Life, the science of biology PDF
Preview Life, the science of biology
1 An Evolutionary Framework for Biology Monster frogs—what a great topic for an undergraduate research project! That’s what Stanford University sophomore Pieter John- son thought when he was shown a jar of Pacific tree frogs with ex- tra legs growing out of their bodies. The frogs were collected from a pond on a farm close to the old Almaden mercury mines south of San Jose, California. Scientists from all over the world were reporting alarming de- clines in populations of many different kinds of frogs, so perhaps these “monster” frogs would hold a clue to why frogs all over the world are in trouble. Possible causes of the deformities could have been agricultural chemicals or heavy metals leaching out of the old mines. Library research, however, suggested other possibilities to Pieter. Pieter studied 35 ponds in the region where the deformed frogs had been found. He counted frogs in the ponds and measured chemicals in the water. Thirteen of the ponds had Pacific tree frogs, but deformed frogs were found in only four ponds. To A Monster Phenomenon As a college sophomore,Pieter Johnson studied ponds Pieter’s surprise, analysis of the water samples failed to reveal higher amounts of that were home to Pacific tree frogs (Hyla pesticides, industrial chemicals, or heavy metals in the ponds with deformed frogs. regilla),trying to discover a reason for the Also surprisingly, when he collected eggs from those ponds and hatched them in the presence of so many deformed frogs. What appears in the inset to be a tail is laboratory, he always got normal frogs. The only difference he observed among the an extra leg. ponds he studied was that the ponds with the deformed frogs also contained fresh- water snails. Freshwater snails are hosts for many parasites. Many parasites go through complex life cycles with several stages, each of which requires a specific host animal. Pieter focused on the possibil- ity that some parasite that used fresh- water snails as intermediate hosts was infecting the frogs and causing their de- formities. Pieter found a candidate with this type of life cycle: a small flatworm called Ribeiroia, which was present in the ponds where the deformed frogs were found. Pieter then did an experiment. He collected frog eggs from regions where there were no records of deformed frogs or of Ribeiroia. He hatched the eggs in the laboratory in containers with and without the parasite. When the parasite was present in the contain-
The list of books you might like
