competition among individuals of the same species, where competition is most intense due to similar needs and environments.
   • Interspecific Struggle: Competition between different species living together.
   • Struggle with the Environment: Organisms also face challenges from adverse environmental conditions like floods, cold, and heat waves.
4. Survival of the Fittest (Natural Selection): Individuals with characteristics that provide a survival or reproductive advantage are more likely to survive and pass on those traits.
5. Gradual Modification of Species: Over long geological periods, species gradually change through the accumulation of these small modifications.

Criticism of Darwinism

Despite its revolutionary impact, Darwin’s theory faced criticism:

• Darwin did not explain the origin of variations, a key component of evolution.
• His concept of “survival of the fittest” did not account for the “arrival of the fittest.”
• Darwin did not distinguish between somatic (body) and germinal (reproductive) variations, leading to confusion about heritability.
• The theory struggled to explain non-adaptive characters, overspecialized organs, and the evolution of terrestrial animals from aquatic ancestors.
• Critics also argued that Darwin underestimated the importance of mutations and overemphasized the role of natural selection without sufficient experimental evidence.

Lamarckian Theory of Evolution

Jean-Baptiste Lamarck, in his book “Philosophic Zoology,” proposed an early theory of evolution that is often referred to as the theory of inheritance of acquired characteristics.

Postulates of Lamarckism

1. Elan Vital (Inner Urge): Organisms have an internal drive to grow and develop, leading to changes in size and complexity.
2. Environmental Pressure: Changes in the environment create needs in organisms, leading to the formation of new organs to meet these needs.
3. Use and Disuse: Organs develop with use and deteriorate with disuse, such as the long necks of giraffes from stretching to reach leaves or the loss of limbs in snakes from not using them.
4. Inheritance of Acquired Characteristics: Traits acquired during an organism’s lifetime are passed on to the next generation.

Criticism of Lamarckism

Lamarck’s theory, although influential, was largely discredited by later scientific research:

• Weismann’s Experiment: August Weismann’s experiments showed that cutting off the tails of mice for generations did not result in tailless offspring, disproving the idea that acquired characteristics are inherited.
• Lack of Genetic Basis: Lamarck did not differentiate between somatic and germinal cells, which modern genetics shows is crucial for understanding inheritance.
• Failure to Explain Certain Traits: Lamarck’s theory could not explain the development of traits like the human heart or the structure of the eye, which do not change significantly through use or disuse.

Despite these shortcomings, Lamarck’s work was important for its early attempt to explain how organisms change over time and adapt to their environments.

Post-Darwinian Theories of Evolution

After Darwin, several new theories emerged, further refining the understanding of evolution.

Mutationism

Proposed by Hugo de Vries in 1901, mutationism suggested that mutations, rather than natural selection, were the primary drivers of evolution. According to this theory, evolution occurs in two steps: the random occurrence of mutations, followed by the persistence or elimination of these mutations through selection. De Vries introduced the concept of “macromutations,” which are large, sudden changes that can create new species. However, mutationism was largely replaced by the Modern Synthesis (or Neo-Darwinism) in the 1930s, which integrated genetics with Darwin’s natural selection.

Mendelian Genetics

Gregor Mendel’s work on the inheritance of traits in garden peas laid the foundation for modern genetics. Mendel’s discovery of “factors” (now known as genes) that control hereditary traits and his formulation of the laws of segregation and independent assortment were crucial in understanding how traits are inherited. Mendel’s work led to the development of the Synthetic Theory, which combines Darwin’s natural selection with Mendelian genetics, providing a comprehensive explanation of evolution.

Conclusion

Theories of organic evolution, from pre-Darwinian speculations to Darwin’s natural selection and post-Darwinian developments, have significantly shaped our understanding of life on Earth. Each theory, despite its limitations, contributed valuable insights that helped build the foundation of modern evolutionary biology. Today, the Modern Synthesis integrates these ideas into a unified framework that explains the mechanisms of evolution, making evolution one of the most powerful concepts in science.

1. How did Mendelian genetics contribute to the development of the Modern Synthesis in evolutionary biology? (250 words)
2. In what ways did Lamarck’s theory of evolution differ from Darwin’s natural selection, and what were the key criticisms of Lamarckism? (250 words)
3. Discuss the impact of mutationism on evolutionary theory and how it was eventually integrated into the Modern Synthesis. (250 words)