Biology Reference
In-Depth Information
Speciation may occur rather rapidly under some circumstances.
Higgie et al.
(2000)
showed that artificial selection can produce the kind of isolation that sep-
arates species in the wild, and that it can do so within nine generations under
laboratory conditions.
Drosophila serrata
and
D. birchii
are sibling species in
Australia that are very similar in morphology and can produce viable and fertile
hybrid progeny in the laboratory. In the field, these two species can be found
in the same area, but rarely interbreed. Where their geographic ranges over-
lap, the two differ in the mix of hydrocarbons found on their cuticle, which is
important in mate choice. Populations of
D. serrata
found in regions of Australia
where
D. birchii
occur have a different set of hydrocarbons. This suggests that
selection to reduce mating between the species has occurred where the two
populations overlap (sympatry). Under laboratory conditions, cuticular hydrocar-
bons of allopatric
D. serrata
populations collected from the field evolved within
nine generations to resemble those of the sympatric populations when held
with populations of
D. birchii
. This experiment in artificial sympatry indicates
how rapidly mate-recognition systems can change. However, the experiment
does not indicate whether selection on mate recognition was a component of
the actual speciation event resulting in
D. serrata
and
D. birchii
.
Drosophila melanogaster
populations from Zimbabwe and populations
from other continents were shown to be reproductively isolated. In the presence
of males of their own kind, females from Zimbabwe do not mate with males from
elsewhere; reciprocal mating is reduced as well. The genes for this behavior appar-
ently are found on autosomes II and III (
Wu et al. 1995
). The data suggest these
populations are in the “early stages of speciation” and that it is “driven by sexual
selection” (
Wu et al. 1995
). Because
D. melanogaster
is so well known genetically,
analyzing speciation should be especially tractable (
Buckley et al. 1997
).
A speciation gene, called
Odysseus
, was cloned, sequenced and compared
between two closely related
Drosophila
species (
simulans
and
mauritiana
) (
Ting
et al. 2000
).
Ting et al. (2000)
were testing the hypothesis that genomes may con-
tain ancient polymorphisms, or gene introgression could have occurred, so that
molecular phylogenies may not reflect reproductive isolation accurately. Rather,
speciation genes may be better indicators of phylogenetic history.
Odysseus
is the
cause of hybrid male sterility in the
D. simulans
clade;
Ting et al. (2000)
compared
Odysseus
and microsatellite sequences from
simulans
,
mauritiana
, and
sechelia
(with
D. melanogaster
as an out group). The results indicated the genome (as indicated
by 39 microsatellite loci) can “indeed be a mosaic of regions of different gene-
alogies among closely related species, because of shared ancient polymorphism
and/or introgressions.” The sequences of
Odysseus
, by contrast, provided a clear
resolution of species because there were extensive amino-acid differences. The
