Consider a gene with a particular function. Mutation X and mutation Y each cause defects in the function of the encoded protein, yet a gene containing both mutations X and Y encodes a protein that works even better than the original protein

Question

The odds are exceedingly small that a single mutational event will generate both mutations X and Y. Explain a simple way that an organism with a mutant gene containing both mutations X and Y could arise during evolution.

Answer 1

The simplest way to evolve the new gene is by duplication and divergence. If the gene is duplicated, then the cell or lineage can maintain one functional, intact old copy of the original gene and can thus tolerate the disabling mutations in the other copy. The other copy can first be modified by the X or Y mutation that impairs its function; second, at some later time, the gene with the single mutation can acquire the additional mutation to yield the doubly mutant X + Y gene with the new or improved function.