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Interestingly, it turns out that both important elements of structure, but also of sequence,
have been conserved in this important RNA molecule over the huge evolutionary distance
of two billion years during which the three organisms baker’s yeast (S. cerevisiae), brew
er’s yeast (S. pombe) and humans have evolved from each other. Of course, this is not so
much the case with less important molecules, and there are enough molecules that are only
found in humans but not in either of the yeasts and so on.
Importantly, however, looking at RNA folding and RNA sequence allows us to see
important conserved structures in organisms by comparison, and based on that, how evolu
tion causes such structures to form and adapt.
It is easy to imagine a mutation swapping one or more letters. And it is indeed the case
that the sequence already changes much faster than the structure in a relatively short time
(“short evolutionary distance”). Yet short evolutionary distances mean millions of years. If
a letter of such an important molecule is successfully exchanged, it takes thousands of
generations until this happens by chance and is not immediately eliminated by disadvan
tages for the cell (negative mutation) (because the organism dies). Thus, over “short evo
lutionary distances” (typically millions of years, many thousands of generations), a few
nucleotides can be exchanged, but the structure remains the same (as can easily be seen in
the figure). Over even further distances (many millions of years – like humans and yeast
cells, which separated their evolutionary lineages about two billion years ago), even the
structure can change. This happens so slowly because when the structure changes, the
partner molecules must also adapt. Such combined mutations take time to occur randomly
in the generational sequence. The easiest to understand is a combined mutation that does
not change the structure of the single molecule at all, a so-called compensatory mutation.
10.3 Measuring Evolution: Sequence and Secondary Structure