SNAKES AND LADDERS. “Given the fractions of conserved and lineage-specific genes in modern archaeal genomes, this translates into approximately 2,500 genes in the ancestral genome, which is a larger genome than most of the extant archaea possess. The reconstructed pattern of gene loss and gain in archaea is non-trivial: there seems to have been some net gene gain at the base of each of the major archaeal branches that was almost invariably followed by substantial gene loss…this could be a general pattern of genome evolution.”
http://onlinelibrary.wiley.com/doi/10.1002/bies.201300037/full