Brogna et al. 2001). However, even if the Adh genes of drosophilids and tephritids are
paralogous, they differentiated at an unusually rapid rate. The age of the duplicates
(estimated assuming that the fastest, most conservative rate of 53.1× 10 −^10 /site per year
between Ceratitis and Bactrocera was the pervasive evolutionary rate after the duplication)
would be 491 Ma, which precedes the origin of insects.
AMD
The average rate within Drosophila species groups is 22.91× 10 −^10 /site per year. But the
willistoni group rate is 36.2, twice as fast as the melanogaster and obscura group rate, which
is 16.3. In turn, the rate between Drosophila subgenera is 38.1× 10 −^10 /site per year, but
between drosophilid genera reduces to 28.6× 10 −^10 /site per year. Comparisons between
the two families cannot be made, because no AMD sequences are available for the
Tephritidae.
DDC
The average rates of this gene are quite uniform in dipterans. The rate ranges from 19.
3 × 10 −^10 /site per year between Drosophila groups, to 23.3× 10 −^10 /site per year between
dipteran families (Table 1.2). However, the rates between species of the same group are
considerably lower, ranging from six to 11.5 (see row 1, Table 1.2). Note in Table 1.2
that the fairly equitable rates of Ddc contrast with the greater variation observed in the
closely linked paralogue Amd.
GPDH
The rate of replacement is ≤ 2 × 10 −^10 /site per year between Drosophila species; 2.5 times
greater (~5× 10 −^10 ) between species of different genera; and more than 10 times greater
(~23× 10 −^10 ) between species of different families.
SOD
The rate increases as progressively more distantly related dipterans are compared, but the
increase is much less than for GPDH. The rate between Drosophila species is ~20 × 10 −^10 ,
twice as fast when the taxonomic window is enlarged to drosophilids (~40 × 10 −^10 ), and 2.
5 times faster when drosophilids are compared with Ceratitis (~50× 10 −^10 ). The average
rate of replacement between Drosophila species in SOD, however, conceals important
differences among species groups. Thus, the mean rate between species of the obscura
group (4.3× 10 −^10 ) is about half the rate in the melanogaster group (9.4) or between the
two Chymomyza species (8.8), and 10 times slower than the rate between species of the
willistoni group (44.8).
MOLECULAR CLOCKS: WHENCE AND WHITHER? 15