|MadSci Network: Evolution|
"Descent with modification" is a phrase used by Charles Darwin in The Origin of Species by means of Natural Selection (6th edition, available online from Project Gutenberg) to describe the process by which different species can be descended from a common ancestor. The idea is that as genes are passed from an organism to its children to its children's children, accidental modifications to the genes (errors in copying) can accumulate to such an extent that after many thousands of generations some of the descendants no longer resemble their ancestor of thousands of generations before.
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