The Scenario: Grass Species Around a Mine

Imagine a species of grass growing in an area surrounding a mine. The soil near the mine is contaminated with heavy metals, while the surrounding soil remains uncontaminated. Some populations of the grass develop tolerance to the heavy metals and thrive in the contaminated patches, while others remain non-tolerant and grow in the clean soil. Despite being continuously distributed—meaning there are no physical barriers separating the populations—these groups start to diverge.

Over time, the tolerant and non-tolerant grass populations evolve different flowering times, further reducing the chances of interbreeding. Eventually, they become reproductively isolated and are recognized as separate species.

What is Parapatric Speciation?

Parapatric speciation occurs when populations are distributed along an environmental gradient or across a habitat with varying conditions, but not separated by a physical barrier. Gene flow is possible but reduced, usually because individuals are more likely to mate with neighbors in similar environments. Divergent selection pressures—such as heavy metal contamination—drive the populations to adapt in different ways. Over time, reproductive isolation mechanisms, like changes in flowering time, solidify the split into distinct species.

Key Features of Parapatric Speciation

• Continuous Distribution: Populations are adjacent or overlapping, with no complete geographic separation.

• Environmental Gradient: There is a sharp change in environmental conditions (e.g., soil contamination) across the range.

• Reduced Gene Flow: Mating is more likely within similar environments, leading to divergence.

• Reproductive Isolation: Over time, mechanisms such as different flowering times or mating behaviors prevent interbreeding.

The Grass Example: A Classic Case of Parapatric Speciation

In the grass species near the mine, the contaminated soil acts as a strong selective pressure. Only those individuals with genetic mutations allowing them to tolerate heavy metals can survive and reproduce in the contaminated patches. Meanwhile, the non-tolerant populations continue to thrive in uncontaminated soil.

As these two groups adapt to their specific environments, gene flow between them is reduced—not because of a physical barrier, but because the plants are more likely to mate within their own microhabitat. Eventually, differences such as flowering time emerge, creating a reproductive barrier. Even though the two types of grass are growing side by side, they are now distinct species.

Why Not Allopatric, Sympatric, or Bottle-Neck Effect?

• Allopatric Speciation: Requires a physical barrier (like a mountain or river) that completely separates populations. In the grass example, no such barrier exists.

• Sympatric Speciation: Occurs within a single, randomly mating population, often through genetic changes like polyploidy or behavioral shifts. There is no environmental gradient or partial separation.

• Bottle-neck Effect: Refers to a sharp reduction in population size due to a catastrophic event, leading to genetic drift. It is not a mechanism of speciation in this context.

Real-World Examples of Parapatric Speciation

• Metal-tolerant grasses: Similar to the mine scenario, several species of grasses in Europe have evolved metal tolerance in areas near mines, leading to the formation of new species.

• Mollusks and snails: In some coastal environments, snails living on different parts of the shore adapt to varying levels of wave exposure, eventually becoming distinct species.

• Plants along roadways: Road salt can create environmental gradients, leading to the evolution of salt-tolerant plant populations adjacent to non-tolerant ones.

The Importance of Parapatric Speciation

Parapatric speciation highlights the power of environmental gradients and selective pressures in driving evolutionary change. It shows that physical barriers are not always necessary for new species to arise. Instead, adaptation to different microhabitats and the evolution of reproductive barriers can be enough to split a population into distinct species.

This process is especially important in plants and animals that inhabit patchy or changing environments, such as areas affected by human activity, pollution, or climate change.

Conclusion

The evolution of grass species around a mine, where populations adapt to heavy metal contamination and eventually become reproductively isolated through changes like flowering time, is a classic example of parapatric speciation. This form of speciation demonstrates how environmental gradients and selective pressures can lead to the emergence of new species, even in the absence of physical barriers.

Correct answer:
(3) Parapatric speciation