Functionality is when a device works to achieve a purpose.
Something that is functional is something that contributes to the completion of a desired task. A wagon with 4 wheels is functional, for example, because it moves. A wagon with 3 wheels is not functional because it does not move adequately.
A major source of controversy about functionality developed after the human genome was completely sequenced in 2003 (and partially in 2001). Whereas previously the biologists had expected to identify about 100,000 human genes, they in fact found only about 20,000. Even before that, these scientists were talking about “junk DNA”, since they could not identify a function for most of the 3 billion or more nucleotides in the genome. They did not ask what the rest of the genetic material might be doing, they just declared that it must have been left over from a long history of evolution and was therefore junk and not functional. The existence of junk DNA, they said, proved that evolution had occurred.
Then the ENCODE consortium started to study what the rest of the human genome might be doing. They published the results of this multiyear study, which involved scientists from around the world in 2012 and they declared that the human genome was 80% functional [Nature 489 #7414 pp. 57-74.] This report (entitled ENCODE II) resulted in a major backlash from many biologists who were particularly attached to evolution theory. Then the results of ENCODE III were published in 2020 [Nature 583 #7818 pp. 699-710]. The study authors did not back down on their claims for function that the previous study reported. Thus, in the latter study the claim was made that “It has become apparent that, by virtually any metric, elements that govern transcription, chromatin organization, splicing and other key aspects of genome control and function are densely encoded in many parts of the human genome sequence.” p. 709
In February 2021, articles celebrating 20 years of the Human Genome Project in Nature also declared: “Thanks in large part to the HGP, it is now appreciated that the majority of functional sequences in the human genome do not encode proteins. Rather, elements such as long non-coding RNAs, promoters, enhancers and countless gene-regulatory motifs work together to bring the genome to life.” [Nature 590 #7845 pp. 212-215 See p. 214] DNA that does not code for a protein is called non-coding, and most of it was previously considered to be junk.
The people doing the actual studies on the human genome concur that the vast majority of our DNA is functional and not junk at all, even if it is non-coding for proteins! The bottom line is that function is an important criterion for good design. The human genome and other creatures’ genomes all show the hallmarks of good design! See junk DNA, ENCODE and Human Genome Project, non-coding DNA.