Problem 6 - Campbell Biology Concepts & Connections

Question

Which of the following is an example of a postzygotic reproductive barrier? a. One Ceanothus shrub lives on acid soil, another on alkaline soil. b. Mallard and pintail ducks mate at different times of year. c. Two species of leopard frogs have different mating calls. d. Hybrid offspring of two species of jimsonweeds always die before reproducing.

Step-by-Step Solution

Answer

d. Hybrid offspring of two species of jimsonweeds always die before reproducing.

Step 1: Understanding the Term 'Postzygotic Barrier'

A postzygotic reproductive barrier occurs after fertilization and typically results in non-viable or sterile offspring.

Step 2: Option Analysis: a

Option a describes two shrubs living on different soils. This is an example of a prezygotic barrier known as habitat isolation.

Step 3: Option Analysis: b

Option b describes ducks mating at different times. This is an example of temporal isolation, a prezygotic barrier.

Step 4: Option Analysis: c

Option c describes frogs with different mating calls. This is an example of behavioral isolation, another prezygotic barrier.

Step 5: Option Analysis: d

Option d describes hybrid offspring that die before reproducing. This is an example of a postzygotic barrier because it occurs after fertilization and affects the viability of the hybrid offspring.

Key Concepts

Postzygotic and Prezygotic Barriers Explained

When we discuss reproductive barriers, it's essential to understand the difference between postzygotic and prezygotic barriers. Postzygotic barriers occur after fertilization. These barriers typically result in hybrid offspring that are non-viable or sterile. In contrast, prezygotic barriers happen before fertilization, preventing the occurrence of mating or fertilization.

Considering the examples provided in the exercise, option d represents a postzygotic barrier. The hybrid offspring of two species of jimsonweeds cannot reach reproductive age, making it a clear example of a postzygotic barrier at work.

Habitat Isolation

Habitat isolation is a form of prezygotic reproductive barrier where two species live in different environments or habitats, making it unlikely for them to meet and mate. This type of isolation prevents the formation of hybrids because the populations are physically separated.

For example, consider the Ceanothus shrubs from option a in the exercise. One shrub lives on acidic soil, while the other lives on alkaline soil. These distinct habitats mean the shrubs rarely, if ever, encounter each other, effectively preventing them from mating. Habitat isolation is a crucial concept in understanding how different species maintain their unique identities and avoid crossbreeding in nature.

Temporal Isolation

Temporal isolation is another type of prezygotic barrier. It occurs when species mate or flower at different times—whether it be different times of day, seasons, or years. This time-based separation ensures that even if species live in the same habitat, they cannot interbreed because their reproductive periods do not overlap.

In the exercise, option b mentions Mallard and Pintail ducks that mate at different times of the year. Even if they share the same habitat, the timing of their mating seasons ensures that they do not interbreed. Temporal isolation is vital to understanding how species avoid hybridization through differences in their biological clocks.

Behavioral Isolation

Behavioral isolation is a prezygotic mechanism that relies on species-specific behaviors or courtship rituals to prevent interbreeding. Differences in mating calls, dances, or other behaviors can act as barriers, ensuring that mating only occurs within the same species.

An example from the exercise is option c, where two species of leopard frogs have different mating calls. These calls are crucial for attracting mates. If a female frog does not recognize the call, she will not mate with that male. This type of isolation helps species maintain their distinct evolutionary paths by reducing the likelihood of hybridization.