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 habitats:

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 species.

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
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.

limnetic 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, freshwater lakes.

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.

lake 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 particular habitat.

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 environment?

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.