Examine closely Figure Recall that mutations were introduced at 140 randomized nucleotide...

Examine closely Figure Recall that mutations were introduced at 140 randomized nucleotide positions. By the ninth round of selection, 4 nucleotides were responsible for most of the evolutionary change. Look at the other 136 nucleotides. Many of these have reverted to their original state. Why?

Figure Allele frequency changes in an evolving RNA population This histogram, built on a crude representation of the folded secondary structure of the 413-nucleotide Tetrahymena ribozyme, shows the frequenciesof mutations in the test-tube RNA population after nine rounds of selection and reproduction. Four mutations, at nucleotides 94, 215, 313, and 314, have risen to frequencies over 50% and are mainly responsible for the new phenotypic characteristics of the population. Reprinted with permission from Beaudry and Joyce(1992).