Causes of Variations

• Variation: The process by which closely related organisms differ from one another (e.g., offspring from the same parents are not exactly alike).
• It is the fundamental starting point for evolution.
• Key Rule: Only heritable variations contribute to the evolution of a new species.

• Cause: External influences experienced during an organism's lifetime (e.g., availability of food, temperature changes, predators).
• Nature: These are acquired traits that affect only the physical body (soma) of the individual.
• Evolutionary Role: Because they do not affect the genes, they are not passed to offspring and play no role in evolution.

• Cause: Alterations in the gene combinations inherited from parents.
• Evolutionary Role: These are passed down to offspring and are responsible for evolutionary change.

Primary Mechanisms:

A. Genetic Recombination: Occurs during sexual reproduction, ensuring offspring inherit unique genotypes. Driven by:

• Random distribution of homologous chromosomes during meiosis.
• Crossing over.
• Dual parentage and random union of gametes during fertilisation.

B. Mutation: A sudden, inheritable change in genetic makeup (not caused by simple recombination). It is the ultimate source of all new genetic material.

• Gene or Point Mutation: A change in the nucleotide sequence of DNA at a single specific gene.
• Chromosomal Mutation (Aberration): Changes in the morphology (structure) or number of chromosomes during cell division.
• Genomic Mutation: An increase in the dose of the entire genome (entire sets of chromosomes).

• Variation refers to the differences among closely related organisms and serves as the fundamental basis for evolution only when the traits are heritable.
• Environmental conditions induce non-heritable, somatogenic variations that affect only the individual's body and do not contribute to evolutionary processes.
• Evolutionary change strictly depends on heritable variations resulting from alterations in the inherited gene combinations.
• Sexual reproduction constantly generates genetic diversity through recombination mechanisms such as crossing over, random chromosomal distribution, and random fertilisation.
• Mutations act as sudden, inheritable genetic changes - classified as point, chromosomal, or genomic - serving as the ultimate source of novel genetic material in a population.