Model Answer
0 min readIntroduction
Organic evolution is the process of change in the characteristics of a species over several generations and relies on the inheritance of traits. It’s a cornerstone of modern biology, explaining the diversity of life on Earth. Initially, explanations were largely philosophical, but the 19th and 20th centuries saw the development of scientific theories attempting to explain the ‘how’ of evolution. These theories, ranging from Lamarck’s inheritance of acquired characteristics to the modern synthesis incorporating genetics, have shaped our understanding of life’s history. This answer will briefly discuss these key theories, supported by examples and evidence.
Lamarckism (Inheritance of Acquired Characteristics)
Proposed by Jean-Baptiste Lamarck in 1809, this theory suggests that organisms acquire traits during their lifetime in response to environmental pressures, and these acquired traits are then passed on to their offspring. Two main principles underpin Lamarckism: Use and Disuse – organs used extensively become larger and stronger, while those not used deteriorate; and Inheritance of Acquired Characteristics – modifications acquired during an organism’s life are heritable.
Example: Giraffes, according to Lamarck, developed long necks because their ancestors constantly stretched to reach high leaves, and this stretched neck was passed on to subsequent generations.
Evidence & Limitations: While intuitively appealing, Lamarckism lacks empirical support. Experiments have shown that acquired characteristics are generally not inherited. For instance, a mouse with its tail cut off will still produce offspring with tails.
Darwinism (Theory of Natural Selection)
Charles Darwin, in his 1859 book "On the Origin of Species," proposed the theory of natural selection. This theory posits that individuals within a population exhibit variations, and those with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring. This leads to gradual changes in the population over time.
Key Principles:
- Variation: Individuals within a population differ in their traits.
- Inheritance: Traits are passed from parents to offspring.
- Selection: Individuals with advantageous traits are more likely to survive and reproduce.
- Adaptation: Over time, populations become better adapted to their environment.
Example: The evolution of Darwin’s finches on the Galapagos Islands. Different beak shapes evolved in response to different food sources.
Evidence & Limitations: Darwin’s theory was supported by observations from his voyage on the HMS Beagle and fossil evidence. However, it lacked a mechanism to explain the source of variation and how traits were inherited.
Mutation Theory
Hugo de Vries, in the early 20th century, proposed the Mutation Theory. He observed that new species arose suddenly due to large, discontinuous variations called mutations. He studied the evening primrose (Oenothera lamarckiana) and observed significant variations appearing in a single generation.
Key Principles: Mutations are the source of new variations, and these mutations are random and heritable.
Evidence & Limitations: While mutations are indeed a source of variation, de Vries underestimated their frequency and the importance of gradual variations. Most mutations are harmful or neutral, and mutation theory alone cannot explain the complexity of evolutionary change.
Modern Synthetic Theory
The Modern Synthetic Theory, developed in the 1930s and 1940s, integrates Darwin’s theory of natural selection with Mendelian genetics. It explains the source of variation (mutations and genetic recombination), the mechanism of inheritance (genes), and the process of adaptation (natural selection).
Key Components:
- Genetic Variation: Mutations and genetic recombination create genetic variation within populations.
- Gene Pool: The total collection of genes in a population.
- Hardy-Weinberg Equilibrium: A principle describing the conditions under which gene frequencies remain stable in a population.
- Natural Selection: Acts on genetic variation, favoring individuals with advantageous traits.
- Speciation: The process by which new species arise.
Example: The peppered moth (Biston betularia) in England. During the Industrial Revolution, dark-colored moths became more common due to increased pollution, which darkened tree bark. This is a classic example of natural selection acting on pre-existing genetic variation.
Evidence: Supported by extensive evidence from genetics, paleontology, biogeography, and molecular biology.
| Theory | Key Principle | Source of Variation | Inheritance Mechanism |
|---|---|---|---|
| Lamarckism | Inheritance of acquired characteristics | Use and disuse | Not well defined |
| Darwinism | Natural selection | Gradual variations | Not well defined |
| Mutation Theory | Sudden, large mutations | Mutations | Mendelian inheritance |
| Modern Synthetic Theory | Integration of Darwinism & Genetics | Mutations & Genetic Recombination | Mendelian inheritance |
Conclusion
In conclusion, our understanding of organic evolution has progressed from early philosophical ideas to sophisticated scientific theories. While Lamarckism and Mutation Theory contributed to the initial discourse, Darwin’s theory of natural selection, refined and expanded by the Modern Synthetic Theory, provides the most comprehensive and well-supported explanation for the diversity of life. The integration of genetics with natural selection has revolutionized our understanding of how species adapt and evolve over time, shaping the biological world we observe today.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.