Core Requirements of Natural Selection

Steps:

• Identify the four key conditions for natural selection: variation in traits, heritability of traits, differential survival and reproduction, and overproduction of offspring.
• Match these to the question's focus on what is "essential," meaning the foundational element without which selection cannot proceed.
• Eliminate options that are consequences or non-requirements of selection.
• Confirm the primary essential factor as the starting point in Darwin's theory.

Not enough information: Choices B, C, and D are blank, preventing specific analysis of why they are wrong.

Final answer: A