“Ring species” are species that are distributed across a habitat in a ring-like distribution. Every population is able to breed with neighboring populations, but where the ring closes, the populations at the two “ends” do not interbreed. One example of a ring species is Ensatina escholtzii, a species of salamander in California. The species began as a single population in the North, and spread South on either side of the California Central Valley. Each color on the map shows the distribution of different named population of E. escholtzii. Hybrids created by interbreeding of the populations marked with the circle are sterile. If hybrid sterility is present, why do you think these populations are still considered the same species?

• A. To be considered separate species there must be pre- and postzygotic barriers.
• B. Because each population interbreeds with its neighbor, gene flow is possible throughout the entire species.
• C. This is a postzygotic barrier; prezygotic barriers are necessary for speciation.
• D. To become distinct species the populations would need to be geographically isolated.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the concept of a ring species. A ring species consists of populations distributed in a ring-like pattern where neighboring populations can interbreed, but populations at the ends of the ring cannot interbreed successfully.
2. Step 2: Observe the map showing the distribution of Ensatina escholtzii populations around the California Central Valley. Each color represents a different population, and the populations at the ends of the ring (marked by the dashed circle) produce sterile hybrids.
3. Step 3: Recognize that hybrid sterility is a postzygotic reproductive barrier, which occurs after fertilization and affects the viability or fertility of offspring.
4. Step 4: Note that despite the postzygotic barrier at the ends of the ring, gene flow occurs between neighboring populations throughout the ring because they can interbreed successfully. This continuous gene flow connects the populations genetically.
5. Step 5: Conclude that because gene flow is maintained throughout the ring and there are no prezygotic barriers (which prevent mating or fertilization), the populations are considered a single species rather than separate species.