The article by Ravi Jain, Maria C. Rivera, and James A. Lake explores the hypothesis of horizontal gene transfer (HGT) among prokaryotic genomes. They analyze six complete prokaryotic genomes to test two theories: the continual horizontal transfer hypothesis and the early massive horizontal transfer hypothesis. The authors find that operational genes (involved in housekeeping functions) have been horizontally transferred continuously since the divergence of prokaryotes, rather than in a few massive events early in their evolution. They also show that differences in evolutionary rates between operational and informational genes (involved in transcription and translation) are minimal, suggesting that other factors, such as gene complexity, are responsible for the observed differences in HGT rates. The complexity hypothesis posits that informational genes, being part of large, complex systems, are less likely to be horizontally transferred compared to operational genes, which are often smaller and more modular. The study supports the continual horizontal transfer hypothesis and highlights the significant role of HGT in prokaryotic genome evolution.The article by Ravi Jain, Maria C. Rivera, and James A. Lake explores the hypothesis of horizontal gene transfer (HGT) among prokaryotic genomes. They analyze six complete prokaryotic genomes to test two theories: the continual horizontal transfer hypothesis and the early massive horizontal transfer hypothesis. The authors find that operational genes (involved in housekeeping functions) have been horizontally transferred continuously since the divergence of prokaryotes, rather than in a few massive events early in their evolution. They also show that differences in evolutionary rates between operational and informational genes (involved in transcription and translation) are minimal, suggesting that other factors, such as gene complexity, are responsible for the observed differences in HGT rates. The complexity hypothesis posits that informational genes, being part of large, complex systems, are less likely to be horizontally transferred compared to operational genes, which are often smaller and more modular. The study supports the continual horizontal transfer hypothesis and highlights the significant role of HGT in prokaryotic genome evolution.