Darwin clarified his theory of common descent by stating that it was a theory of "common descent with modification." What does the phrase common descent with modification mean?

Answer 1

Common descent refers to organisms coming from a common ancestor. For example, siblings have the same parents, grandparents, etc., so the have common descent. But they are still the same species (although teen-aged brothers and sisters might argue that point at times). There has not been any significant change in their genomes that they would have diverged into different species.

For evolution to occur, there must be a heritable change in the genome, and that change must be passed down to future generations. This introduces new alleles, or even new genes, into the genome, and that is the modification part. Changes that introduce new traits but do not change mating success would be evolution within a species. Changes that do change mating success are the tyupe of changes that can lead to a new species.