"Buchnera only have 600 genes, compared to about 4,000 or 5,000 for E. coli ... This is a recurring pattern in the genomes of both bacterial symbionts and pathogens, but why do they get so small? ... while part of the reduction is due to adaptation, a lot of it just reflects genetic drift ... It's just a consequence of long-term evolution in a restricted environment with small population sizes."