Fish of a fin swim together

	Tamra Mendelson peers into tanks filled with different populations of darter fish to see how they're getting along.

A male brushes by a female, hoping she finds him attractive. And who knows, maybe they'll even be compatible.
This may sound like a scene from a singles' bar on a Friday night, but this budding romance is actually taking place in a tank in Iacocca Hall, where pretty darter fish are checking each other out as part of Tamra Mendelson's research on speciation.
As long as an animal has a nervous system and a brain, it has an opinion as to who it mates with. And this decision, according to Mendelson, assistant professor of biological sciences, appears to help drive the formation of new species.
When two populations that were once the same species begin to diverge from one another, it's often the behavioral differences -- like preferences in a mate -- that change first. And that's what I'm most interested in studying, says Mendelson, whose related research on the Laupala cricket was recently published in Nature magazine, one of the world's most respected scientific publications.
The individual factors that drive mate preference differ from species to species. The darter fish are visual communicators, so they use their colors to attract the opposite sex during courtship. The crickets, on the other hand, are acoustic and chemical communicators, so they flaunt scent and song.
But no matter what the means used to woo, the evolutionary goal is the same: to get as many mates as possible.
The big question Mendelson hopes to answer with her fish is this: How does species formation proceed and what are the first things that change? The standard dogma of the formation of new species is that a geographically isolated population experiences a new environment as compared to its sister populations, she says. For instance, an animal that lives in an area with deeper dirt will develop longer arms for digging and, presto, they've become a new species.
But Mendelson is exploring the possibility that behavioral evolution in the form of mate selection may be independent of the environment.
It all starts when a species becomes divided. For instance, take two populations of the same species that become geographically separated from each other; one is in western Tennessee and the other in eastern Tennessee. They obviously don't breed together because geographically, they're too far apart.
If those geographic barriers stay in place for a long time you'll get evolutionary changes in these populations that will cause them to be different, so different that at some point, they'll become different species, Mendelson says. And the way we mostly define them as being different species is when they can no longer reproduce together.
But before the two species reach the point of sterility, they become disinterested in each other. It's not that they can't reproduce with each other, it's that they won't. They lose the desire to interbreed before they lose the capacity, she says.
To see whether fish that are turned off to each other can actually produce offspring, Mendelson has done some fish in-vitro fertilization. I've found when I strip off eggs and sperm from different species and put them in a dish and raise them up, they make perfectly viable offspring. But I don't know yet whether the hybrids are fertile or sterile, and that's my big research push right now.
Mendelson has returned to the darter fish -- the subjects of her dissertation -- after a post-doc project involving the crickets. After working with acrobatic and energetic lemurs for her first year of graduate school, she says she chose the fish because they are refreshingly easy to observe and beautiful to boot.
Beyond fish and crickets, Mendelson's research has implications for other species as well, particularly those in which males and females look distinctly different. In other species like birds and frogs, we expect behavioral differences in mating communication and courtship to evolve before other differences, she says.
--Elizabeth Shimer
Lehigh Alumni Bulletin
Spring 2005