Natural Selection Image Map
The dinosaurs are gone. They, along with many non-dinosaur species, disappeared from land, the seas, and the air about 65 million years ago. The only mammals present at that time, according to fossils that have been found, were small rodent-like creatures that lived in the shadows of the giant dinosaur beasts (but there were small dinosaur species too). Dinosaurs were at the top of the animal kingdom at that time. They dominated the habitats available to animals. 

We are not sure what killed the dinosaurs in such massive numbers that all those species became extinct. It might have been a worldwide plague of infectious disease, which we can't prove because such a plague could occur without leaving a fossil record. The latest theory is that a giant meteor struck the earth in the southern Gulf of Mexico.  Such a crash would have thrown up so much dust and debris into the earth's atmosphere that all light would have been blocked out, making the earth suddenly very cold and plants could not grow. This idea is popular but not yet accepted by all scientists.

Whatever the cause for dinosaur extinction, we are confronted with the well-documented fact that many new species of mammals appeared soon after the extinction of the dinosaurs. Many of these primitive mammals also died out, but apparently some of their descendants still exist today. Can you name some of these species? How about:

  • monotremes (like platypus and echidnas)

  • marsupials (like opossums)

  • moles

  • armadillos


Are there certain general forces of nature that can favor some species at the expense of others? How does nature influence which species survive and which become extinct?  How do humans influence species survival?

You might say that nature selects which species survive and which become extinct. And you can go further and suggest that natural selection is the process by which new species might emerge. 

This is exactly what Charles Darwin did. Darwin did not invent evolution. He discovered that natural selection was the way evolution occurs. The idea is that each species has a range of genetic variability, and genetic mutations can introduce even more variation. The environment favors certain traits in a species, and those individuals that have such traits are more likely to survive and have more offspring that will also have these traits. Eventually, given enough time (like millions of years) the genes controlling the advantageous traits may dominate the population and a new species may emerge.

Changes in environment may put existing species at risk, because their traits are no longer suitable for the new environment. If the genes of the species no longer help the species survive, extinction of species may then occur.

After completing this lesson, each student should be able to:

  • Explain what we mean by "Natural Selection"
  • Explain how natural selection is the mechanism for the scientific view of how new species arise.
  • Give examples of how human activity changes natural selection forces




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