GENE DUPLICATION

Recall that most mutations are selected against. Sometimes, however,
an extra copy of a gene is present. One copy may be modified,
but as long as the second copy furnishes the essential protein,
the organism is likely to survive. Gene duplication, the accidental
duplication of a gene on a chromosome, is one way that extra genetic
material can arise.
Vertebrate hemoglobin and myoglobin are believed to have
arisen from a common ancestral molecule. Hemoglobin carries
oxygen in red blood cells, and myoglobin is an oxygen storage
molecule in muscle. The ancestral molecule probably carried out
both functions. However, about 1 billion years ago, gene duplication
followed by mutation of one gene resulted in the formation of
two polypeptides: myoglobin and hemoglobin. Further gene duplications
over the last 500 million years probably explain why most
vertebrates, other than primitive fishes, have hemoglobin molecules
consisting of four polypeptides.