Problem 6 - AP Biology Premium Prep 2021

Question

The Middle East blind mole rat ( Nannosplalax ehrenbergi) lives in the Upper Galilee Mountains of Israel. Two groups of these mole rats live in the same region, but scientists discovered that there is a \(40 \%\) difference in mitochondrial DNA between these two groups. These rats do not seem to interbreed in the wild. This is an example of (A) allopatric speciation (B) sympatric speciation (C) pre-zygotic barrier (D) hybrid zone

Step-by-Step Solution

Answer

(B) sympatric speciation

Step 1: Define Key Terms

The key terms in this exercise are as follows: 1. Allopatric Speciation: It is a process of speciation that occurs when a physical barrier separates populations, preventing gene flow and eventually leading to the evolution of distinct species. 2. Sympatric Speciation: In this process, speciation occurs within a shared geographical area as a result of reproductive isolation or genetic divergence. 3. Pre-zygotic Barrier: It is an obstacle that prevents different species from mating and producing fertile offspring by inhibiting the formation of zygotes. These barriers can include habitat isolation, temporal isolation, or behavioral isolation. 4. Hybrid Zone: It is a region where two closely related species come into contact and interbreed. The gene flow between them often leads to the formation of hybrids, which may have reduced fitness compared to their parent species.

Step 2: Analyze the Mole Rat Population

We are given the following information about the mole rat population: 1. Two groups of mole rats live in the same region. 2. There is a 40% difference in mitochondrial DNA between these groups. 3. They do not seem to interbreed in the wild.

Step 3: Compare the Given Information with Key Terms

With the given information, let's assess which concept out of the four options best matches the mole rat population. 1. Allopatric Speciation: Since the two groups of mole rats live in the same region, there is no physical barrier separating them. Thus, this option is not the most appropriate. 2. Sympatric Speciation: Speciation occurred within the shared geographical area without a physical barrier, and the two groups have a significant mitochondrial DNA difference. This option is a potential match. 3. Pre-zygotic Barrier: The mole rats do not seem to interbreed in the wild. This information indicates that there might be some kind of pre-zygotic barrier preventing them from breeding. This option could also be a match. 4. Hybrid Zone: The information does not indicate any interbreeding or the formation of hybrids between the two groups, so this option is not likely the best match.

Step 4: Choose the Best Match

Based on the comparison in Step 3, the most appropriate concept among the given options is sympatric speciation (Option B). Even though the pre-zygotic barrier concept could also fit the description, it is important to note that sympatric speciation might involve pre-zygotic barriers as one of the mechanisms that lead to reproductive isolation. Since these mole rats live in the same region and do not interbreed, sympatric speciation is more comprehensive as an explanation for their situation, encompassing not only the possibility of a pre-zygotic barrier but also the genetic divergence that results in two distinct groups with a 40% difference in mitochondrial DNA. Therefore, the answer is: (B) sympatric speciation

Key Concepts

Allopatric Speciation

Allopatric speciation describes the process by which new species arise due to a physical barrier that separates populations. This geographical separation prevents gene flow—a critical factor that maintains similarities within a species—leading to genetic divergence over time.

Imagine a population of animals living along a mountain range. An earthquake occurs, creating a deep chasm that splits the population in two. Because these resulting two populations can no longer mix, they may evolve independently. They adapt to their unique environments and accumulate genetic changes that eventually lead to the formation of two distinct species.

This kind of speciation emphasizes the role of physical distance and environmental differences as primary drivers of biodiversity. It is the traditional view of how species diverge, especially in instances where colonization of a new habitat, migration, or other physical barriers comes into play.

Pre-zygotic Barrier

Pre-zygotic barriers are mechanisms that prevent different species from mating successfully before the formation of a zygote, which is the initial cell resulting from reproduction. These barriers can be biological or behavioral in nature.

• Temporal Isolation: Populations may mate or flower at different times of the year.
• Habitat Isolation: Even when living in the same location, species may prefer different habitats and thus do not meet.
• Behavioral Isolation: Unique mating calls or rituals may be unrecognizable or unattractive to other species.
• Mechanical Isolation: Physical incompatibilities can prevent successful mating.
• Chemical Isolation: Molecular signals may not be exchanged properly, thwarting the fertilization process.

Such barriers are essential because they maintain species identity and help in reproductive isolation, which is crucial in the speciation process.

Mitochondrial DNA

Mitochondrial DNA (mtDNA) is genetic material found in the mitochondria—often termed the powerhouses of the cell. Unlike the DNA in a cell's nucleus, which comes from both parents, mtDNA is inherited only from the mother.

Studying mtDNA is useful in evolutionary biology and speciation because it mutates at a relatively consistent rate. This allows scientists to estimate the time since two populations shared a common ancestor. A significant difference in mtDNA between two groups, like in the case of the Middle East blind mole rat, suggests that they have been genetically isolated for a substantial amount of time.

MtDNA gives us a window into the maternal lineage and can reveal fascinating tales of migration, population dynamics, and, importantly, speciation events. By comparing mtDNA sequences, researchers can infer relationships between species and their potential divergence.

Reproductive Isolation

Reproductive isolation is a collection of mechanisms, behaviors, and physiological processes critical for speciation. It prevents members of different species or populations from interbreeding and producing fertile offspring even if they live in the same environment.

These isolating mechanisms are broadly categorized into two groups: pre-zygotic, which prevent fertilization from occurring, and post-zygotic, which occur after fertilization and tend to lead to the inviability or infertility of the offspring.

Reproductive isolation is integral to the process of sympatric speciation, like with the mole rats mentioned in the exercise. Although these mole rats in the Galilee Mountains live in the same location (sympatric) and are not separated by a physical barrier, they may still undergo speciation due to reproductive isolation, resulting from mechanisms that prevent them from interbreeding, thus allowing genetic divergence.