A gene known as eda helps determine
how much armor a three-spined stickleback has (bony plates stained red at
midsections). Armor changes, in turn,
may split one species into two.
While high school biology courses
may emphasize the role of geographic
barriers — big mountains and wide rivers — in driving would-be species apart,
some scientists are more interested in
the genetic story behind such divisions.
During any one evolutionary split, several, or maybe even hundreds, of genes
can contribute a tiny push.
Researchers are now focusing in on a
handful of promising speciation heavyweights, with more expected over the
next few years. While the phlox color
genes act directly to keep the annual and
pointed versions distinct, most newly
found candidate speciation genes have
arisen as by-products of evolutionary
pressures not related to mating at all.
Genetic tweaks inspired by environmental shifts, for example, may be cleaving
apart species as diverse as stickleback
fish and monkeyflowers. And a newly
identified fruit fly gene demonstrates
that competition among genes themselves, not just environmental changes,
can drive one species into two.
The search for speciation genes
isn’t just an evolutionary scavenger
hunt. Discovering such genes brings
scientists closer to solving a biological
mystery, says Nosil: “How easy is it to
create a new species?”
Governesses on duty
As is the case with the phloxes, appearance doesn’t always distinguish one
species from another. Deciding what
makes a species distinct from its evolutionary neighbors can get fuzzy, and
there are about as many different definitions as there are biological disciplines.
An old and often cited explanation
comes down to sex. In true Romeo-and-Juliet style, members of separate species
don’t mate, at least not successfully.
When the annual and pointed phlox
do manage to interbreed, their hybrid
children rarely produce mature seeds.
Because of this reproductive dead end,
individuals extra wary of interbreeding
should flourish over others that are less
discriminatory. “If a new mutant stops
them from hybridizing, that’s favored,”
says Robin Hopkins, an evolutionary
biologist at Duke University.
Previous work showed that the pur-ple-to-red change slows mating between
the two species by up to two-thirds.
Hopkins and Duke’s Mark Rausher discovered that several enzymes are behind
that shift. Breeding and genetic experiments revealed that those enzymes are
produced under the watch of two genes.
The researchers identified the enzyme
culprits in January in Nature but still
have to locate the specific genes.
Wherever they are, the two genes don’t
seem to make the phlox smell sweeter or
better catch the eye of passing butterflies.
The yet-to-be-identified governess genes
do one and only one thing, Hopkins says:
stop sex between the two species. As far
as speciation genes go, few examples put
their stamp so directly on mating.
Most new species are, instead, accidental monsters. The genes that give
rise to these creatures come about
through mundane evolution — such as
improvements to cellular machinery or
antipredator defenses — that impacts
Switch to scarlet in areas
where the annual phlox’s range overlaps with that of a close relative, the
phlox’s color switches from light purple
to scarlet. this shift — driven by two
yet-to-be-identified genes — appears
to prevent interbreeding with the
pointed phlox relative (also a light
mating only through twists of fate. Successful reproduction is a very important
thing for organisms, so why evolution would tolerate two populations of
would-be parents that can’t interbreed
has been hotly debated among biologists.
“Every single piece of machinery
that is necessary to keep you fertile and
alive already exists,” says evolutionary
geneticist Nitin Phadnis of the Fred
Hutchinson Cancer Research Center
in Seattle. “So the big question is why
something functioning perfectly well
changes through evolutionary time.”
The speciation flood
For Dolph Schluter, the answer begins
with a flood. Around 10,000 to 20,000
years ago, receding glaciers doused the
Pacific Northwest with meltwater, forming a spider web of new streams and lakes.
Animals rushed in to take advantage of
these new habitats. Most important, from
the point of view of Schluter and other
evolutionary biologists, was the appearance of a silver fish, usually no longer
than a credit card, called the three-spined
stickleback (Gasterosteus aculeatus).
Though not separate species, the
sticklebacks living in streams today and
those still living in the ocean are two very
different beasts with varied behaviors
source: d. levin/EVOLUTION 1985
typical color of both the
annual phlox and its relative,
the pointed phlox.
color of the annual phlox
where its range overlaps with
that of the pointed phlox.