Darwin noticed that organisms in similar habitats across the world looked alike. This illustrates

Step 1: Analyze the question The question says organisms in similar habitats across the world (often unrelated and in different places) look alike. We need the evolutionary concept that explains this.

Step 2: Explain the concepts

• Convergent evolution: Unrelated organisms living in similar environments develop similar features because they face the same selection pressures. Example: sharks (fish) and dolphins (mammals) both have streamlined bodies and fins. Another is the cactus in American deserts and the Euphorbia in African deserts.
• Adaptive radiation: One ancestral species diversifies into many different forms to fill different niches, so organisms become different from each other. Example: Darwin's finches in the Galápagos.
• Co-evolution: Two species influence each other's evolution, such as flowers and their pollinators.
• Descent with modification: Species change over generations from common ancestors. This is Darwin's general theory, not specifically about similar-looking organisms in similar habitats.

Step 3: Solve

• Similar habitat leads to similar selection pressures, which leads to similar adaptations and a similar appearance, even in unrelated groups in different parts of the world.
• This matches convergent evolution.
• Adaptive radiation produces differences, so B is wrong.
• Co-evolution involves two interacting species, so C is wrong.
• Descent with modification is too general and is about common ancestry, so D is wrong.

Step 4: Answer The correct option is A. convergent evolution.

Exam tip: Remember "Converge = come together." Different ancestors, similar environment, similar features. For adaptive radiation, think "radiate = spread out" into many different forms from one ancestor.