Editor's Note: This post was first published as a two-part series on April 15-16, 2013.
Is the contemporary theory of evolution an example of good science? The answer to this question depends on how you define “science,” and what you think makes science “good.”
Good science has an addiction to theories, and for science to be good science, it must deal with good scientific theories. What constitutes a good scientific theory? That is a very involved question, but a user’s view of good scientific theories looks something like this:
- A scientific theory is not a guess or suspicion. For example, “I have a theory about who shot President Kennedy,” reflects the colloquial meaning of the word “theory,” and not the meaning conveyed by scientists when they use the word “theory.”
- Scientific theories are convincing explanatory frameworks that efficiently integrate a large body of evidence about the world. Good scientific theories have the capacity to make sense of a wide range of data that made less sense before the introduction of the theory.
- In order to be called a scientific theory, it must have been successfully tested and retested many times.
- A scientific theory must be falsifiable in order to be truly scientific. The theory has to live constantly at risk from new data.
- A theory must have predictive power.Good theories allow scientists to make predictions based on the theory that, when tested, turn out to be at least roughly correct.
These are not the only characteristics of a scientific theory, but they probably represent the most important features for practitioners of science.
If we hold contemporary evolutionary theory to these standards, how well does it do? Since the inception of evolutionary theory by Charles Darwin in 1859 with the publication of On the Origin of Species, there are four characteristics of evolutionary theory that have endured 150 years of further research:
- Living species are descendants of other species that lived in the past.
- These past species lived in populations that underwent gradual transformation so that the individuals in these populations changed their appearance, behaviors, metabolisms, and life histories over long spans of time.
- New forms of life arose by means of a process called speciation in which one lineage splits into two distinct lineages. This continual splitting of organismal lineages leads to a nested genealogy of species. This nested genealogy forms a veritable tree of life, whose root represents the first species to arise and whose twigs represent the millions of species living today. If you trace back any pair of twigs from the modern species you will find that their histories merge at some node on the tree where the two species share a common ancestor.
- This process of biological change that takes place throughout the advance of geologic time, or evolution, occurs by means of variation in organisms (which we know today is due to genetic mutations) that is acted on by either random genetic drift or natural selection. Those individuals with variations better suited to the current environment leave more offspring, thus changing the average appearance of the population over time and making it a better fit to the environment. This improving fit between organisms and their environment gives the appearance of organisms having been well designed for their milieu.
What is the evidence for these aspects of evolutionary theory? The evidence is actually immense, but I will restrict this discussion to just a few items.
First there is the fossil record. If life results from a natural process such as biological evolution, then we should observe a progression of fossil organisms that proceed from relatively simple, single-celled organisms in the oldest rocks to more complex, multicellular organisms in younger rocks. When paleontologists examine the geologic column, they perceive that some of the oldest and deepest layers of the geologic column contain fossils of microorganisms, and then marine invertebrates in younger layers above those, and then much later and higher up in the geologic column fish appear, followed later and higher still by amphibians, and then by reptiles, mammals, and birds. Thus, the general presentation of the fossil record in the rock record comports exactly with what the theory of evolution predicts.
However, the fossil story gets even better, because scientists can trace evolutionary trends throughout the fossil record. For example, horses get bigger, fuse their leg bones and toes into a single bone with a thick hoof and grow the thickness of their tooth enamel; Cenozoic brachiopod shells get narrower, decrease their rib numbers and beak angle; diatoms get bigger; and primate fossils reduce the size of their teeth and expand the size of their brains.
Additionally, Darwin predicted that there should be organisms preserved in the fossil record that possess features found in two different types of creatures. Such organisms are “transitional forms” that bridge the gap between different types of organisms. However, the fossil record of Darwin’s time provided little evidence of such transitional forms. Therefore, Darwin gambled that future paleontological research would provide sufficient evidence to corroborate his theory. How did this gamble turn out? Since Darwin’s time, paleontologists have discovered transitional fossils that are part fish and tetrapod, part amphibian and part reptile, part dinosaur and part bird, and part reptile and part mammal. Once again, we would predict such paleontological trends and the existence of such transitional fossils if life came about through a process of organic evolution. Clearly paleontological research since Darwin’s time has powerfully vindicated his theory.
The second piece of evidence for evolution 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. 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. The same can be said for the hind limbs of snakes, which form limb buds during embryonic development, but die off later. 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. 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. 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.
A third piece of evidence for evolution comes from biogeography. 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. 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. 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. 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 seawater for long periods of time, and again, Darwin was shown to be right. 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. 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.”
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.
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.