I'm watching a TED Talk by John Searle who is claiming that consciousness is a biological function. Is there any indisputable evidence for or against this assertion?
Consciousness is a broad concept.
In a binary world (conscious/not conscious), biological evidences are irrefutable. For example, a 2009 Brain article stated that "Impaired consciousness during temporal lobe seizures is related to increased long-distance cortical-subcortical synchronization." (Arthuis et al.). A 2012 Lancet Neurology article had the following sentence "the consciousness system,a specialised set of cortical-subcortical structures, maintains alertness, attention, and awareness" (Blumenfeld) - a system that is described on wikipedia. Here, we can see that the binary view of consciousness can be divided into smaller pieces.
Dehaene et al. ("Conscious, preconscious, and subliminal processing: a testable taxonomy", Trends in cognitive science, 2006) proposed a more complex view within a 2 by 2 frame: consciousness is a product perception (weak and strong) and top-down attention (absent and present) - the fully conscious experience arising in the "strong perception/present attention" case. Here again, biological evidences are strong, but the type of the published article (opinion, not a review-type article) says that consciousness is far from being well understood - not even well defined.
An interesting aspect: the first two articles cited here (1st paragraph) are written by neurologists - meaning pure biology. Dehaene et al. (2nd paragraph) are cognitive psychologists and philosophers - a wee bit more of humanities but still biologically grounded. When it comes to philosophy alone, consciousness can completely loose its biological link: William Lycan, for example, argued in his book Consciousness and Experience that at least eight clearly distinct types of consciousness can be identified (organism consciousness; control consciousness; consciousness of; state/event consciousness; reportability; introspective consciousness; subjective consciousness; self-consciousness) (from wikipedia). And when it comes to non-science, the views on consciousness are purely spiritual (The 144 Levels of consciousness).