Classifying Organisms

One important branch of biology investigates biodiversity, the variety of organisms considered at all levels from populations to ecosystems. In general, the scientific catalogue of all biodiversity on Earth has increased over time. The number of identified species has grown from hundreds to millions. And scientists are certain that many more remain unidentified.

Since the 1980s, ecologist Terry Erwin and others have been working to catalog insect species in plots of tropical rain forest. To do this, researchers may fog the treetops with insecticide and then catch the falling insects in a net, as shown in Figure 17-1. Then, the scientists will count and classify the insects. Using such methods, Erwin collected over 1,000 species of beetles from just 19 trees of the same species in a limited area of rain forest. Furthermore, Erwin used his data to estimate the total possible number of insect species worldwide. He proposed that there could be more than 30 million species of insects on Earth!

Every year, biologists discover thousands of new species and seek to classify them in a meaningful way. Over the centuries, classification systems for organisms have been proposed and modified to include an increasing understanding of how organisms are structured and how they evolved. Biologists are challenged to organize their knowledge of such diversity in a way that makes sense.

For example, consider the pangolin, a species that is covered with scales and catches ants on its sticky tongue. Should pangolins be grouped in a category with other scaly animals, such as lizards and crocodiles? Biologists have decided against such a classification, because pangolins share many more similarities with mammals such as dogs and cats than with lizards or crocodiles. Should pangolins be grouped in a category with other mammals that use sticky tongues to eat ants? Again, biologists have decided against this grouping, because pangolins share more similarities with dogs and cats than with the other ant-eating mammal species.

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