Both of these workers are alcohol dehydrogenases, but they're substantially different from each other. The solid-colored links are different between the two: green for conservative substitutions, yellow for other ones. (The light blue molecule is NAD.) The worker on the left is Al, the one on the right is alcohol dehydrogenase IV sigma.

Al and sigma are related to each other. A single Instruction for an older alcohol dehydrogenase was duplicated, and the two copies diverged into Al and sigma. After millions of years of evolution, mutations have caused about 31% of the links to differ between Al and sigma: about 10% of the mutations have been conservative, and 20% have been non-conservative. These changes have produced workers specialized for different environments: sigma is 100 times faster than Al at changing ethanol into acetaldehyde, but works best in the stomach's strong acid. Sigma also has a much broader specificity, which lets it work on more kinds of alcohols. (Ethyl alcohol isn't the only alcohol found in food and drink.). Al's tighter specificity, on the other hand, lets him work on breaking down left-over digestive molecules as well as ethyl alcohol.