First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of natural selection. It is a general statement that does not need other sentences for context. It does not start with a conjunction or use pronouns. It also does not use the definite article 'the' for a defined subject. Why other lines are not chosen? The other paragraphs are not chosen because: 1. Paragraph 2 starts with 'It', which is a pronoun and needs context. 2. Paragraph 3 starts with 'Over time', which suggests a sequence and is not independent. 3. Paragraph 4 starts with 'Thus', which is a conjunction and needs prior context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about natural selection. It uses the phrase 'Thus', which indicates a conclusion. It summarizes the main points and shows the overall result of natural selection.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what natural selection suggests. This is a logical order where the first paragraph introduces the concept and the second paragraph provides details about it. This follows the 'Broader to Specific Strategy'.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what happens to the traits mentioned in Paragraph 2. This shows a 'Cause and Effect Relationship' where the survival leads to traits becoming common.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it concludes the idea of natural selection by showing its result. This follows the 'Logical Order' where the outcome of the traits becoming common leads to the diversity of life.