Family trees and waves

A branch requires an innovation

A family tree represents descent from a common ancestor and groups daughters by innovations inherited from an intermediate ancestor. A feature retained from a much older ancestor can occur in several branches without defining a new subgroup. Likewise, two neighboring languages can share a borrowed form. Clackson’s discussion of Indo-European subgrouping emphasizes diagnostic shared innovations and warns that lost intermediate dialects can hide the actual path of a change.

A proposed subgroup is strongest when several independent phonological and morphological innovations agree. A single lexical item is vulnerable to copying and replacement. The Austronesian Comparative Dictionary uses branch distribution requirements for its named reconstruction levels for exactly this reason. A tree is a model of one kind of historical relation, not a map of political identity or a claim that contact stopped after branching.

Overlapping histories

In a dialect continuum, innovations can spread across neighboring communities and produce intersecting geographic boundaries. A wave model tracks that diffusion; the pattern may not fit a single clean split. Historical glottometry describes how overlapping innovations arise and why a tree still captures descent while waves show interaction. To infer a diffusion direction, one needs dated forms or other evidence; a modern map alone shows distribution, not motion.

A South American example shows what an actual subgrouping diagnostic looks like. A study of sound grouping notes the loss of a glide in a reconstructed *aja sequence, yielding a long vowel, as a criterion used in identifying Central Quechua. The starred sequence is a published reconstruction, not an inscription. To use it as a subgroup test, inspect multiple cognate sets and whether other Quechua branches lack the same innovation. Do not infer the entire Quechua family structure from one quoted sound change.

Practice

For three related varieties, mark two shared retentions, two proposed shared innovations and one feature that may have spread by contact. Draw a tree supported only by innovations. Then sketch where a wave could cross its branches. Explain which dated or geographic evidence would distinguish the two histories.