On Mon, 7 Aug 1995 01:33:21 -0400 you wrote to Art Chadwick:
WP>Your argument about Hawaiian Drosophila is fascinating.
>You say that the flies are all Drosophila, so no real evolution
>has occured. Consider the following. Morphologically, some
>of the Hawaiian species are dramatically different from, say,
>Drosophila melanogaster. But the real key is genetic. In the last
>one million years approximately, the Drosophila species in the
>Hawaiian Islands have gone through about 10,000,000 generations.
>In the 5 to 7 million years since humans and chimpanzees shared
>a common ancestor, fewer than 500,000 generations have passed.
>One might therefore expect that the measured genetic distances
>(Nei's method or any of the other widely used methods) between
>some species of Hawaiian Drosophila would be greater than the
>measured genetic distances between humans and chimpanzees.
>
>Thus on both morphological and genetic distance scales, Hawaiian
>Drosophila have undergone greater evolutionary changes than that observed
>between humans and chimpanzees. If all Hawaiian Drosophila are merely
>flies that have undergone no significant evolution, then there is no significant
>evolutionary change between humans and chimpanzees. I suggest you look
>at Drosophila heteroneura as a start, and see if you still think no evolution
>has occurred in Hawaiian Drosophila.
Surely this is too simplistic? Gould points out that based on genetic
similarity, chimps and man could be "sibling species", but they are
not even the same genus:
"When two species scarcely differ in morphology but function as
separate and reproductively isolated populations in nature,
evolutionary biologists speak of "sibling species." Sibling species
generally display far fewer genetic differences than pairs of species
placed in the same genus but clearly different in morphology
("congeneric species"). Now chimps and humans are obviously not
sibling species; we are not even congeneric species by conventional
taxonomic practice (chimps belong to the genus Pan; we are Homo
sapiens). But King and Wilson have shown that the overall genetic
distance between humans and chimps is less than the average for
sibling species and far less than in any tested pair of congeneric
species." (Gould S.J., "Ever Since Darwin", 1977, Penguin, p53)
There is clearly much more than genetic similarity than mere
biochemical distance. The organisation of those similar biochemicals
is clearly all-important. Gould goes on to point out some of these
genetic organisational differences:
"The genetic differences between humans and chimps are minor, but they
include at least ten large inversions and translocations. An
inversion is, literally, the turning around of a chromosomal segment."
(Gould, p55).
All drosophila, on the other hand, despite a greater number of genetic
changes, presumably still remain within the same genus.
Regards.
Stephen
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