Lamarckism, Neo-Lamarckism, Darwinism and Neo-Darwinism

Organic evolution describes the process by which living organisms change over generations through the modification of their genetic characteristics. Various theories have been proposed to explain the mechanism of this change.

Lamarckism

Proposed by Jean-Baptiste Lamarck in 1809, this theory is often termed the theory of inheritance of acquired characters. Lamarck argued that organisms evolve due to their own efforts and changes in their environment.

Core Principles

• Use and Disuse: Organs that are used extensively grow stronger and larger, while those that remain unused gradually atrophy and vanish.
• Inheritance of Acquired Characters: Any physical change acquired by an individual during its lifetime through environmental influence or self-effort is passed on to its offspring.

Examples and Criticisms

• The classic example is the giraffe, which purportedly acquired a long neck by stretching to reach higher leaves, passing this trait to its descendants.
• August Weismann disproved this theory with his germplasm experiment, where he cut the tails of mice for 22 generations. The offspring were always born with tails, proving that physical changes to somatic cells do not affect inherited germ cells.

Neo-Lamarckism

Neo-Lamarckism emerged to rescue the theory by suggesting that environment does influence heredity but only through specific mechanisms.

Key Modifications

• Supporters argue that environmental changes affect the germplasm indirectly through chemicals or other external pressures.
• Some modern studies in epigenetics—where gene expression is altered without changing the DNA sequence—are sometimes cited as providing a biological basis for neo-Lamarckian concepts.

Darwinism

Charles Darwin and Alfred Russel Wallace independently proposed the theory of natural selection in 1858. Darwin detailed this in his book, On the Origin of Species.

Core Principles

• Overproduction: Organisms produce more offspring than the environment can support.
• Struggle for Existence: Since resources are limited, individuals compete for food, space, and mates.
• Variations: Individuals within a population exhibit small, heritable variations.
• Natural Selection: Individuals with favorable variations are better adapted to their environment and have a higher chance of survival and reproduction.
• Survival of the Fittest: Those who survive pass their favorable traits to the next generation.

Limitations

• Darwin could not explain the origin of variations or the mechanism of inheritance.
• The theory focused on small, continuous variations, failing to account for discontinuous, sudden variations.

Neo-Darwinism (Modern Synthetic Theory)

Neo-Darwinism is the integration of Darwin’s theory of natural selection with modern insights from genetics, particularly Mendelian inheritance and population genetics.

Core Principles

• Genetic Variation: Evolution is driven by changes in the gene pool of a population.
• Sources of Variation: Mutations, genetic recombination during sexual reproduction, and gene flow introduce new traits.
• Natural Selection: It acts on these genetic variations, increasing the frequency of alleles that improve fitness.
• Isolation: Reproductive isolation prevents interbreeding between populations, leading to speciation.

Comparative Overview of Evolutionary Theories

Key Evolutionary Facts

• Evolutionary biology relies on specific definitions to distinguish between theories. Microevolution refers to changes in allele frequencies within a population over a short period. Macroevolution refers to larger changes that lead to the origin of new species.
• The term germplasm refers to the reproductive cells containing genetic information. Changes here are heritable, whereas changes in somatic cells, which constitute the body, are not.Hardy-Weinberg equilibrium provides a mathematical framework for evolutionary study. It states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary influences like mutation, selection, or migration. Evolution occurs when these conditions are disrupted.
• Mutation is the sudden, permanent change in the DNA sequence. It is the raw material for evolution. If a mutation provides a survival advantage, it is selected by natural selection. Most mutations are neutral or harmful, but rare beneficial ones drive adaptation.
• Speciation is the process by which a new species evolves. Allopatric speciation occurs when a population is physically separated by a geographic barrier, such as a mountain or river. Sympatric speciation occurs when a new species evolves from a single ancestral species while inhabiting the same geographic region.

Adaptive radiation is a process where organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available. A famous example is Darwin’s finches in the Galapagos Islands, where different beak shapes evolved to exploit different food sources.