When called first, a checksum entry is generated for all species with data in the selected alignment. For alignment 'ali_16s' this entry is named 'checksum_ali_16s'.
When called again later, the stored checksums are compared with the current checksums. If a checksum change is detected, an entry describing the change is stored in the field 'seq_history'.
The first checksum in the 'checksum_xxx'-entries is calculated with gaps ignored (i.e. it reflects changes to the sequence data), the second checksum is calculate using gaps (i.e. it reflects changes to the alignment of the sequence).