Speciation and Extinction

What processes influence whether species and clades survive or become extinct?

The study of life's history leaves no doubt that life has changed over time. Many of those changes occurred within species, but others occurred in larger clades and over longer periods of time. These grand transformations in anatomy, phylogeny, ecology, and behavior, which usually take place in clades larger than a single species, are known as macroevolutionary patterns. The ways new species emerge through speciation, and the ways species disappear through extinction, are among the simplest macroevolutionary patterns. The emergence, growth, and extinction of larger clades, such as dinosaurs, mammals, or flowering plants are examples of larger macroevolutionary patterns.

Macroevolution and Cladistics Paleontologists study fossils to learn about patterns of macroevolution and the history of life. Part of this process involves classifying fossils. Fossils are classified using the same cladistic techniques, based on shared derived characters, that are used to classify living species. In some cases, fossils are placed in clades that contain only extinct organisms. In other cases, fossils are classified into clades that include living organisms.

Remember that cladograms illustrate hypotheses about how closely related organisms are. Hypothesizing that a fossil species is related to a living species is not the same thing as claiming that the extinct organism is a direct ancestor of that (or any other) living species. For example, Figure 19–9 does not suggest that any of the extinct species shown are direct ancestors of modern birds. Instead, those extinct species are shown as a series of species that descended, over time, from a line of common ancestors.

In Your Notebook Explain what macroevolution is and how fossils can show macroevolutionary trends.

FIGURE 19–8 Paleontologists at Work The white covering protects the fossils until they can reach a museum.