Bacterial evolution: Since Escherichia coli and Salmonella diverged evolutionarily, which major genomic process best explains their present differences?

Introduction / Context:
Closely related enteric bacteria such as E. coli and Salmonella provide classic case studies in how bacterial genomes evolve. Gene gain, loss, and rearrangement driven by mobile elements and phage contribute to pathogenesis, metabolism, and host interaction differences.

Given Data / Assumptions:

• E. coli and Salmonella share a common ancestor.
• Genomic analyses reveal numerous genomic islands and prophages in E. coli strains.
• We select the best single characterization of divergence.

Concept / Approach:

Horizontal gene transfer (HGT) through plasmids, transposons, integrative elements, and bacteriophages introduces new functions (toxins, adhesion, metabolism). E. coli lineages exhibit mosaic genomes with many acquired loci, whereas “little change” or “50% loss” are unsupported extremes.

Step-by-Step Solution:

Verification / Alternative check:

Comparative pan-genome studies show substantial accessory gene pools in E. coli consistent with frequent HGT events.

Why Other Options Are Wrong:

“Little change”: contradicts extensive variability among E. coli strains.

“50% loss”: not supported; E. coli does not show such extreme reduction relative to its ancestor.

“None of these”: incorrect because HGT is well documented.

Common Pitfalls:

Assuming a uniform genome across a species; many E. coli strains are highly variable due to HGT and recombination.

Final Answer:

E. coli has acquired many genes via horizontal transfer