SOLVED PROBLEM 1. Two closely related species of bacteria are found to be fixed for two different electrophoretically detected alleles at a locus encoding an enzyme involved in breaking down a nutrient. How could you test experimentally whether the divergence in enzyme sequences may have caused differences in function and fitness?

Solution

In order to test whether the enzymes have different functional properties, one could devise both in vitro and in vivo experiments. If the substrates and properties of the enzyme are known, one could purify the enzyme from each species and measure directly whether there are functional differences. Alternatively, an indirect test would be whether each species grew as well on the particular nutrient that the enzyme broke down.

Ideally, in order to measure fitness differences, one would replace the enzyme-coding region of one species with the enzyme-coding region from the second species and vice versa. Then the growth of each wild-type and transgenic strain could be compared on the same nutrient-containing media, with growth being an indicator of fitness. If there are differences in the relative fitness of the transgenic and wild-type strains, then it is possible that the enzymes have diverged under natural selection. If not, then it is likely that the enzymes have evolved neutrally or that the effect of selection is too small to measure experimentally.