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Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1
Question 10
Read the following passage. Lake Malawi in East Africa has more species of fish than any other lake in the world. Many of these species have evolved from a common a... show full transcript
Worked Solution & Example Answer:Read the following passage - AQA - A-Level Biology - Question 10 - 2021 - Paper 1
Step 1
Suggest and explain how this speciation may have occurred.
Answer
The speciation in Lake Malawi may have occurred due to the geographical isolation of certain fish populations when the lake fragmented into smaller bodies of water. Over time, these isolated populations underwent genetic divergence as they adapted to their specific environments, leading to the development of different species. The lack of gene flow between these populations allowed distinct alleles to be selected, resulting in unique adaptations and ultimately reproductive isolation.
Step 2
Calculate the mean loss of forest cover in km² per week during this time period.
Answer
To calculate the mean loss of forest cover, first determine the forest cover in 1990 and 2016:
- In 1990, 41.4% of 94,800 km² = 39,207.2 km²
- In 2016, 26.4% of 94,800 km² = 25,027.2 km²
The loss of forest cover is: 39,207.2 km² - 25,027.2 km² = 14,180 km².
The time period is 26 years, which is 26 x 52 weeks = 1,352 weeks.
The mean loss per week is: ( \frac{14,180 \text{ km}^2}{1,352 \text{ weeks}} \approx 10.48 \text{ km}^2 \text{ per week} ).
Step 3
Explain why.
Answer
The loss of nutrients into Lake Malawi can lead to decreased oxygen levels due to increased microbial activity. When excess nutrients, such as nitrates and phosphates, enter the lake, it can result in algal blooms. These blooms eventually die off and are decomposed by saprobic microorganisms, which consume oxygen during respiration. The subsequent drop in dissolved oxygen levels can make it difficult for fish to respire, leading to declines in fish populations.
Step 4
Explain how.
Answer
The mark-release-recapture method involves several steps: First, a sample of fish is captured, marked with a harmless substance, and then released back into the lake. After allowing sufficient time for the fish to redistribute, a second sample is captured. The number of marked fish in the second sample is counted, and this proportion is used to estimate the total population size using the formula: ( N = \frac{(M \times C)}{R} ) where ( N ) is the total population, ( M ) is the number of marked fish in the first sample, ( C ) is the total number of fish in the second sample, and ( R ) is the number of marked fish recaptured.
Step 5
Suggest why the mark-release-recapture method can produce unreliable results in very large lakes.
Answer
In very large lakes, the movement of fish may lead to uneven distribution after release, causing marked fish to either congregate in specific areas or remain near their release location. This behavioral variation can result in a biased second sample that does not accurately represent the overall population. Additionally, external factors, such as predation and environmental changes, may affect fish survival rates, further complicating the reliability of estimates obtained from this method.