Which of the following is not an observation or inference on which natural selection is based? a. There is heritable variation among individuals. b. Poorly adapted individuals never produce offspring. c. Species produce more offspring than the environment can support. d. Only a fraction of an individual's offspring may survive.

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1. Identify the key concepts of natural selection: Natural selection is based on the observations that there is variation among individuals within a species, that more offspring are produced than can survive, and that only some offspring will survive and reproduce based on their inherited traits.
2. Analyze each statement: a) There is heritable variation among individuals - this is a true observation for natural selection as it provides the material for selection. c) Species produce more offspring than the environment can support - this is true and leads to competition among offspring for survival. d) Only a fraction of an individual's offspring may survive - this is also true and reflects the survival of the fittest.
3. Identify the incorrect statement: b) Poorly adapted individuals never produce offspring - this statement is not always true as poorly adapted individuals can still produce offspring, though their offspring might have lower survival rates.
4. Understand the implications of the incorrect statement: The statement in option b suggests an absolute scenario which is not consistent with the principles of natural selection. Natural selection suggests that while poorly adapted individuals may have fewer offspring or their offspring may have lower survival rates, it does not imply they never reproduce.
5. Conclude the correct answer: Based on the analysis, option b is the statement that is not an observation or inference on which natural selection is based.