Question

Prezygotic and postzygotic barriers to reproduction have the effect of a. reducing genetic differences between populations. b. increasing the chance of hybridization. c. preventing speciation. d. reducing or preventing gene flow between species.

Short answer

d. reducing or preventing gene flow between species.

Step-by-step solution

Understand the Terminology

Prezygotic barriers occur before the formation of a zygote, preventing mating or fertilization between species (e.g., temporal isolation, habitat isolation, behavioral isolation). Postzygotic barriers occur after zygote formation, preventing hybrid offspring from surviving or reproducing (e.g., hybrid inviability, hybrid sterility).

Eliminate Options that Do Not Fit

We need to find the option that describes the main effect of these barriers. a. reducing genetic differences – these barriers actually increase genetic differences between populations. Remove this option. b. increasing hybridization – these barriers reduce or prevent hybridization. Remove this option. c. preventing speciation – these barriers contribute to speciation by preventing gene flow. Remove this option.

Confirm the Correct Answer

The remaining option is: d. reducing or preventing gene flow between species. By preventing interbreeding, both prezygotic and postzygotic barriers reduce or prevent gene flow, maintaining species boundaries.

Key concepts

prezygotic barriers

Prezygotic barriers are mechanisms that prevent different species from mating and producing hybrid offspring. These barriers occur before fertilization and can take several forms:

• Temporal Isolation: Species mate or flower at different times of the day, season, or year.
• Habitat Isolation: Species live in the same general area but in different habitats, so they rarely encounter each other.
• Behavioral Isolation: Different species have different mating rituals that aren't recognized by other species.
• Mechanical Isolation: Structural differences in reproductive organs prevent mating.
• Gametic Isolation: Even if mating occurs, sperm and egg cells from different species are incompatible.

These barriers are essential to preventing gene flow between species, thus fostering speciation by maintaining genetic distinctions between populations.

postzygotic barriers

Postzygotic barriers come into play after fertilization has occurred. These barriers prevent hybrid offspring from developing into viable, fertile adults and ensure that any hybrids that do form are not successfully propagating. Examples include:

• Hybrid Inviability: Hybrids fail to develop or reach maturity. They may die at an early embryonic stage or soon after birth.
• Hybrid Sterility: Hybrids develop into adults, but they are sterile and cannot produce offspring. An example is the mule, a hybrid between a horse and a donkey.
• Hybrid Breakdown: First-generation hybrids are fertile, but when they mate, their offspring are weak or sterile.

These barriers further prevent gene flow between species and solidify the genetic differences that contribute to speciation.

gene flow

Gene flow is the transfer of genetic material from one population to another. It is a crucial process for maintaining genetic diversity within a species. When gene flow occurs frequently among populations, it can reduce genetic differences and prevent populations from diverging into separate species.
Prezygotic and postzygotic barriers impede gene flow by preventing successful interbreeding between populations of different species. Without gene flow, species are more likely to develop distinct traits and adapt to their specific environments, leading to speciation. Therefore, gene flow plays a significant role in both maintaining genetic diversity and facilitating or hindering speciation, depending on the context.

speciation

Speciation is the evolutionary process where populations evolve to become distinct species. It usually involves a reduction or elimination of gene flow between populations, allowing genetic differences to accumulate over time. There are several types of speciation:

• Allopatric Speciation: Occurs when populations are geographically isolated from each other, leading to genetic divergence.
• Sympatric Speciation: Takes place without geographic isolation. Instead, other factors like ecological niches or behavioral isolation drive speciation.
• Parapatric Speciation: Neighboring populations evolve into distinct species while maintaining contact along a common border.

Prezygotic and postzygotic barriers are integral to speciation because they prevent gene flow, ensuring that populations remain separate and genetically distinct. As these barriers strengthen over time, they solidify the differences between species, completing the process of speciation.