Ch. 22.3 The Origin of Species

Question 1

Hybrid Zone

Answer 1

Occurs when species with incomplete reproductive barriers come into contact with one another, and form a region in which members of different species meet and mate, producing at least some offspring of mixed ancestry.

Question 2

Hybrid Zones and Environmental Change

Answer 2

A change in environmental conditions can alter where the habitats of interbreeding species meet. When this happens, an existing hybrid zone can move to a new location, or a novel hybrid zone may form.

A hybrid zone can be a source of novel genetic variation that improves the ability of one or both parent species to cope with changing environmental conditions.

Question 3

Hybrid Zones Over Time: Reinforcement

Answer 3

Involves reinforcing reproductive barriers. Hybrids often are less fit than members of their parent species. In such cases, natural selection should strengthen the pre-zygotic barriers to reproduction, reducing the formation of unfit hybrids.

Question 4

Hybrid Zones Over Time: Fusion

Answer 4

Barriers to reproduction may be weak when two species meet in a hybrid zone. So much gene flow may occur that reproductive barriers weaken further and the gene pools of the two species become increasingly alike. In effect, the speciation process reverses, eventually causing the two hybridizing species to fuse into a single species.

Question 5

Hybrid Zones Over Time: Stability

Answer 5

Many hybrid zones are stable in the sense that hybrids continue to be produced, in some cases because the hybrids survive or reproduce better than members of either parent species.

In some stable zones, hybrids are selected against.