the major refinements in Darwin's theory that stem from modern finding?

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Answer 1

The weakpoint in Darwin's theory was "Variations".
He hypothesised that variations within a specific group of animals increased their ability to survive. Darwin could not explain why these variations ocurred.
The modern theory of evolution explains how these variations might occur: Recombination of chromosomes at fertilization; Gene (biochemical) mutations; Chromosome Mutations, including - nondisjunction, translocation of nonhomologs,inversions. deletions, additions, polyploids, etc.

Although variations can now be explained , it must be noted that most (99+%) of mutations are NOT beneficial.