Saturday, December 31, 2011
Darwin's theory and proof
I wrote this post in response to a discussion in my Philosophy of Research class, and thought I would share it with you.
“The struggle with Darwin's theory is that it makes claims that are not easily falsifiable. Is there a way to test Darwin's assumptions?”
This is actually an outdated perspective promoted by the intelligent design movement. When Darwin originally propounded the theory of evolution, the mechanisms by which natural selection operated were not known, and so his opponents objected that his theory was not falsifiable. In fact, one of the principle objections to his theory is the idea that certain elements of organisms present “irreducible complexity,” that is, they cannot operate at all if you remove even one element from them (Miller, 2008).
However, advances in earth sciences, paleontology, molecular biology, and genetics have refuted this attitude in nearly every case. The fossil record, for example, is filled with examples of intermediate species at every stage of life’s history. “The earliest reptiles look remarkably amphibian-like, and the earliest mammals are actually known informally as the ‘reptile-like mammals.’ They were preceded by a group called the ‘mammal-like reptiles’ lest the point be missed. Within other groups of organisms the pattern is even more evident, especially for animals in the more recent past, for which the fossil record is more complete” (Miller, 2008, p. 47).
The evolution of the horse family is a good example, evolving over the past 55 million years from a group of relatively small animals that browsed on vegetation through 30 million years of diversifying into many species to include modern horses, donkeys, and zebras. The irony is that all of the remaining species of horse fit into a single genus, Equus, and the diversity of the past have died out as various species failed to adapt. Many attempts appear in the record, but only one ended up as adaptive. Multiply this by the many other species of mammals, reptiles, birds, insects, plants, and so forth, and the idea that a designer had any intent involved in the final forms becomes ridiculous (Miller, 2008).
We see evolution in action in the resistance various parasites and diseases evolve to the drugs we use to treat them. For example, Plasmodium, the parasite that causes malaria, has evolved a resistance to chloroquinine, the main drug used to treat the disease. Similarly, some humans who live in malaria-prone areas have evolved a resistance to the parasite (Miller, 2008). (Sickle-cell anemia provides a degree of protection against malaria, giving those with it an adaptive advantage when living in malaria-prone environments. It only becomes a disease when those people move to areas where malaria is no longer a problem.)
Another example of evolution observed in action involved a species of moth in England during the Industrial Revolution. The species originally had white wings with spots, but as the smoke from coal fires darkened the wood of trees where the moths lived and other surfaces they lit upon, being white was no longer adaptive. They were gradually replaced by a dark-winged variant that blended more effectively with the background. This variant remained the dominant form of the moth until efforts to clean up the environment got underway in the 1960s and 70s, at which point the moth reverted to its white-winged form, which was once more adaptive.
Molecular genetics have begun solving problems such as the origins of clotting factors in the blood, and the flagellum on a number of different kinds of bacteria, identifying precursors of the components of the systems in older species. Most proteins in cells are used for more than one purpose, so this really shouldn’t be surprising. Laboratory experiments are showing more and more how the genetic code can reshuffle itself and allow new forms to arise—the ability of antibodies to adapt to new diseases is one example of this (Miller, 2008).
The bottom line is that we have many ways of testing Darwin’s assumptions. The fossil record is only one. Molecular genetics, analysis of proteins, the study of related animals and plants, and the co-evolution of predators and prey are just some of the many examples. Evolution is no longer merely a theory that has no structure to permit testing; it is the basis of the explosion of methods in modern biology. As scientists, it is important that we not continue to promote notions that are outdated and inaccurate, but to challenge those perspectives with more recent data and evidence (Miller, 2008).
Miller, K. R. (2008). Only a theory: Evolution and the battle for America’s soul. New York, NY: Penguin Books.