• Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. The species is the most fundamental unit of classification.

This activity also provides all students with opportunities to develop abilities of scientific inquiry as described in the National Science Education Standards. Specifically, it enables them to:

  • identify questions that can be answered through scientific investigations;

  • use appropriate tools and techniques to analyze and interpret data;

  • develop descriptions, explanations, predictions, and models using evidence;

  • think critically and logically to make relationships between evidence and explanations;

  • recognize and analyze alternative explanations and predictions; and

  • communicate scientific procedures and explanations.

Finally, this activity provides all students with opportunities to develop understandings about inquiry and the nature of science. Specifically, it incorporates the following concepts:

  • Different kinds of questions suggest different kinds of scientific investigations.

  • Current scientific knowledge and understanding guide scientific investigations.

  • Technology used to gather data enhances accuracy and allows scientists to analyze and quantify results of investigations.

  • Scientific explanations emphasize evidence, have logically consistent arguments, and use scientific principles, models, and theories.

  • Science distinguishes itself from other ways of knowing and from other bodies of knowledge through the use of empirical standards, logical arguments, and skepticism, as scientists strive for the best possible explanations about the natural world.

Background Information for Teachers

A remarkable implication of the theory of evolution is that all of the species that exist on the earth today are descended from common ancestors. In other words, if several species are compared, an evolutionary tree can be drawn that suggests how those species are related by common ancestry.

In this exercise, students gain familiarity with the idea of common ancestry and descent by investigating the evolutionary relationships of 18 species of drosophilid flies that live on the Hawaiian islands. Morphological and genetic data indicate that the approximately 800 species of drosophilid flies in the Hawaiian islands are descended from a single colonizing species that came to the islands millions of years ago. In studying the evolution of the Hawaiian drosophilids, biologists have focused on a group of about 100 species that can be grown in the laboratory and have large wings with distinctive black markings. Known as the picture-winged drosophilids, these species lay their eggs in rotting stems and bark and in bark moistened by the saps, or fluxes, exuded by trees. After the eggs hatch, the larvae feed on bacteria in those substances before pupating and becoming adult flies.

For the drosophilid flies of Hawaii, biologists have a particularly informative marker to trace their evolutionary relationships. During the larval stage of a fly’s life, the salivary glands produce cells containing a special kind of giant chromosome known as a polytene chromosome. If extracted from the salivary cells and stained, these polytene chromosomes exhibit a



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