Where does coalescence collide?

In population genetics, coalescence is the merging of gene lineages as they are traced backward in time: any sample of copies of a gene can be followed into the past until, pair by pair, the lineages join at common ancestors, until all reach a single most recent common ancestor. Kingman's 1982 The coalescent defined this genealogical process as a rigorous stochastic model and showed that the waiting times between successive coalescent events follow a simple distribution set by population size. The referent is the random ancestral tree of a set of genes, read from present-day variation, and it has nothing to do with two drops fusing into one or two speech sounds merging, beyond the shared image of separate things joining.[1]

Unit

The waiting time, in generations, until lineages join at common ancestors

Recovery means

Not applicable

Measured by

Inferred from patterns of DNA sequence variation in a sample, fitted to coalescent models to estimate population size and history

In fluid mechanics, coalescence is the merging of two liquid drops (or bubbles) into a single larger one once their surfaces touch, as surface tension drives a tiny liquid bridge between them to widen explosively fast. Eggers, Lister, and Stone's 1999 Coalescence of liquid drops analysed the singular early dynamics of this bridge and derived how its radius grows with time as the drops fuse. The referent is a hydrodynamic merging event governed by surface tension, viscosity, and inertia, measured in the laboratory by high-speed imaging, and it is a wholly different thing from ancestral gene lineages joining or two vowels fusing in speech.[2]

Unit

The radius of the liquid bridge as it grows during merging

Recovery means

Not applicable

Measured by

Observed by high-speed imaging of the growing neck between merging drops and compared with hydrodynamic scaling laws

In historical linguistics, coalescence is a sound change or morphological process in which two adjacent segments merge into a single new one, as when two vowels across a lost consonant fuse into one long vowel, or a stem and an affix fuse so their boundary is no longer separable. Campbell's Historical Linguistics: An Introduction catalogues coalescence among the regular types of sound change and distinguishes it from simple loss or assimilation. The referent is the diachronic merging of linguistic units in a language's history, reconstructed by comparing related forms across time, and it shares only the word with fusing drops or joining gene lineages.[3]

Unit

A merger of two segments or morphemes into one new unit

Recovery means

Not applicable

Measured by

Reconstructed by comparing cognate and older forms to show that two earlier units have become one