These images from the Smithsonian Institution depict Nancy Knowlton's work with snapping shrimp in Panama. Knowlton found that the closing of the isthmus -- dividing the Pacific Ocean from the Caribbean -- resulted in new species of shrimp.

Biology 2e is designed to cover the scope and sequence requirements of a typical two-semester biology course for science majors. The text provides comprehensive coverage of foundational research and core biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes various types of practice and homework questions that help students understand—and apply—key concepts. The 2nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and the textbook features additional assessments and related resources.

By the end of this section, you will be able to do the following:

Describe the different types of variation in a population
Explain why only natural selection can act upon heritable variation
Describe genetic drift and the bottleneck effect
Explain how each evolutionary force can influence a population's allele frequencies

Describes three of the four forces of evolution: mutation, gene flow, and genetic drift.

A free CK-12 account is required to view all materials.

Discusses how sexual reproduction leads to genetic variation. [Free Registration/Login may be required to access all resource tools.]

A free CK-12 account is required to view all materials.

This site from the University of Arizona runs a two-step simulation of Genetic Drift. There are teacher's notes included. A great site to aid in understanding Genetic Drift.

Learn how genetic variation is the raw material of evolution, and learn how restricted gene flow can lead to the formation of new species. [11:52]

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

In this tutorial students can learn the definition of species, how new species evolve, and the processes that can cause new species to arise. At the end of the tutorial students can take a quiz to evaluate what they learned about speciation.