Final answer:
Darwin claimed variations among species members are due to biological inheritance and adaptation to the environment, which are key components of his natural selection theory.
Step-by-step explanation:
Charles Darwin claimed that the reason why no two members of a species were exactly alike was due to variations caused by biological inheritance and adaptation to the environment. This is encapsulated in his theory of natural selection, which is based on the principles that organisms inherit characteristics from their parents, more offspring are produced than can survive leading to competition for limited resources, and that there is a variation in inherited traits among individuals. Offspring with inherited traits that improve their chances of competing for resources are more likely to survive and reproduce, thus passing on these advantageous traits to the next generation in a process Darwin termed 'descent with modification'.