The connotation of the word "adaptability" is too positive for this story of the downfall of both a man (Okonkwo) and the society to which he belongs. Achebe is not chronicling the Igbo's adaptability but the demise of their culture at the hands of the English settlers. This culture clash in one of the major themes of the novel. At the beginning of the story, the Igbos are described as a prosperous, successful culture which has survived for hundreds of years. Once the white men arrive, they do not honor any of the tribes customs or beliefs. In fact, they immediately begin to imply that the white man's ways are better. First, they introduce their religion and then their government. The effect on the Igbos is to split them apart. Ancient tribal customs are abandoned. Eventually, the white men dominate the Igbo culture so much that they can imprison and even kill the natives at will. This is more than "adaptation" of a culture. It is the submission and destruction of one culture by another culture that considers itself superior.
Isn't it ironic that Achebe's grandfather was one of the first converts?
I totally agree about the destroying of culture part though.