If an amino acid sequence is formed incorrectly (i.e. DNA point mutations or frameshift mutations) then the enzyme can either be dysfunctional or non-functional; both of which lead to the end point of enzyme deficiency and overt disease. For example, the mutation of the enzyme glucose-6-phosphate dehydrogenase or G6PD decreases the production of antioxidants which can lead to increased hemolysis of red blood cells causing the clinical manifestation of jaundice (since the end point of metabolism of hemoglobin is bilirubin).
The stronger survived and the weaker passed on... Those with a weaker immune system had trouble with sickness and usually ended up passing on but those with strong immune systems were stronger and healthier and lived.
