Answer: The observation that led to Darwin's major inferences is: A. Members of a population vary in their heritable traits.
Step-by-step explanation:
1. Variation in traits: Darwin observed that individuals within a population exhibit variation in their inherited traits. For example, some individuals may have longer beaks, while others have shorter beaks in a population of birds.
2. Heritability of traits: Darwin recognized that these traits are passed down from parents to offspring. Offspring tend to resemble their parents in certain traits, indicating that these traits are heritable.
3. Natural selection: Based on the variation and heritability of traits, Darwin inferred that individuals with traits that are advantageous in their specific environment are more likely to survive and reproduce. This is known as natural selection.
4. Differential survival and reproduction: Darwin observed that not all individuals within a population can survive and reproduce. Due to limited resources and competition, only a fraction of the population can successfully pass on their traits to the next generation.
Option B, "Body parts that are not used deteriorate over time," is incorrect. Although Darwin did observe variations in traits, he did not make inferences about the deterioration of body parts based on lack of use.
Option C, "Organisms only go extinct when catastrophes occur," is also incorrect. While extinction events do happen, Darwin's major inferences were not specifically based on catastrophic events leading to extinction.
Option D, "Although organisms can produce huge numbers of offspring, many of these offspring do not survive," is partially correct. Darwin did observe that organisms produce more offspring than can survive, leading to competition for limited resources. However, this observation alone does not capture the essence of his major inferences.
In summary, the observation that members of a population vary in their heritable traits was the key observation that led to Darwin's major inferences about natural selection and the survival and reproduction of individuals with advantageous traits.