Is it possible for a change of a single nucleotide in DNA to have an observable effect on phenotype?

No change occurs in phenotype.

Some mutations don't have any noticeable effect on the phenotype of an organism.

racism and segregation.

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.