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Phylogenetic Clustering of Indian philosophical schools:
They originate from same culture and parent civilization. Hence, although they amongst themselves are different belief systems, they belong to same family and share lot of characteristics with each other. Lets perform a phylogenetic assessment of Indian religions.
Shaiva and Vaishnava are different belief systems. however, they both accept the Vedic philosophy and authority. Although different religions, they belong to same family and are comparatively very close to each other, as compared to say Buddhism and Vaishnavism. Buddhism and Shaiva, although different belief systems, are comparatively very close to each other when compared to Buddhism and Islam.
Its like Gorilla, Chimpanzee and Humans, belong to same family and are genetically very close to each other. In fact, almost 98% of genetic similarity between chimps and humans. But, remaining 2 percent makes them different species. Hence they are clubbed under the family name of Hominidae. On the other hand, Tiger belongs to feline (Cat) family and is very far away from Hominids in terms of genetics. Even amongst the family Hominidae, Humans are more closer to Chimps than they are to gorillas.
Similarly, Shaiva is more closer to Vaishnava, than it is to say Buddhism. Each of them are different species, but yet, we have such clusters on basis of common characters. This is not possible between a Cheetah and Orang-utan, just like it is not possible to cluster Jainism and Christianity.
Furthermore, if we extend this logic - say if an intelligent lion enters territory of hominids for the first time, with intent of conquest, he will identify everybody with similar characters as one group. Although if he is smart enough he will observe the difference between Human and chimp, but, since him aim is conquest and all of them are his enemies, he will cluster them as hominids :-
Just like Islam clubbed Indian religions as Hindu, simplicity for persecution.
There was no need for clustering between Humans, Chimps, Gorillas, Orang-utans, Bonobos when they were living amongst themselves. Everybody knew and identified their own kind. It was only after the encounter with member from whole different family, who came with intent of conquest and conversion of ape land to cat-land, did this necessity arise.
There were other conquerors like Greeks, Kushans, Huns, Scythians, they came, they won, lost, got assimilated in one of the many philosophical schools of India eventually. Muslims came to change the identity of the India as Islamic land, not just to cherry-pick India's wealth. Hence need of clustering.
However, in retrospect, that makes sense, because, all the species of Ape land are in fact very close to each other and have similar genetic and behavioural patterns in some respect and different in some other respect.Hence when comparing with members of other families, it makes sense to treat them as one family, instead of separate species, because, there are so many of different species of Hindu religions, that it just becomes too complex if we treat this at species level.
Hence the term Hindu religions OR Dharmic religions. Hindu denotes a family, not a individual species which is has name as the family in modern times. Hindu is synonymous to Indian in Persian.
Better understanding of evolution will tell that this is simplification to have better resolution of extreme complexity. The differences between Saamkhya, Yoga, Vedanta, Shaiva, Vaishnava, Buddhism, Jainism and others are too subtle for a crude eye of an outsider to understand. Furthermore, all these school of thoughts, started as intellectual movement by a person or group of person having a different take on nature of universe, but coming from same background. Buddha himself was a Kshatriya and by that standards, he must have learnt Vedas and other vedic philosophy existing prior to him in his formative years. Same is the case with Mahavir. Using this education, they built a different opinion about universe.
Hence the similarities. Differences are due to their different takes on the issue.