Theories of Organic Evolution

Evolution, the process through which current organisms are descended from old ancestors, is backed by a wide range of evidence. The mechanisms behind the process, however, have been a matter of vigorous scientific investigation. A number of leading theories have been advanced to explain how evolution takes place.

1.0Theory of Inheritance of Acquired Characteristics (Lamarckism)

• Supporter: Jean-Baptiste Lamarck (1744-1829)
• Key Idea: Lamarck suggested that animals evolve with time by passing the characteristics they develop in their lifetime to their offspring. It implies that alterations that an organism acquires in its lifetime depending on its environment are transmitted to its offspring.
• Core Principles:

Use and Disuse: Organs which are repeatedly used become stronger and larger, and the unused ones become weaker.

Inheritance of Acquired Characteristics: Acquired characteristics of an organism are passed on to its offspring.   

• Example: Lamarck's own example was the elongation of giraffes' necks. Lamarck assumed that giraffes had stretched their necks so as to access leaves on high branches, and this acquired trait was passed on, leading to longer and longer necks from generation to generation.
• Current Status: Lamarckism is largely discounted. Modern genetics has shown that acquired characteristics are not normally transmitted to the offspring. Genetic changes (mutations) within the reproductive cells (sperm and eggs) are the primary source of variation that can be inherited.

2.0Theory of Natural Selection (Darwinism)

• Supporters: Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913)
• Key Idea: Darwin's theory of natural selection proposes that evolution is achieved through the survival and reproduction differentially of individuals with advantageous heritable traits.
• Core Principles:
• • Variation: Members of a population have variations in their characteristics.
  • Inheritance: Certain variations are heritable, or can be transmitted to offspring.
  • Struggle for Existence: More offspring are produced by organisms than the environment is able to support, leading to competition for the available resources.
  • Natural Selection: Individuals with favorable traits tend to live longer and produce offspring, passing on those traits.
  • Adaptation: Natural selection makes the desirable traits accumulate over time, resulting in adaptation to the environment.   
• Example: Darwin's research on the finches on the Galapagos Islands showed how beak forms varied according to the source of food available, illustrating adaptation through natural selection.
• Current Status: Darwin's natural selection forms the foundation of modern evolutionary theory. It is highly supported by incredible evidence from genetics, paleontology, and biogeography.

3.0Modern Synthesis (Neo-Darwinism)

• Key Idea: Neo-Darwinism, or the new synthesis, combines Darwin's theory of natural selection with Mendel's genetics. It provides a fuller description of evolution by incorporating the contribution of genetic variation.  
• Key Concepts:
• • Genetic Variation: Recombination of genes and mutations are the primary reasons for variation.
  • Gene Pool: Entire set of genes in a population.
  • Natural Selection: Acts on genetic variation in the gene pool, favoring alleles that increase reproductive success.
  • Genetic drift: Unpredictable alteration in allele frequency, most important in small populations.
  • Gene Flow: Gene movement between populations, which can introduce new alleles.
  • Speciation: How species become new, usually as a result of reproductive isolation.
• Current Status: Neo-Darwinism is the prevailing evolutionary theory of contemporary biology. It has a robust paradigm for describing the mechanisms of evolutionary change.

4.0Mutation Theory

• Proponent: Hugo de Vries (1848-1935)
• Key Idea: De Vries emphasized the significance of mutations (sudden, profound genetic changes) as the dominant force in evolution.
• Core Principles:
• • Mutations as the Cause of Variation: Mutations are the source of raw material for evolutionary change.
  • Saltation: Evolution in great leap-like steps instead of continuous change.
• Present Status: While mutations do generate variation, they are not typically considered to be part of the evolutionary process per se so much as one of a number of factors. Neo-Darwinism therefore reconciles mutations with natural selection and other influences. Mutations are the origin of the variation upon which natural selection acts.

Evolutionary theory has progressed greatly from the earliest idea of Lamarck. The most general, best-supported description of how evolutionary change operates is Darwin's natural selection, codified by the modern synthesis (Neo-Darwinism). An understanding of these theories is necessary to realize the diversity and interdependence of life on this planet.