Behavioural adaptation is vastly more efficient. immediate and non-random- are some qualities of behavioural adaptation.

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.

'Behavioural adaptation' is the subject and in the excerpt it is said that behavioural adaptation is more efficient than genetic adaptation. I believe 'being immediate and non-random' is a postmodifier but, whilst I am a native speaker, I have not studied English grammar extensively so don't quote me on that. It does not change the meaning of the sentence which is that 'behavioural adaptation is more efficient than genetic adaptation,' but it does give you more information about behavioural adaptation, namely that behavioural adaptation is 'immediate and non-random.'

In this case it is used to state properties or qualities of the brain. Immediate response is a trait of the brain making it more efficient. Some examples of usage: The cat is being cute - the cats is acting in a cut way. John is known for being organised - everyone knows that John is organised. It is a skill or behaviour that he has.