Final answer:
The phylogenetic species concept is the best way to define species for bacteria and archaea that reproduce asexually.
Step-by-step explanation:
The BEST way in which to define species, particularly in the case of bacteria and archaea that exchange DNA but do not reproduce using conventional sexual reproduction, is with the phylogenetic species concept. The phylogenetic species concept defines a species as a group of organisms that share a common ancestor and have a unique set of characteristics that distinguish them from other species. This concept is based on evolutionary relationships and genetic similarities, making it particularly suitable for organisms that reproduce asexually and do not exhibit typical reproductive behaviors.