Evolution__3rd_Edition

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Biologists are interested in theories
of “speciation genes”
Our modern understanding of
speciation shows several advances
since Darwin
Dobzhansky–Muller theory, any gene can cause isolation, provided it can have an
epistatic interaction with other genes in the genome. However, the genes that drive speciation
will be the genes that have changed in evolution. An unchanging, conserved
gene cannot cause isolation between two species. The genes driving speciation will be
the first genes to change a that is, the genes that evolve fastest. Maybe they will be genes
like Odysseus, which does not normally evolve fast but happened to in one population.
One gene may have an evolutionary spurt in one lineage, and cause speciation there.
Another gene may spurt in another lineage, and cause speciation there. The “speciation
genes” will be those that happened to evolve fast in a particular lineage. Or it could be
that some genes in the genome evolve faster than average in all life forms. Then, these
fast evolving genes may be the speciation genes. One suggestion of this sort is that genes
expressed in the reproductive system may evolve faster than other genes (see Swanson
& Vacquier (2002) for the facts). Then speciation will more often be caused by evolution
in the genes of the reproductive system than in genes of (for example) the nervous
or digestive system.
These ideas about speciation genes are currently conjectural. However, they are an
example of the kind of general idea about speciation that we should be able to investigate
as modern genetic techniques are used to identify the genes that are causing reproductive
isolation in particular species.
14.13 Conclusion
CHAPTER 14 / Speciation 417
At the beginning of the chapter we saw that there are two theories of how reproductive
isolation evolves: the “by-product” theory and reinforcement. When Darwin discussed
the topic in On the Origin of Species (1859) he favored what is here called the byproduct
theory, saying that the sterility of interspecies hybrids is “incidental on other
acquired differences.” He devoted a chapter to arguing the point. He was less interested
in the geographic circumstances of speciation, but argued for something like what we
would now call sympatric speciation rather than allopatric speciation. Competition
between forms within an area would force them to diverge, he reasoned.
The impressive evidence that we now have from artificial selection experiments
(Section 14.3.1) plugs one hole in Darwin’s case. Darwin had no evidence that reproductive
isolation evolved between domestic varieties that had been selected apart.
“The perfect fertility [he wrote] of so many domestic varieties, differing widely from
each other in appearance, for instance of the pigeon or the cabbage, is a remarkable
fact.” It is not so remarkable now, because more careful, and better controlled, measurements
have shown that reproductive isolation often evolves between artificially
selected varieties.
After Darwin, the evolutionary biologists of the “modern synthesis” added four or
five main claims, in the period from about 1930 to 1950. One claim, argued for by Mayr
(1942), was that new species arise allopatrically rather than sympatrically. Associated
with this was a second claim, that speciating populations tend to be small and that
genetic drift is particularly important in speciation. Thirdly, Dobzhansky and others
argued that reinforcement also contributes to speciation. (Dobzhansky (1970) gives a