To continue the tree analogy, a person can start from the tip of any arbitrarily chosen twig and follow it back to the point where it joins another twig, thecommon ancestor? of the two (or more) twigs. That now thicker twig or branch can then be traced back to where it joins another branch. This process can be repeated until that person ultimately reaches the trunk. Applied to evolution, each twig and branch represents a particular lineage; the points where there is a joining of those twigs and branches represents the common ancestor of all the lineages that can be traced back to that point. Note that the descent with modification hypothesis can be tested independently of any ideas about how it happened, the mechanism behind it. Of course, the mechanism introduced by Darwin was that of natural selection (along with the additional view that natural selection proceeded in a gradual, steady fashion).
Darwin proposed that evolution could be explained by the differential survival of organisms following their naturally occurring variation, a process he termednatural selection.? According to this view, the offspring of organisms differ from one another and from their parents in ways that are heritable; that is, they can pass on the differences genetically to their own offspring. Furthermore, organisms in nature typically produce more offspring than can survive and reproduce given the constraints of food, space, and other environmental resources. If a particular off spring has traits that give it an advantage in a particular environment, that organism will be more likely to survive and pass on those traits. As differences accumulate over generations, populations of organisms diverge from their ancestors. .
A particularly compelling example of speciation involves the 13 species of finches studied by Darwin on the Galapagos Islands, now known as Darwin's finches. The ancestors of these finches appear to have emigrated from the South American mainland to the Galapagos.
