The term DNA refers to deoxyribonucleic acid. It is a blueprint which specifies exactly how the person is formed, every single detail of them! Every aspect of the human body is dictated by the DNA within each cell. The amount of information in a pinhead of DNA is the equivalent of a pile of paperback books 500 times higher than the distance from the earth to the moon. And the information in each book is unique!
DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Each human cell contains about 3 billion bases of DNA. This is a huge amount of information. It is inconceivable that it could occur by random chance.
There are control mechanisms built into DNA. DNA can replicate itself as it does when cells divide and the cell contains an exact copy of the DNA in the old cell.
Another control mechanism operates to identify damaged DNA and to repair the damage. So changes in the DNA are seen as faults and the cell attempts to repair itself. Therefore the idea that new genetic material can appear in the DNA to create a new species is contrary to the mechanisms supposedly created by the evolutionary process!
Damaged DNA could be passed on to the next generation. In most cases this is detrimental, even harmful. We see all sorts of consequences in people because of incorrect DNA programming resulting in some aspect of their body being formed wrongly. But this is alway a negative consequence often resulting in ruined lives and early deaths.
What we simply do not see are 'improvements' in people. There are approximately six billion people alive on the earth at this moment. None of them were born with any new features that were an improvement on the current model of the human being.