This is a difficult sentence. I will take it apart and then put the parts together again. That way you can see how they fit, and how IS and BEING work here. There are three different ideas here. (1) Brains are behavioural organs. (2) Behavioural adaptation IS immediate and non-random. (3) Behavioural adaptation IS vastly more efficient than genetic adaptation. In sentence (3), "IS" is the verb and "behavioral adaptation" is the subject. That's the "IS" you are asking about. We can put sentences (2) and (3) together by using a subordinate clause. (2) + (3) Behavioural adaptation, WHICH IS immediate and non-random, IS vastly more efficient than genetic adaptation. But the author has done something else. The author has used a participial clause instead of a subordinate clause. The participle is "BEING". (2) + (3) NEW VERSION Behavioural adaptation, BEING immediate and non-random, IS vastly more efficient than genetic adaptation. Participles are adjectives, and so they refer to nouns or noun phrases. Here' "BEING" refers to "behavioural adaptation." This is the BEING you asked about. Finally, the author puts (1) and (2) + (3) together with "and" to get the whole sentence. (1) + (2) + (3) Brains are behavioural organs, and behavioural adaptation, BEING immediate and non-random, IS vastly more efficient than genetic adaptation.