This paper reports acoustic-phonetic data on three speaking styles: Informal conversational speech, “clear” speech and Baby Talk. These sets of observations illustrate the fact that a speaker's pronunciation of a given linguistic form can undergo rather drastic physical transformations, particularly in the wide range of contexts presented by spontaneously produced speech. Despite their extensive variations, vowel formant measurements showed a high degree of predictability. The findings bring to the fore a classical issue of speech research: signal variability and phonetic invariance. While the present results do not conclusively preclude the possibility that the investigated speech samples are organized around a core of signal invariants, the extent as well as the systematicity of the variability observed lend support to a different perspective. The paper proposes that the variegated acoustic pattern of speech be seen as products of adaptation. According to this interpretation, phonetic gestures and signals are modulated and tuned adaptively in accordance with on-line communicative and socio-linguistic demands (e.g., controlling the “social distance” between speakers, preserving intelligibility, performing “phatic” and “emotive” functions, etc). Furthermore, it is argued that the linguistic task of the phonetic signal is not to encode invariants but to complement information already available to the speech processing system of the listener. Accordingly, intra-speaker phonetic variations need not be seen as invariants embedded in linguistically irrelevant variability. They rather represent genuine behavioral adaptations that may jeopardize or demolish signal invariance but that transform speech patterns in essentially principled ways.