Unit 7 FRQ (Evolutionary Advantages)

AP Bio Unit 7 FRQ: Evolutionary Advantages

Explore Evolution and Adaptation in the Mexican Tetra

👋 Welcome to the AP Bio Unit 7 FRQ (Evolutionary Advantages).
These questions are more in-depth, so grab some paper and a pencil, or open up a blank page to work through them. These FRQs are similar to what you might find on the exam, so it’s great practice!

⏱ Time Management Tip: The AP Biology exam contains 6 free-response questions that you’ll need to complete in 90 minutes. This means you should allocate about 15 minutes to answer each FRQ.

🤔 Need to refresh your knowledge? Check out the Unit 7 Overview for key evolutionary biology concepts before diving in.

😩 Feeling stuck? You can consult all available Unit 7 resources to get a clearer understanding.

Scenario

The Mexican tetra (Astyanax mexicanus) is a unique species of freshwater fish found primarily in rivers, streams, and caves along the eastern coast of Mexico. This species has two notable forms: the surface-dwelling form, which has well-developed eyes and darker pigmentation, and the cave-dwelling form, which lacks eyes entirely and has reduced pigmentation. The cavefish are commonly called “blind cavefish.”

Figure 1 shows the distribution of five subtypes of Mexican tetras, each with distinct morphological traits adapted to their respective environments.

Questions

(a) Propose an evolutionary advantage of the blind cavefish over the surface form in its respective environment.

Response Guide:

• Evolutionary Advantage:
  • In the dark, low-light cave environments, vision is not beneficial. Eyes can be vulnerable to injury and infection, especially in complete darkness. By losing the eyes, the blind cavefish avoid the energy cost associated with the development and maintenance of unnecessary organs. This allows them to allocate more energy to other senses (such as enhanced lateral line systems for detecting vibrations) that are more valuable in their environment.

(b) Identify the nonselective process that caused different groups of the fish to differentiate rapidly and with major phenotypic differences.

Response Guide:

• Nonselective Process: Genetic Drift.
  • Genetic drift is a nonselective process that affects small populations, like those living in isolated caves. When populations are geographically isolated, genetic drift can cause random changes in allele frequencies, leading to the rapid differentiation of phenotypes over time, especially in the absence of selective pressures found in larger populations.

(c) Discuss what would have happened if the mutation that led to blindness occurred in a school of fish living at the surface rather than a school of fish residing in low-light caves.

Response Guide:

• Mutation Impact on Surface-Dwelling Fish:
  • If the mutation that caused blindness occurred in a surface-dwelling population, it is likely that it would not be beneficial. Surface-dwelling fish rely heavily on vision for avoiding predators, finding mates, and locating food. A mutation causing blindness would put an individual at a significant disadvantage, making it less likely to survive and reproduce. Consequently, natural selection would likely remove this mutation from the population.

(d) Draw a cladogram demonstrating the relationships between the five types of Mexican tetras.

Draw it on a separate sheet of paper, take a picture, and attach it to your answer document.

Response Guide:

• Cladogram Guide:
  • The cladogram should reflect the evolutionary relationships between the five subtypes of Mexican tetras. Use the different traits, such as the presence or absence of eyes and pigmentation, to determine branching points.
  • For example, the blind cavefish may branch away from the surface-dwelling fish based on the absence of eyes and reduced pigmentation, indicating adaptation to the cave environment.

(e) In the original population of Mexican tetras, scale color may be light or dark (d or D, respectively). Scale color is the result of complete dominance. If 15% of Mexican tetras have light scales, calculate the frequencies of the dominant and recessive alleles.

Response Guide:

• Step 1: Define the Given Information.

  • Light scales (dd): 15% of the population = 0.15
  • Since light scales are recessive, q² = 0.15, where q represents the frequency of the recessive allele (d).

• Step 2: Calculate q.

  • q = √(0.15) ≈ 0.387

• Step 3: Calculate p.

  • p + q = 1, where p represents the frequency of the dominant allele (D).
  • p = 1 – 0.387 ≈ 0.613

• Step 4: State the Allele Frequencies.

  • Frequency of Dominant Allele (D): p ≈ 0.613
  • Frequency of Recessive Allele (d): q ≈ 0.387

Summary of Key Concepts:

1. Evolutionary Advantage: Blind cavefish have evolved to save energy by not developing eyes, which are unnecessary in dark environments, allowing them to adapt efficiently to their habitat.
2. Genetic Drift: A nonselective process that leads to random changes in allele frequency, playing a role in the rapid differentiation of isolated populations.
3. Mutation in Surface Fish: In a surface environment, a mutation causing blindness would likely be detrimental due to the reliance on vision, and would be removed by natural selection.
4. Cladogram Construction: Depicting evolutionary relationships between subtypes using shared and derived traits.
5. Allele Frequencies Calculation: Use Hardy-Weinberg principles to calculate the frequencies of alleles given phenotypic data.