The rise of evolutionary biology - What is evolution?
Descent with modification
When the members of a population breed and produce the next generation, we can imagine a lineage of populations, made up of a series of populations through time. Each population is ancestral to the descendant population in the next generation: a lineage is an 'ancestor - descendant' series of populations. Evolution is then change between generations within a population lineage. Darwin defined evolution as "descent with modification" and the word 'descent' refers to the way evolutionary modification takes place in a series of populations that are descended from one another.
Figure: Darwinian evolution: lineages change in form over time. Notice that evolution is tree like: as lineages split and extinction occurs.