and appearances. In the lab, marine and
freshwater fish can knock boots and,
hence, the populations can exchange
genetic information. But in nature, the
two groups may have trouble mating.
If a stickleback from a stream wanders back into the big ocean, it will very
much be a fish out of water. If this unlucky
immigrant can’t dodge attacks by predatory fish or compete for food with the
locals, then it may die before getting the
chance to breed with a native. So, even if
its newly acquired traits don’t touch mating directly, they could prevent sex indirectly. Over time, a lack of mixing between
the populations might allow more differences to build up, making it so that the fish
couldn’t mate even if they got the chance.
Flower flip some of the differences
between two populations of yellow monkey
flower are driven by genes that sit on an
area of a chromosome that has been flipped
(below). such flips, called inversions, may
help keep the two populations different.
As long as genes for stream living slow
interbreeding, “then the genes for speciation and the genes for adaptation are one
and the same,” says Schluter, of the University of British Columbia in Vancouver.
One promising candidate for an adap-tation-turned-speciation gene is called
ectodysplasin or eda. It’s one of a handful of genes that give rise to the stickleback’s extravagant armor. Ocean fish are
covered in an array of plates and spines,
looking something like swimming
knights. Stream fish, which have a different eda version, are much smoother.
“Most freshwater populations are
low-plated, and in every one of those
low-plated populations that have been
investigated so far, eda is partly or largely
responsible,” Schluter says.
The success of the smooth gene variant seems to stem, in part, from the edge
it gives stream fish against a new breed
of predator. Young fish with less armor
were more likely to survive to adulthood
in artificial ponds stocked with insect
diners, Kerry Marchinko, a colleague of
Schluter’s, reported in 2009 in Evolution.
Sticklebacks with less protection seem to
grow faster, quickly becoming too big for
stream-dwelling bugs to catch.
But discovering whether eda alone
could affect survival enough to slow
interbreeding and drive the creation
of a new species would require further,
expensive experiments, Schluter says.
Without that data, eda’s status as a speciation gene remains as slippery as a
Not too far from the stickleback’s cozy
streams, a little yellow flower illustrates
how changes that go beyond individual
genes can ensure that two populations on
the verge of speciation remain different.
Like sticklebacks in separate locales,
populations of yellow monkeyflowers (Mimulus guttatus) living along the
Pacific Coast are physically different from
members of the same species dwelling
east into central California and Oregon.
At the most basic level, coastal monkeyflowers are robust and live for multiple
years, whereas inland monkeyflowers are
spindly and live for just one.
About 20 to 30 percent of the
differences bet ween the two populations
arise from genes sitting within a single
region of one chromosome, evolution-
ary geneticists John Willis of Duke and
David Lowry, now at the University of
Texas at Austin, reported in 2010 in PLoS
Biology. But this region isn’t an ordinary
genetic plot of land: It’s what’s called an
inversion. At some point in the past, this
chromosome chunk did a flip in one of
the two populations, landing with the
side that used to be up pointing down.
“Genes in the genome are chasing moving
targets in the environment,” says Daven
Presgraves, an evolutionary geneticist at
the University of Rochester in New York.
“The other part of the environment that