ap bio chapter 22: Concept 22.3: Hybrid zones reveal factors that cause reproductive isolation

What is a hybrid zone?

A region where members of different species mate and produce hybrids.

What causes the formation of hybrids in a hybrid zone?

Incomplete reproductive barriers between species.

Are hybrids able to reproduce?

Yes, though they are usually less healthy, strong, or fertile than parent species.

What is the general fitness of hybrids compared to parent species?

Hybrids often have reduced fitness.

What does “fitness” mean in biology?

The ability to survive and reproduce, contributing your genes to the next generation.

What does a single-band hybrid zone look like?

It’s where two populations (e.g., A and B) meet and interbreed along a narrow region.

Example of a single-band hybrid zone in nature?

Two species of Bombina toads interbreeding in a long, narrow hybrid zone.

What allele frequency characterizes yellow-bellied toads?

Close to 1.

What allele frequency characterizes fire-bellied toads?

Close to 0.

What allele frequency indicates a hybrid toad?

Somewhere in the middle between 0 and 1.

What happens when parent species are found in patches within the same region?

Hybrid zones can become complex, with many different zones forming.

What are the three possible outcomes when species meet in a hybrid zone?

Reinforcement, Fusion, and Stability.

What is reinforcement in a hybrid zone?

Strengthening of reproductive barriers, reducing the formation of hybrids over time.

What type of barriers are reinforced during reinforcement?

Prezygotic or postzygotic barriers.

What is fusion in a hybrid zone?

Weakening of reproductive barriers, allowing two species to merge into one.

What condition allows fusion to occur?

When hybrids are healthy and fit, and substantial gene flow happens.

What is stability in a hybrid zone?

Continued production of hybrids without strengthening or weakening reproductive barriers.

What happens to hybrid numbers during reinforcement?

Hybrid numbers decrease over time.

In which case are reproductive barriers stronger: sympatric or allopatric species?

Sympatric, because they live in the same area but must remain distinct.

What example illustrates fusion due to weakened reproductive isolation?

Pollution in Lake Victoria reducing female cichlids’ ability to distinguish males → species intermix.

How does pollution lead to fusion in cichlids?

Cloudy water disrupts sexual selection → different species begin breeding together.

What leads to a stable hybrid zone?

Gene flow from outside overwhelms selection for stronger reproductive isolation.

Do hybrids outnumber parental species in a stable hybrid zone?

No, they persist over time but remain fewer in number.