Read the following passage about black rhinoceros and genetic variation, and then answer the question: according to the passage, what is described as the cornerstone and most important factor for evolution?

Introduction / Context:
This question is based on a reading comprehension passage about black rhinoceros populations in sub Saharan Africa and the role of genetic diversity in evolution. The passage explains how many genetic lineages of black rhinoceros have been lost and links this loss to the idea of genetic erosion. A key line in the passage explicitly states which factor is considered the cornerstone of evolution. To answer correctly, you must recognise this key statement and distinguish it from other general ideas related to evolution and conservation biology.

Given Data / Assumptions:
• The passage states that genetic variation is the cornerstone of evolution. • It also explains that without genetic variation, there can be no natural selection. • The passage mentions that low genetic diversity reduces the ability of a species to survive and reproduce. • Several countries where black rhinoceros lived are listed, and their genetic lineages are discussed.

Concept / Approach:
Reading comprehension questions like this often ask directly about a phrase that appears in the text. The phrase "cornerstone of evolution" is a strong signpost. The strategy is to locate the sentence containing that phrase and read it carefully. Since the passage is about genetics and conservation, you might be tempted by related ideas such as survival of the fittest or large population, but the correct choice must match the wording and meaning of the passage. Therefore, the key is to rely on what the passage explicitly states rather than general knowledge alone.

Step-by-Step Solution:
1. Skim the passage to find the sentence that contains the important phrase "cornerstone of evolution". 2. Read that sentence in full, noting that it states that genetic variation is the cornerstone of evolution. 3. Note the supporting explanation: without genetic variation there can be no natural selection, and low genetic diversity harms survival and reproduction. 4. Compare this information with the options and look for the choice that exactly matches this wording. 5. Recognise that option A, "Genetic variation", repeats the phrase used in the passage. 6. Confirm that none of the other options use exactly the same idea as the passage for the cornerstone of evolution.

Verification / Alternative check:
To verify the answer, imagine covering the options and asking yourself, based only on the passage, what is described as the most important factor for evolution. The passage author states that genetic variation is the cornerstone of evolution. When you later reveal the options, option A clearly matches this phrase. Although other ideas like survival of the fittest are related to evolution, the passage does not call them the cornerstone. This check helps confirm that you are choosing based on the text instead of outside assumptions.

Why Other Options Are Wrong:
Option B, "A large population size", can help conservation efforts, but the passage never calls it the cornerstone of evolution. Option C, "Mixing of different species", is not mentioned in the passage and is biologically inaccurate as a general rule because evolution mainly works within species. Option D, "Survival of the fittest alone", is part of the process of natural selection, but the passage explains that without genetic variation there can be no natural selection. Therefore, survival of the fittest depends on genetic variation and cannot be the cornerstone itself.

Common Pitfalls:
A common mistake is to rely on prior textbook knowledge and assume that phrases like survival of the fittest must be the most important concept in evolution. However, reading comprehension questions always want the answer that aligns with the passage, not with general beliefs. Another pitfall is skimming too quickly and missing key signalling phrases such as "is the cornerstone of". Training yourself to notice and underline such phrases can significantly improve your accuracy in similar questions.

Final Answer:
The passage clearly states that the cornerstone of evolution is Genetic variation.