Question

Evidence suggests that sympatric speciation may have occurred or may be in progress in three of the following four cases. Select the exception. a. apple maggot fly b. squirrels on opposite sides of the Grand Canyon c. cichlid fishes d. polyploid plants (or their ancestors)

Short answer

Squirrels on opposite sides of the Grand Canyon (b)

Step-by-step solution

Understand Sympatric Speciation

Sympatric speciation occurs when new species evolve from a single ancestral species while inhabiting the same geographic region. It often involves genetic differences, such as polyploidy or ecological niche differentiation.

Analyze Each Case

Review each case to determine if sympatric speciation is a plausible explanation.- **Apple Maggot Fly (a):** These flies have shifted from hawthorn to apple trees, potentially leading to genetic divergence without geographic separation.- **Squirrels on Opposite Sides of the Grand Canyon (b):** These squirrels are separated by the physical barrier of the Grand Canyon, suggesting geographic isolation rather than sympatric speciation.- **Cichlid Fishes (c):** Cichlid fishes often undergo sympatric speciation through divergent ecological niches and behaviors in the same lake.- **Polyploid Plants (d):** Polyploidization can lead to new species arising within the same geographic region, commonly seen in plants.

Identify the Exception

Among the four cases, only the squirrels on opposite sides of the Grand Canyon are separated by a physical barrier. This indicates allopatric (geographic) speciation rather than sympatric speciation.

Conclusion

The exception is the squirrels on opposite sides of the Grand Canyon (b), as they are not undergoing sympatric speciation due to the physical geographic barrier.

Key concepts

speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. This can occur in a few different ways, primarily through allopatric and sympatric speciation.
Speciation involves several mechanisms:

• Genetic divergence
• Geographic isolation
• Ecological niche differentiation

When populations of the same species become isolated, they can no longer interbreed, which leads to genetic changes over time.
In sympatric speciation, new species evolve from a single ancestral species while living in the same geographical region. This can happen through behavioral changes, differences in ecological niches, or genetic mutations such as polyploidy, especially in plants.

genetic divergence

Genetic divergence refers to the process in which two or more populations of an ancestral species accumulate independent genetic changes (mutations, gene flow interruptions, etc.) through time, often leading to speciation. Over generations, this can create significant genetic differences that prevent the now-divided populations from interbreeding.
Key factors contributing to genetic divergence include:

• Random mutations
• Natural selection
• Genetic drift

For example, in sympatric speciation, the apple maggot fly showcases genetic divergence without geographic separation, as flies have specialized in different fruit hosts (apples vs. hawthorns), leading to reproductive isolation.

geographic isolation

Geographic isolation occurs when physical barriers prevent populations from interbreeding, leading to allopatric speciation. This type of speciation happens when populations are separated by barriers such as:

• Mountains
• Rivers
• Deserts
• Oceans

Due to these barriers, gene flow between the populations is reduced or eliminated, leading to genetic divergence.
A classic example from the exercise involves squirrels on opposite sides of the Grand Canyon. They cannot interbreed due to the physical barrier, which is leading them to diverge genetically into distinct species.

ecological niche

An ecological niche is the role an organism plays in its ecosystem, including its habitat, resource use, and its relationships with other organisms.
Niche differentiation can lead to sympatric speciation, as organisms adapt to different niches within the same environment, reducing competition and promoting reproductive isolation. Just like the cichlid fishes in lakes, which evolve to occupy various niches such as different feeding strategies and breeding behaviors, sympatric speciation can occur.
Ecological niches can include:

• Diet and feeding behavior
• Reproductive strategies
• Microhabitats

Niche differentiation helps minimize direct competition, allowing multiple species to coexist and evolve separately even within the same geographical areas.