Evolution Occurs by Natural Selection
In 1973, evolutionary biologist Theodosius Dobzhansky penned an essay titled "Nothing in Biology Makes Sense Except in the Light of Evolution." By now, thousands of students across the globe have heard this title quoted to them by their biology teachers.
And for good reason, too. The power of evolution comes from its ability to explain both the unity and diversity of life; in other words, the theory describes how similarities and differences between species arise by descent from a universal common ancestor. Remarkably, all species have about one-third of their genes in common, and 65% of human genes are similar to those found in bacteria and unicellular eukaryotes (like algae and yeast).
One of the most fascinating examples of common descent is the evolution of the gene responsible for the final step in vitamin C synthesis. Humans have this gene, but it is broken. That is why we have to drink orange juice or find some other external source of vitamin C. By sequencing this gene and tracking mutations, it is possible to trace back exactly when the ability to synthesize vitamin C was lost. According to this phylogenetic tree (see diagram on right), the loss occurred in an ancestor which gave rise to the entire anthropoid primate lineage. Humans, chimpanzees, orangutans and gorillas all possess this broken gene, and hence, all of them need an external source of vitamin C. (At other points in evolutionary history, bats and guinea pigs also lost this vitamin C gene.) Yet, many mammals don't need vitamin C in their diet because they possess a functioning copy and are able to produce it on their own; that's why your dog or cat gets by just fine without orange juice.
The most satisfying explanation for these observations is descent with modification from a common ancestor.
Source: Galileo's Finger: The Ten Great Ideas of Science by Peter Atkins
Source: Guy Drouin, Jean-Rémi Godin, and Benoît Pagé. "The Genetics of Vitamin C Loss in Vertebrates." Curr Genomics 12(5): 371–378. (2011) doi: 10.2174/138920211796429736