Evolution__3rd_Edition

416 PART 4 / Evolution and Diversity
D. simulans D. mauritiana
Ods (s)
Odysseus has evolved fast in these
species
Y
X
Ods (m)
Y
X
Ods (m)
Y
Male sterilized
X
Insert Ods (m)
in male D. simulans
Sibling
species
Figure 14.15
A gene that has been identified and that causes reproductive
isolation. Ting et al. (1998) experimentally inserted the Drosophila
mauritiana version of the gene Odysseus into a D. simulans genetic
background. The result was that the males were sterilized, just as
in normal hybrid crosses between these two species. Odysseus
probably causes reproductive isolation between these two species.
Odysseus is a gene on the X chromosome in fruitflies. It is a
homeobox gene, likely expressed in spermatogenesis. It has also
evolved exceptionally fast between these two (sibling) species:
it is more different between the two than it is between rodents
and nematode worms.
They looked into Odysseus some more. Odysseus contains a “homeobox,” a sequence
found in genes that regulate development (Section 20.6, p. 582). It is expressed in the
development of the male reproductive system. The sterility of D. mauritiana × simulans
hybrids may be caused by an incompatibility between the mauritiana form of the
Odysseus gene and a simulans gene that is also expressed in the male reproductive
system.
One striking feature of Odysseus is its rate of evolution. Odysseus, like most homeobox
genes, normally evolves slowly. But it has undergone a sudden spurt of evolution
in these fruitflies. Indeed Odysseus is more different between D. mauritiana and
D. simulans, which share a common ancestor about half a million years ago, than it is
between a worm and a mouse, which share a common ancestor at least 700 million
years ago. The rate of evolution of this gene has zoomed up over 1,000-fold in these
fruitflies. And associated with that, it causes postzygotic isolation.
We can fit these observations in with a general idea about speciation: the idea of
“speciation genes.” Speciation genes can be defined as genes that differ between a pair
of species, and cause reproductive isolation between them. (A more demanding
definition would be that speciation genes are genes that differ between a pair of species
and drove speciation between them. However, empirical research can usually only
show that a gene causes reproductive isolation a and we remain uncertain whether the
gene also drove speciation.)
Biologists discuss various hypotheses about speciation genes. We can distinguish a
strong and weak claim. The strong claim would be that some genes in the genome may
be particularly likely to drive speciation. That is, we can look at the genome in advance
of speciation and say “if gene X changes, speciation will follow.” For instance, changes
in the genes concerned with courtship or mate choice might be more likely to drive speciation
than changes in other genes. If true, genes that influence courtship and mate
choice would be “speciation genes.” Other possible examples include genes on the X
chromosome, or genes such as the segregation distorter genes (Section 11.2, p. 294), or
chromosomal mutation. But none of these kinds of genes have been shown to drive
speciation in general, and the strong claim about speciation genes may well be false.
Alternatively, changes in almost any gene might be able to drive speciation. Then
we can talk about speciation genes in a weaker sense a simply to refer to the genes
that happen to cause reproductive isolation in a particular pair of species. In the
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