Explain how just a change of one amino acid has such an effect.

Proteins are made by a sequence of amino acids, if one amino acid is changed, the whole function of the protein changes, or it becomes a non-functional protein that has to be destroyed.

Explanation:

Every protein is made by a specific sequence of amino acids, this is what we call the primary structure of the protein. If just one of the amino acids in the polypeptide chain changes, the fold of the chain, which is the secondary and tertiary structure, will be different, leading to a different function of the protein, or giving a non-functional protein that has to be destroyed.

Internal fertilization is the process when the syngamy (union of male and female gamete) occurs inside the female body after insemination using copulation. In contrast, External fertilization is the syngamy outside the female body, that is in the outer environment especially in water bodies.