Mapping evolution

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“Nutcracker” fish shows evolution in action

In biology, odd is awesome. Charles Darwin, famously, struggled with the peacock’s outlandish tail. You might have wondered why the rhinoceros has a horn. We wonder why the narwhal has a tusk-like tooth and looks like an underwater unicorn.

Christopher Martin, a Ph.D. student at the university of California at Davis, wants to know why the common pupfish, found in wetlands from Massachusetts to Venezuela, does not share its ponds with related pupfish species.

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Small, silver fish swimming near bottom of water body, surrounded by short grasses and suction-cup-like plants.

Image courtesy of Chris Martin
This generalist pupfish, in its natural habitat on San Salvador Island, Bahamas, doesn’t have much of a nose. Check out the impressive hood ornaments on the closely-related fish down below!

The lone exception occurs in the Bahamas, where two sister pupfish species evolved from a common ancestor in ponds that are just 10,000 years old.

Three pupfish species live in the pond in question, says Martin, who was first author of the new study:

one generalist that eats a wide variety of food, including seaweed

a parasite that (ecch!) eats scales from other pupfish

a shellfish-eating carnivore with a honking big “nose”

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Two fish. Left: iridescent brown with enlarged bottom jaw. Right: light orange with enlarged nose structure.

Images courtesy Tony Terceira
Left: The Cyprinodon scale-eater is a unique pupfish that eats scales from other fish. Right: Cyprinodon durophage is a specialist pupfish with unique, protruding nasal appendage suited to eating shellfish (clams, anyone?). These pupfish are endemic to San Salvador Island, Bahamas.

The shellfish eater “is bizarre. It’s essentially a fish with a nose; it has a swollen nasal region and the jaw tucks up underneath,” says Martin, and “no one knows what it is used for. Possibly the nose stabilizes the jaw to help crush prey, or females use it to discriminate among males, or it could be a sensory organ. The nose is not just a nutcracker.”

These pupfish are a recent example of adaptive radiation, a rapid evolution of species that occurs after an organism reaches new habitat with empty ecological niches. But why do distinct new species arise, instead of a mishmash of in-between traits?

Trouble is…

How, exactly, does adaptive radiation take place? Offspring of genetically dissimilar parents usually lack the abilities of their parents, and die out, which makes it difficult to transition toward a new ability, like stripping scales or shucking clams.

To study how changes in phenotype (genetically determined behavior and structure) affect survival, biologists create graphs that look like mountains maps.

Survival of fish in wild plotted against jaw shape shows higher survival for two of the three fish studied.

San Salvador Island, Bahamas, courtesy Christopher Martin
This plot relates the survival of three types of pupfish (vertical axis) to the shape of their jaws and bodies (horizontal axes). These fish resulted from adaptive radiation during the past 10,000 years.

On these graphs, peaks show phenotypes with enough offense, defense and presumably reproductive ability to pass along their genes to posterity. But when diverse individuals mate, the hybrids must “jump between the peaks.” They may form a new species, but are more likely to drop into “death valley.”

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Open pond surrounded by vegetation, with study plot at the edge enclosed by black netting supported by white, plastic pipe.

Image courtesy of Chris Martin
Martin used this field enclosure, in Crescent Pond on San Salvador Island, Bahamas.

To figure out how two new pupfish evolved from their common ancestor (the generalist shown in the top photo), Martin cross-bred members of the three pupfish species. (The species could cross-breed because they are still closely related, after such a brief evolution.)

But the offspring were oddballs, says Martin, who worked with Peter Wainwright, a professor of evolution and ecology at Davis. “Eighty percent of the hybrids look nothing like what you find in the wild.” When he returned, the hybrids that had survived “nearly exactly matched the original species.” In other words, there was little room for blended phenotypes; these fish occupied the deadly lower regions of the graph.

Adaptive radicals

The study addresses a mystery from Hawaii and the Galapagos Islands, hotbeds of adaptive radiation. “We have no idea” why some birds, through adaptive radiation, left many related species, while other species did not, Martin says.

The new maps could explain the rarity of specialists in pupfish ponds, Martin says. “When people talk about adaptive landscapes, classically, they think the population should move uphill” to a higher level of adaptation.

But even though the shellfish eaters were more fit in the Bahamas, “The generalists are isolated by a ‘fitness valley’ between them.”

“It’s remarkable,” Martin says. The experiments were done in small pens, “and this means there are two ways of being a pupfish and surviving, even in this small enclosure.”

— David J. Tenenbaum

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Terry Devitt, editor; Emily Eggleston, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Multiple Fitness Peaks on the Adaptive Landscape Drive Adaptive Radiation in the Wild, Christopher H. Martin and P.C. Wainwright, Science, 11 Jan. 2013.
  2. Salt or fresh? How fish survive in different types of water
  3. Taking the long view: Fish as they first appeared on earth
  4. What’s the latest news on fish evolution?
  5. The Story of the Prehistoric Fish