Population size
Some species have open (or extensive) pangenomes, while others have closed pangenomes. For species with a closed pan-genome, very few genes are added per sequenced genome (after sequencing many strains), and the size of the full pangenome can be theoretically predicted. Species with an open pangenome have enough genes added per additional sequenced genome that predicting the size of the full pangenome is impossible. Population size and niche versatility have been suggested as the most influential factors in determining pan-genome size.