Predicted versus actual responses for sets of codons with identical composition. Each line is the sum of eight codons with the same GC content (by position). Each solid circle is a species. Lines of open circles are the theoretical predictions based on the four-parameter model. (a) All-GC (blue) and all-AT (red) codons in prokaryotes. (b) Codons with two G or C and one A or T, the minority base being at the first (blue), second (green), or third (red) position. Note that the third-position slope is actually of opposite sign to the first- and second-position slopes. The orange line is what would be expected if there were no position dependence (that is, P(GC)2P(AT) as in ). (c) As in (b), but for codons with two A or T and one G or C. In this case, the orange line is P(AT)2P(GC). (d) As in (c), but for eukaryotes. (e) As in (d), but now each point is a randomly chosen gene in Drosophila.