An Evolutionary Framework for Biology

Monster frogs—what a great topic for an undergraduate research P^V* project! That's what Stanford University sophomore Pieter Johnson thought when he was shown a jar of Pacific tree frogs with extra 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 declines 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 Pieter's surprise, analysis of the water samples failed to reveal higher amounts of pesticides, industrial chemicals, or heavy metals in the ponds with deformed frogs. Also surprisingly, when he collected eggs from those ponds and hatched them in the laboratory, he always got normal frogs. The only difference he observed among the ponds he studied was that the ponds with the deformed frogs also contained freshwater 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 possibility that some parasite that used freshwater snails as intermediate hosts was infecting the frogs and causing their deformities. 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-

Frog Deformities Parasites

A Monster Phenomenon As a college sophomore, Pieter Johnson studied ponds that were home to Pacific tree frogs (Hyla regilla), trying to discover a reason for the presence of so many deformed frogs. What appears in the inset to be a tail is an extra leg.

Frog Deformities ParasitesFrog Deformities Parasites

ers, 85 percent of the frogs developed deformities. Further experiments showed why not all the frogs were deformed: The infection had to occur before a tadpole started to grow legs. When tadpoles with already developing legs were infected, they did not become deformed.

Pieter's project started with a question based on an observation in nature. He formulated several possible answers, made observations to narrow down the list of answers, and then did experiments to test what he thought was the most likely answer. His experiments enabled him to reach a conclusion: that these deformities were caused by Ribeiroia. Pieter's project is a good example of the application of scientific methods in biology.

Biology is the scientific study of living things. Biologists study processes from the level of molecules to the level of entire ecosystems. They study events that happen in millionths of seconds and events that occur over millions of years. Biologists ask many different kinds of questions and use a wide range of tools, but they all use the same scientific methods. Their goals are to understand how organisms (and assemblages of organisms) function, and to use that knowledge to help solve problems.

In this chapter, we will take a closer look at what biologists do. First, we will describe the characteristics of living things, the major evolutionary events that have occurred during the history of life on Earth, and the evolutionary tree of life. Then we will discuss the methods biologists use to investigate how life functions. At the end of the chapter, we will discuss how scientific knowledge is used to shape public policy.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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