Theories of Evolution: Lamarckism, Darwinism, and Neo-Darwinism
Understanding the theories of evolution is crucial for mastering NEET Biology, particularly in the Cell Biology and Genetics sections. These theories explain the mechanisms by which life on Earth has diversified over millions of years. We will explore the foundational ideas of Lamarck, Darwin, and the modern synthesis, Neo-Darwinism.
Lamarckism: The Inheritance of Acquired Characteristics
Jean-Baptiste Lamarck, in the early 19th century, proposed one of the first comprehensive theories of evolution. His theory was based on two main principles:
Use and Disuse: Organisms develop traits based on their needs and usage.
Lamarck suggested that parts of the body that are used more frequently become stronger and larger, while those that are not used tend to deteriorate.
For instance, he hypothesized that giraffes developed long necks by stretching to reach higher leaves, and this acquired trait was passed down to their offspring.
Inheritance of Acquired Characteristics: Traits acquired during an organism's lifetime are inherited by its progeny.
This principle states that changes an organism undergoes due to environmental influences or behavior can be passed on to its offspring.
This is the most controversial aspect of Lamarckism, as modern genetics has shown that acquired traits are generally not encoded in germ cells (sperm and egg) and therefore not heritable.
Lamarck's theory, while influential, is largely discredited due to the lack of evidence for the inheritance of acquired characteristics.
Darwinism: Natural Selection
Charles Darwin, along with Alfred Russel Wallace, proposed the theory of evolution by natural selection in the mid-19th century. This theory is supported by several key observations and inferences:
Variation: Individuals within a population exhibit variations in their traits.
No two individuals are exactly alike. These variations can be in physical characteristics, behavior, or physiology.
These variations arise randomly through processes like mutation and genetic recombination.
Inheritance: Many of these variations are heritable.
Offspring tend to resemble their parents, meaning traits are passed down from one generation to the next.
This is a fundamental concept that Darwin observed, though he didn't know the genetic mechanisms behind it.
Overproduction and Struggle for Existence: More offspring are produced than can possibly survive.
Organisms produce more offspring than the environment can support, leading to competition for limited resources.
This competition creates a 'struggle for existence'.
Differential Survival and Reproduction (Natural Selection): Individuals with advantageous traits are more likely to survive and reproduce.
In the struggle for existence, individuals possessing traits that make them better adapted to their environment are more likely to survive and pass on those traits.
This leads to a gradual change in the population over time, as favorable traits become more common.
Natural selection acts on existing variations within a population; it does not create new traits.
Neo-Darwinism: The Modern Synthesis
Neo-Darwinism, also known as the Modern Synthesis, emerged in the mid-20th century. It integrates Darwin's theory of natural selection with modern genetics, particularly Mendelian genetics and population genetics. Key components include:
Genetic Basis of Variation: Mutations and genetic recombination are the primary sources of heritable variation.
Unlike Lamarck, Neo-Darwinism identifies the molecular mechanisms (mutations in DNA, gene flow, genetic drift) that create and shuffle genetic variation.
These variations are random with respect to the needs of the organism.
Natural Selection Acts on Phenotypes: Selection favors individuals with advantageous phenotypes, which are the result of their genotypes.
The environment 'selects' individuals based on their observable traits (phenotypes), but it's the underlying genetic makeup (genotype) that is passed on.
This process leads to changes in allele frequencies within a population over generations.
Population as the Unit of Evolution: Evolution is defined as a change in the genetic makeup of a population over time.
Individual organisms do not evolve; rather, populations evolve as the frequency of genes changes across generations.
This concept is central to population genetics.
This diagram illustrates the core concepts of Darwinian evolution: variation within a population, inheritance of traits, overproduction leading to a struggle for existence, and differential survival and reproduction where individuals with advantageous traits are more likely to pass them on, leading to adaptation over time. The process is driven by random genetic variation and non-random selection.
Text-based content
Library pages focus on text content
| Feature | Lamarckism | Darwinism | Neo-Darwinism |
|---|---|---|---|
| Source of Variation | Use and Disuse | Random variation (unspecified) | Mutation & Genetic Recombination |
| Inheritance | Acquired Characteristics | Heritable traits | Heritable traits (genes) |
| Mechanism of Change | Directed by need | Natural Selection | Natural Selection + Genetic Drift + Gene Flow |
| Unit of Evolution | Individual | Individual (leading to population change) | Population |
Key Takeaways for NEET
Inheritance of acquired characteristics.
Natural selection.
Mutations and genetic recombination.
The population.
Learning Resources
This blog post clearly outlines the fundamental differences between Lamarck's and Darwin's theories, highlighting their core principles and contrasting their mechanisms.
A comprehensive video explaining the concept of natural selection, its components, and how it drives evolutionary change, presented in an accessible manner.
An in-depth article from Britannica detailing Darwin's life, his voyage, and the development and core tenets of his theory of evolution by natural selection.
This resource provides a clear definition and explanation of Neo-Darwinism, emphasizing its integration of genetics with Darwinian principles.
An educational article from UC Berkeley's Understanding Evolution website that explains the historical development and key components of the Modern Synthesis (Neo-Darwinism).
This tutorial breaks down the four main mechanisms of evolution, providing clear explanations and examples for each, including mutation, gene flow, genetic drift, and natural selection.
A concise explanation of Lamarck's theory, its historical context, and why it was largely superseded by Darwin's ideas, with a focus on the inheritance of acquired characteristics.
This video explains the Hardy-Weinberg principle, a foundational concept in population genetics that is crucial for understanding how allele frequencies change (or don't change) in a population, a key aspect of Neo-Darwinism.
This Nature Education article discusses the various lines of evidence that support evolutionary theory, including fossil records, comparative anatomy, embryology, and molecular biology.
The seminal work by Charles Darwin, offering the original and detailed exposition of his theory of evolution by natural selection. A foundational text for understanding Darwinism.