Also see:
Punctuated Equilibrium

Darwin sailed at age 22 in 1831 on the ship H.M.S. Beagle, a trip that would prove critical in leading Darwin to his understanding of evolution. Paramount was his visit to the many Galápagos Islands 600 miles off the west coast of South America.

Darwin identified 13 species of Finches among the Galápagos Islands that were primarily differentiated by beak size. In contrast, only one species of this bird existed on the mainland South America to the east. Darwin correctly concluded that the different beaks were adaptations to different diets available among the islands.

Darwin ultimately generalized the observation from the finches that any population consists of individuals that are all slightly different from one another. Furthermore, individual organisms having a phenotype characteristic providing an advantage in staying alive to successfully reproduce will pass their phenotype traits more frequently to the next generation. Over time and generations the traits providing reproductive advantage become more common within the population. Darwin called this process "descent with modification". Adaptive radiation, as observed by Charles Darwin in Galapagos finches, is a consequence of allopatric speciation among island populations.

Darwin also correctly understood that the variability allowing adaptation already existed in the finch population, though its genetic (genotype) reason was not yet known by science at the time. Nature was NOT "producing" the variation within the finch populations - it already existed. Rather, nature "selected" from among the population variation the traits that better fostered survival and reproduction, a process known as "natural selection". The process guides evolution across the entire Tree-of Life.

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us." - Charles Darwin (1859)

Charles Darwin correctly inferred much about the process of natural selection when observing finches in the Galápagos island way back in the middle of the 19th century. Now, in a paper appearing Science (Grant P. R., et al. Science, 313. 224 - 226 (2006), Peter and Rosemary Grant, both biologists at Princeton University, New Jersey, have replicated Darwin's empirical observations. They describe their observation of evolution at work in the survival struggle between the medium ground finch (Geospiza fortis) and the large ground finch (Geospiza magnirostris). They observed competition between two species to the beaks of one species to shrink, that is, they observed evolution by means of natural selection.