Some authors tend to think that once upon a time there was one Yoga and that later this has been “altered” or has at least “evolved” into many forms. According to their own stand, they might look at this developments as meaningful adaptations or as soulless metamorphoseis.
Other authors tend to think that there were several trends of Yoga prior to a given point (usually identified with the Yogasūtra (YS) if you agree with Chapple, etc., or with the Pātañjala Yogaśāstra (PYŚ) if you agree with Bronkhorst, Maas, etc.) and that they have been unified into a single system by the author of one or the other text. A long time after that, the same authors claim, new tendencies developed out of this unitary Yoga, much like in the way described by the authors of the fist group.
A minority group of authors contests the idea of a unitary Yoga at all and says that between the various things called Yoga in Classical and Post-Classical India there are at most family resemblances and at least nothing common at all. For these authors, it does not really make sense to host a conference on Yoga with people discussing Buddhist Tantric Yoga, Pāñcarātra Yoga, the Yogasūtra’s, contemporary Yoga practices and so on.
Who is right? Difficult to say. The point is that what we have are only fragments of the whole picture and that our interpretation of it will make us interpret some scattered pieces as belonging to the same puzzle or not. Accordingly, if we assume the first perspective, we will consider a form of Yoga which is far away from Patañjali’s YS (or PYŚ) as still somehow connected with it and detect slight similarities. If we assume the third perspective, we will rather notice the differences between the two.
Similar differences in approach can be detected in the case of Sāṅkhya (where the first scenario is ruled out by the data and scholars either subscribe to the second or to the third approach), Buddhism, the two Mīmāṃsās (Parpola embraces the first scenario, Bronkhorst the third one, there are no clear data in favour of the second one), the two schools of Viśiṣṭādvaita Vedānta and so on. In the latter case, in fact, I only know scholars subscribing to the first scenario. Mumme (1988) is aware of the fact that there were differences between the two schools even before the official split, but still calls them both Śrī Vaiṣṇava and says that they were “complementary”.
Am I forgetting some further example or some further approach? And which approach do you subscribe to in the cases mentioned?