Genes do not arise de novo." Keeping in view this fact describe the phenomena that must have helped in increasing the number of genes during evolution.

Mechanisms Increasing Gene Number During Evolution

The increase in gene number during evolution isn't a result of spontaneous generation but rather a series of processes that manipulate existing genetic material. These mechanisms can be broadly categorized into duplication events, rearrangement events, and the contribution of mobile genetic elements.

1. Gene Duplication

Gene duplication is arguably the most significant mechanism for increasing gene number. It occurs when a segment of DNA containing a gene is copied, resulting in two or more identical or nearly identical copies of the gene. These duplicates can then evolve independently.

• Types of Gene Duplication:
  • Whole-Genome Duplication (Polyploidy): Duplication of the entire genome. Common in plants (e.g., wheat, *Triticum aestivum* is hexaploid – 6n).
  • Segmental Duplication: Duplication of a portion of a chromosome.
  • Gene Conversion: Non-reciprocal transfer of genetic information between similar sequences.
• Evolutionary Consequences: Duplicated genes can undergo:
  • Nonfunctionalization: One copy becomes a pseudogene.
  • Neofunctionalization: One copy acquires a new function.
  • Subfunctionalization: Each copy specializes in a subset of the original gene's functions.

2. Exon Shuffling

Exon shuffling involves the recombination of exons from different genes to create novel genes with new combinations of functional domains. This process contributes to the evolution of proteins with altered or expanded functions.

• Mechanism: Introns, being non-coding regions, are more prone to recombination. Unequal crossing over between introns can lead to the exchange of exons.
• Example: The evolution of antibody diversity in vertebrates relies heavily on exon shuffling within immunoglobulin genes.

3. Horizontal Gene Transfer (HGT)

Horizontal gene transfer is the transfer of genetic material between organisms that are not parent and offspring. This is particularly common in bacteria and archaea, but can also occur in eukaryotes.

• Mechanisms:
  • Transformation: Uptake of naked DNA from the environment.
  • Transduction: Transfer of DNA via viruses (bacteriophages).
  • Conjugation: Transfer of DNA via direct cell-to-cell contact.
• Impact: HGT can introduce entirely new genes into a genome, rapidly increasing genetic diversity and contributing to antibiotic resistance in bacteria.

4. Retrotransposition

Retrotransposition involves the movement of RNA transcripts back into the genome, where they are reverse transcribed into DNA and inserted at a new location. This process often creates gene duplicates lacking introns.

• Mechanism: Utilizes reverse transcriptase, an enzyme encoded by retrotransposons.
• Consequences: Can lead to the creation of processed pseudogenes (non-functional copies of genes without introns) and, occasionally, functional gene duplicates.

5. Role of Mobile Genetic Elements (Transposons)

Transposons, or "jumping genes," are DNA sequences that can move from one location to another within the genome. They can contribute to gene duplication, exon shuffling, and the creation of new genes.

• Types: DNA transposons and retrotransposons.
• Impact: Transposons can disrupt gene function, alter gene expression, and contribute to genome instability. However, they can also provide raw material for the evolution of new genes.

Mechanism	Process Description	Example
Gene Duplication	Copying of existing genes	Polyploidy in plants (wheat)
Exon Shuffling	Recombination of exons from different genes	Antibody diversity in vertebrates
Horizontal Gene Transfer	Transfer of genes between unrelated organisms	Antibiotic resistance genes in bacteria
Retrotransposition	RNA transcripts converted to DNA and inserted into genome	Creation of processed pseudogenes
Transposons	Movement of DNA sequences within the genome	Genome instability and gene disruption