Not only have the Grants documented natural selection in nature, their data also confirm that competition and environmental change drive natural selection. Traits that don't matter much under one set of environmental conditions became adaptive as the environment changes during a drought. The Grants' work shows that variation within a species increases the likelihood of the species' adapting to and surviving environmental change. Without heritable variation in beak sizes, the medium ground finch would not be able to adapt to feeding on larger, tougher seeds during a drought.

Evaluating Evolutionary Theory Advances in many fields of biology, along with other sciences, have confirmed and expanded most of Darwin's hypotheses. Today, evolutionary theory—which includes natural selection—offers insights that are vital to all branches of biology, from research on infectious disease to ecology. That's why evolution is often called the grand unifying theory of the life sciences.

Like any scientific theory, evolutionary theory is constantly reviewed as new data are gathered. Researchers still debate important questions such as precisely how new species arise and why species become extinct. And there is also significant uncertainty about exactly how life began. However, any questions that remain are about how evolution works—not whether evolution occurs. To scientists, evolution is the key to understanding the natural world.

FIGURE 16–18 Survival and Beak Size This graph shows the survival rate of one species of ground finch, the medium ground finch, Geospiza fortis, during a drought period. Interpret Graphs What trend does the graph show?

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16.4 Assessment

Review Key Concepts

1. 1. Review What is biogeography?

   2. Relate Cause and Effect Why do distantly related species in very different places sometimes share similar traits?

2. 1. Review Why are fossils important evidence for evolution?

   2. Interpret Visuals Use Figure 16–13 to describe how a modern mysticete whale is different from Ambulocetus.

3. 1. Review How do vestigial structures provide evidence for evolution?

   2. Compare and Contrast Explain the difference between homologous and analogous structures. Which are more important to evolutionary biologists? Why?

4. 1. Explain What is the relationship between Hox genes and embryological development?

   2. Draw Conclusions Organisms A and B have very similar Hox genes, and their embryos, in the earliest stages of development, are also very similar. What do these similarities indicate about the ancestry of organisms A and B?

5. 1. Explain Explain What hypothesis have the Grants been testing?

   2. Draw Conclusions How do the Grants' data show that genetic variation is important in the survival of a species?

WRITE ABOUT SCIENCE

Explanation

6. In your own words, write a paragraph that explains how evidence since Darwin's time has strengthened his theories.

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