Note: This is a continuation of an essay posted yesterday. You can read the first part here.
The second piece of evidence is found in living creatures, which are littered with the remnants of their ancestors’ ways of life. Bird and anteater embryos show tooth buds that are later absorbed and never erupt. Baleen whale embryos even develop teeth that are later resorbed. These are relics of their toothed ancestors.1 Flightless kiwi birds have diminutive wings underneath their feathers, which testify to the ability of their ancestors to fly. Many cave-dwelling animals have rudimentary eyes that cannot see, even though eye development initiates in many of these species, but is later aborted.2 The same can be said for the hind limbs of snakes, which form limb buds during embryonic development, but die off later.3 All these are indications that they are descended from sighted and limbed ancestors, respectively.
Such remnants are also found in our genomes. Humans, unlike most mammals, cannot synthesize (make) our own vitamin C, but we carry the genes for synthesizing vitamin C. One of these genes encodes the GLO (L-gulonolactone oxidase) enzyme, and this gene in humans contains inactivating mutations and is therefore a “pseudogene.” This pseudogene and the genes that encode the enzymes of the vitamin C biosynthetic pathway are the remnants of our own evolutionary lineage from an ancestor that was able to synthesize its own vitamin C.4 Furthermore, the GLO pseudogene is just one of a graveyard of inactivated genes, transposons, retroviruses and other non-functional sequences that litter our genome. While some of these sequences have been co-opted for particular functions, many of them have no known function.5 We share many of these non-functional sequences with chimpanzees. The very presence of these genomic and anatomical flotsam and jetsam only makes sense if evolution has occurred.6
A third piece of evidence for evolution comes from biogeography.7 The flora and fauna of islands such as those of the Galápagos and Hawaii are radically unbalanced in that they lack many types of plants and animals but contain a profusion of clusters of similar species. Hawaii, for example, has no native mammals, reptiles, or amphibians, but a profusion of fruit flies and silversword plants.8 One third of the 2,000 species of fruit flies are found on the Hawaiian Islands, which only covers 2 percent of the land on earth. These islands were never connected to the continents and arose as a result of volcanic activity and were, at least initially, completely uncolonized. The colonization of these islands occurred by means of occasional introduction of creatures from the mainland that then rapidly speciated on these islands to fill every available ecological niche. Thus, the organisms most closely related to island species come from the closest mainland areas, and often include those creatures most likely to find their way to islands, such as birds and flying insects.
The Galápagos Islands provide an excellent example of how biogeography provides evidence for evolution. The Galápagos have fourteen species of finch whose closest relative is probably the South American grassquit (Tiaris), yet only four of these finch species feed on seeds as finches normally do, while two others feed on cacti, seven eat insects, and another eats almost exclusively leaves.9 Darwin, while visiting the Galápagos, still thought that species only varied within a particular kind (though he would not have used that terminology) but could adapt to various local environments and become particular subspecies. Therefore, he originally listed the warbler finch (Certhidea olivacea) as a wren and listed the small cactus finch (Geospiza scandens) as a member of the Icteridae or the family of meadowlarks and orioles. Only after Darwin had deposited his Galápagos specimens with the British ornithologist John Gould did Darwin discover (in a meeting with Gould that occurred during March, 1877), that his finch collection included thirteen or fourteen species of unusual finches that were all so closely related, Gould classified them in a single group all their own. This meeting showed Darwin that the immutable barrier between kinds of species did not exist. The Galápagos Islands were not a distinct “center of creation,” but a workshop for evolution in which an ancestral species made it to the yet uncolonized island and underwent a massive degree of speciation to adapt to the environment of the island.10 This is precisely what one would expect if the species of islands had arisen by evolution.
A scientific theory also allows scientists to make predictions, and good theories provide accurate predictions. Can the theory of evolution allow accurate predictions? The answer, once again, is yes. Darwin himself predicted that the earth must be very old for evolution to occur. He did not know the age of the earth, but further research has shown that the earth is 4.55 billion years old, which is plenty of time for evolution to occur. Darwin also predicted that since plants on islands were most closely related to certain mainland plant species, the seeds of these plants should be able to withstand immersion in sea water for long periods of time, and again, Darwin was shown to be right.11 Many decades after Darwin, we now know that variation in organisms is due to mutations in DNA and that these mutations are inherited, just as Darwin predicted.12 Also, Darwin’s principle of natural selection predicts that particular sequences of DNA should behave in a manner that benefits only themselves and not their carriers, which modern research has thoroughly confirmed with the discovery of transposons and other types of “selfish DNA.”13
Is evolutionary theory a good scientific theory? It has been repeatedly tested for over 150 years since its inception, and it has passed those tests successfully. The theory has been modified in response to new data, but the outlines of the theory have remained largely intact. It has existed at risk from new data. During the molecular biology revolution that began with the discovery of the structure of DNA by Franklin, Watson and Crick in 1953, the explosion of new data could have shown contemporary evolutionary theory to be wrong. However, some of the most powerful evidence for the theory of evolution has come from a field of science that did not even exist during Darwin’s time. The ability of a theory to withstand such intense scrutiny is a clear sign it is robust and enduring. As shown, the theory of evolution has predictive power, and it also integrates and makes sense of data from several fields of science, including ecology, paleontology, genetics, historical geology, paleoclimatology, and comparative anatomy and biochemistry. The highly integrative nature of evolutionary theory makes it a fine theory by any measure.
In conclusion, when measured against the standards of a good scientific theory, modern evolutionary biology clearly qualifies as good science. Ongoing debates within evolutionary biology exist about mechanism, rates, and causes, but not over whether evolution occurred. Such a question has been largely settled by the last 150 years’ worth of research. The future certainly looks bright for this field of science and I cannot imagine a more exciting topic to study.
1. Davit-Béal, Tiphaine,Abigail S. Tucker, and Jean-Yves Sire. “Loss of Teeth and Enamel in Tetrapods: Fossil Record, Genetic Data and Morphological Adaptations.” Journal of Anatomy 214, no. 4 (2009): 477–501.
2. Tian, Natasha M. M.-L., and David J. Price. “Why Cavefish are Blind.” BioEssays 27 (2005): 235–38; Yamamoto Y, Stock DW, and Jeffery WR (2004) Hedgehog Signalling Controls Eye Degeneration in Blind Cavefish. Nature 431:844–7; Jeffery, W. R. “Adaptive Evolution of Eye Degeneration in the Mexican Blind Cavefish.” Journal of Heredity 96, no. 3 (2005): 185–196.
3. Bejder, L., and B. K. Hall. “Limbs in Whales and Limblessness in Other Vertebrates: Mechanisms of Evolutionary and Developmental Transformation and Loss.” Evolution and Development 4, no. 6 (2002): 445–58.
4. Lachapelle, M. Y., and G. Drouin. “Inactivation Dates of the Human and Guinea Pig Vitamin C Genes.”Genetica 139, no. 2 (2011): 199–207.
5. Avise, John C. Inside the Human Genome: A Case for Non-Intelligent Design. New York: Oxford University Press, 2010. Romano, C. M., F. L. Melo, M. A. Corsini, E. C. Homes, and P. M. Zanotto. “Demographic Histories of ERV-K in Humans, Chimpanzees and Rhesus Monkeys.” PLoS One 2, no. 10 (2007): e1026.http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001026.
6. Max, “Plagiarized Errors and Molecular Genetics,” http://www.talkorigins.org/faqs/molgen.
7. Coyne, Jerry A. “Intelligent Design: The Faith that Dare Not Peak Its Name.” In Intelligent Thought: Science Versus the Intelligent Design Movement, edited by John Brockman, 3–23. New York: Vintage, 2006.
8. Kricher, John. Galápagos: A Natural History. Princeton, NJ: Princeton University Press, 2006.
9. Grant, Peter R., and Rosemary B. Grant. How and Why Species Multiply: The Radiation of Darwin’s Finches. Princeton, NJ: Princeton University Press, 2011.
10. Sulloway, Frank J. “Why Darwin Rejected Intelligent Design.” In Intelligent Thought: Science Versus the Intelligent Design Movement, edited by John Brockman, 107–25. New York: Vintage, 2006.
11. Darwin, Charles. “On the action of sea-water on the germination of seeds.” Journal of Proceedings of the Linnean Society of London (Botany). 1 (1857): 130–140.
12. Futuyma, Douglas J. Evolution. 3rd ed. Sundbury, MA: Sinauer Associates, 2013.
13. Dawkins, Richard. The Selfish Gene. New York: Oxford University Press, 2006.