Discover! In Depth Fac./Student Interview Student Interview I Student Interview II		Active Learning and Research 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.

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Fishing for clues about evolution

• 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 species.

• Why study stickleback
• Kinds of stickleback
• Surprising evidence

Why study stickleback?

Kinds of stickleback

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

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. 

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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 University Press.   Can they or can't they? What might prevent individuals from two different specifies from producing offspring? They might 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)