Modern Theory of Evolution

• Term Coined By: Julian Huxley (termed it the "modern synthesis of evolution").
• Core Concept: The theory integrates Darwin's natural selection with modern findings from population genetics, palaeontology, and biogeography.
• Unit of Evolution: The population is recognised as the basic unit that undergoes gradual evolutionary change, even though natural selection acts directly on the individual.

According to Stebbins in his book ‘Process of Organic Evolution’, several key factors drive evolutionary processes. These can be classified based on their specific roles:

1. Factors Causing Genetic Variation

• Gene mutations: Alterations in the genetic material.
• Chromosomal changes: Alterations in the structure and number of chromosomes.
• Genetic recombination: The reshuffling of genes during sexual reproduction.
• Hybridisation: Interbreeding among different races, species, or related genera to introduce new genetic variations.

2. Factors Directing the Evolutionary Process

• Natural selection: Acts on existing variations, favouring traits suited for survival.
• Reproductive isolation: Prevents different populations from interbreeding, leading to the development of distinct species.

3. Additional Evolutionary Force

• Migration: The movement of individuals between populations, which introduces new genes into a population.

Most Critical Factors: While all these mechanisms play a role, the three most important forces for driving evolution are mutations, genetic recombination, and natural selection.

• The Modern Synthetic Theory integrates Darwin's natural selection with principles from population genetics, palaeontology, and biogeography.
• It establishes the population, rather than the individual, as the fundamental unit that undergoes gradual evolutionary change.
• Genetic variation within a population is primarily generated by gene mutations, chromosomal alterations, genetic recombination, and hybridisation.
• Evolutionary processes are actively directed by the forces of natural selection and reproductive isolation.
• While migration introduces new genetic variations, mutations, genetic recombination, and natural selection remain the most critical drivers of evolution.