What characteristic of the species level of organization distinguishes two similar organisms such as ducks and geese as
different species?

The inability to interbreed to produce fertile offspring -is the characteristic of the species level of organization that distinguishes two similar organisms such as ducks and geese as different species. This answer has been confirmed as correct and helpful.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.