Humans only have a few eye colors and only four abo-based blood types. how can dna tests definitively identify individuals when many people have brown eyes or type a blood?

DNA testing utilized particular regions in our DNA that are unique to every individual. These regoins are tested using tehcnques such as RFLPs (Random Fragments Length polymorphisms),  SNP (Single Nucleotide Polymorphism), STR (Short Tandem Repeats), Y chromosome (for males) and mitochondria DNA analysis (for females).

the denominator has a very specific role.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.