If, however, the communities have split and are no longer
Linguists rely on systematic sound changes to establish the relationships
between languages. The basic idea is that when a change occurs within
a speech community, it gets diffused across the entire community of speakers
of the language. If, however, the communities have split and are no longer
in contact, a change that happens in one community does not get
diffused to the other community. Thus a change that happened between early
and late Latin would show up in all the 'daughter' languages of Latin, but
once the late Latin speakers of the Iberian peninsula were no longer in
regular contact with other late Latin speakers, a change that happened there
would not spread to the other communities. Languages that share innovations
are considered to have shared a common history apart from other languages,
and are put on the same branch of the language family tree.