I want to know the answer to the following question:
Why are variations distributed along clines?
All the background I'm provided with is that "One crucial innovation in reconceptualizing genotypic and phenotypic variation was the anthropologist C. Loring Brace's observation that such variations, insofar as it is affected by natural selection, slow migration, or genetic drift, are distributed along geographic gradations or clines.". Is that enough to answer the question?
Answer: no