Biologist Susan Foster and her students study the threespine stickleback in the lab and in the field. These small fish provide big insights into evolutionary biology.
Fishing for clues about evolution
Professor Susan Foster's research
Right before the last ice age ended, a small
fish called the threespine stickleback lived in ocean waters. After the glaciers
receded and the ocean level fell, groups of stickleback were left in different
- some remained in the ocean
- some found themselves in shallow freshwater lakes
- others found themselves in deep freshwater lakes
- still others were left in freshwater lakes that had
both shallow and deep areas
Ten thousand years later in the 20th century, biologists like Susan Foster
and her students began studying stickleback, hoping to learn more about how
organisms descending from common ancestors evolve into
- Why study stickleback
- Kinds of stickleback
- Surprising evidence
Why study stickleback?
The post-glacial, lake-dwelling stickleback mentioned above present
an ideal group of populations in which to
study speciation. Having entered
their various lake environments at the same time, the length of the evolutionary
process is the same for each. By comparing populations in different lake
environments (especially differences in behavior and body shape), Foster hopes
to observe these descendents of a common ancestral population in different
stages of the evolutionary process--both before and after speciation, and thus
find clues to the process of speciation.
Kinds of stickleback
Scientists have encountered several important variations in modern threespine
stickleback that seem initially to be a product of available gene
pool and habitat:
ocean-dwelling vs. lake-dwelling
Stickleback that still live in the ocean have
plates along their sides that act as armor to help defend them against
predators. Lake-dwelling stickleback have lost
this armor to varying degrees.
vs. benthic stickleback in lakes
Limnetics feed near the surface of the lake where there is abundant plankton water.
These stickleback are characterized by particular mating behaviors, and a
distinctive mouth shape that helps them gather plankton. They are found in deep,
Benthics feed in shallow water where they can access organisms that live on
the shallow lake floor. They have their own mating behaviors and physical
features different from those of limnetics. Benthics are found in shallow
freshwater lakes, or in shallow bays of deeper lakes.
environments with native predators vs. those without
Stickleback body shapes also differ between these two “ecotypes”.
Benthic fish are deeper bodied and better at turning while limnetic fish
are long and slender, and swim more easily through the water (less frictional
drag). Where predatory fish are
absent, stickleback may have slower escape responses.
Surprising evidence from three different study regions
The stickleback is found in coastal fresh and salt waters of the Northern
Hemisphere. Several scientists including Foster, have, over the years,
independently studied stickleback in a variety of ocean, lake and river
locations (yes, they also live in rivers!). From the evidence described
above, it would seem that habitat should predict whether benthic or limnetic
varieties evolve- and this generally seems to be the case.
However, there are some wonderful surprises.
Throughout northwestern North America, where Foster and her students work,
the stickleback in deep, clear lakes tend to be exclusively plankton eaters (limnetic)
while those in the shallow lakes are benthic, feeding on lake floor organisms.
Because these lakes present two dramatically different habitats, it seems to
make sense that two types of stickleback would have evolved, each adapted to a
A small region in British Columbia holds one of the surprises.
Here, in six small shallow lakes on three islands, there are both benthic
and limnetic sickleback, and they are separate species!
Benthics mate only with benthics and limnetics only with limnetics- even
though they nest and breed in the same area.
One of the mysteries in this system then, is how these pairs of species
came into being.
A second surprise is found in a lake in the Cook Inlet region of Alaska where
Foster and her students often work. In
this lake are found both deep and shallow areas, and an island with steep sides
protrudes from the deep middle of the lake.
Here, fish with limnetic shapes are found at the edge of the island
feeding on plankton, and the breeding territories of limnetic males are found on
the island edge. Benthic fish are
found breeding and feeding in shallow bays along the lake edge.
Perhaps because benthics and limnetics have different mating behaviors
and feeding areas, there doesn't seem to be a lot of interbreeding. Could they
be on the verge of becoming two different species in this more complex lake
One of the goals of the research in Foster’s laboratory is to study the
mating behaviors and preferences of these small fish to understand how new
species arise. They hope to be able
to compare behavior of the benthic and limnetic fish that live in isolation with
that of the forms that co-occur to understand this process, and ultimately. to
contribute to our understanding of the way the remarkable diversity of organisms
we see on earth has come into being.
|The threespine stickleback.
|Approximate range of the threespine stickleback. Enlarge. Adapted with permission from Michael A. Bell and Susan A.
Foster, "Introduction to the evolutionary biology of the
threespine stickleback," in The Evolutionary
Biology of the Threespine Stickleback, eds. Michael A. Bell and Susan A.
Foster, (Oxford: Oxford University Press),
1994, p. 3. ©
Oxford University Press.
|Representative limnetic (top) and benthic (bottom) stickleback
from Lake Enos in British Columbia, Canada. Posted with permission from Paul J. B. Hart and Andrew B. Gill,
"Evolution of Foraging Behaviour int the threespine stickleback," in The Evolutionary Biology of the
Threespine Stickleback, eds. Michael A. Bell and Susan A. Foster, (Oxford:
Oxford University Press), 1994, p. 211. © Oxford
Can they or can't they? What might prevent individuals from two different specifies from producing offspring?
be fertile at different times
need different breeding habitats
have conflicting courtship behaviors that prevent mating
have sexual organs which are incompatible
be physically able to mate, but cannot produce a fertilized egg [zygote], or the egg dies
be able to produce offspring, but the offspring can't reproduce (an example here is the mule, the result of mating between a horse and a donkey)