Final answer:
Mutations often have little phenotypic effect because many are neutral mutations, such as silent point mutations that don't change protein amino acids, or somatic mutations that don't impact the organism broadly.
Step-by-step explanation:
Mutations often have little phenotypic effect because many mutations are neutral mutations. These include silent point mutations that do not change the amino acids in the proteins they encode, thus having no impact on the organism's phenotype or fitness. For example, if a mutation occurs in a non-coding region of DNA or results in a synonymous codon that still encodes for the same amino acid, the mutation is unlikely to affect the protein's function.
In addition to neutral mutations, some mutations may only cause changes to a single cell or to its direct daughter cells, known as somatic mutations, which are typically not passed on to offspring and may not have any significant effect on the organism as a whole. Conversely, mutations in genes important to embryonic development can be severe since embryos are very fragile, and survival requires precise genetic instructions.
It is also worth mentioning that genetic diversity is also increased by processes like sexual reproduction and crossing over during meiosis, ensuring that even if some mutations do not affect an individual's phenotype, there is still a variety of genotypes within a population.
Additionally, while many mutations are neutral, there are also mutations that may be beneficial, such as those enhancing environmental adaptability, or detrimental, leading to decreased fitness and negative selection pressure.