Evolution as Fact, Theory, and Path

Hypothesis, theory, fact, law. Prefaced with “hunch” or “guess”, this list of terms would reflect what many people consider a graded series from least to greatest degree of certainty. This ranking may be appropriate in common usage, but actually makes little sense when these words are employed in a scientific context.

The first of these is being observed and confirmed at this very moment all over the planet. The second can be tested at any time (feel free to confirm it using a nonbreakable object of your choosing). The third is far less intuitive, and in fact, required the genius of Galileo to demonstrate as a scientific principle and subsequent researchers with more sophisticated instruments to confirm. The equal rates of acceleration of objects independent of mass can be demonstrated by using objects of similar shape (to cancel out differences in friction with the air—try, say, a baseball and a basketball; Galileo used balls rolling down inclines), and this can be observed using any objects in an artificially created vacuum. However, one of the most dramatic demonstrations came when Dave Scott, an astronaut on the Apollo 15 mission to the airless surface of the moon, dropped a hammer and a feather and observed the result. “How about that,” he noted, “Mr. Galileo was correct”.

Gravity cannot be observed directly, such that its characteristics must be inferred from observations of its effects. These effects, it turns out, can be predicted with extreme accuracy if specific conditions are identified—in other words, there are laws that can describe the behavior of objects under the influence of gravity. First, the force exerted by gravitation can be described as the product of an object’s mass multiplied by its acceleration. This is known as Newton’s Second Law of Motion. Moreover, Newton’s Law of Universal Gravitation specifies that the force of gravity experienced by two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them. In other words, if one specifies the masses of two objects and their distance from one another, one can calculate with great precision the force of gravity to which they will be subject and how they will behave as a consequence. Deviations from the expected orbit of Uranus based on physical laws allowed the existence and location of another massive object to be predicted—the object in question, the planet Neptune, was discovered in 1846 within 1° of its inferred position.

Of course, acknowledging, describing, or even predicting the effects of gravity do not explain how the phenomenon works or why it has the properties that it does. Facts and laws are insufficient for a deeper understanding of gravitation; to achieve this requires a testable, substantiated, comprehensive explanation that is consistent with all known facts about gravity—in other words, a theory. Many theories of gravitation have been proposed, and most of them have failed tests or have been inconsistent with accepted facts and have therefore been rejected. The reigning theory, Einstein’s Theory of General Relativity, explains gravity as the consequence of the warping of space–time, the fabric of the universe, by mass. Einstein’s theory has triumphed to date because it has been able to account for observations that other theories (e.g., Newton’s) could not, such as the characteristics of the orbit of Mercury and the bending of light by mass. In fact, it was a test of the latter, during an eclipse in 1919, that made Einstein an international celebrity.

Relativity, like any other theory in science, continues to be tested. Notably, Kramer et al. (2006) recently highlighted the utility of a double pulsar system in space as “a good candidate for testing Einstein’s theory of general relativity and alternative theories of gravity”. The fact that global positioning satellite (GPS) systems would not work without correcting for the implications of relativity represents an indirect confirmation as well.

Einstein’s theory is not complete, however. For one, it has not been possible thus far to reconcile relativity with observations (rather, inferences) about the nature of the universe at the subatomic scales at which quantum processes operate. It also does not explain why gravity is such a weak force relative to the other known forces (electromagnetism and the strong and weak nuclear forces). Indeed, a trivial application of electromagnetic force is more than sufficient to counteract the force of gravity exerted by the entire planet, as when a paperclip is lifted with a small magnet. Explaining these properties of gravity remains an active area of research in theoretical physics (Randall 2005).

The notion that species may change through time and that living organisms are related to one another through common descent was not original to Charles Darwin. Ideas regarding evolutionary change, as with ideas about gravity, extend back at least to a few ancient Greek thinkers. There had been much discussion of this topic two generations before Darwin based on the writings of Jean-Baptiste de Lamarck, and Darwin’s own grandfather, Erasmus Darwin, was explicit in his view that species could change. Darwin’s major contribution on this issue was not to introduce the idea, but to assemble a massive compendium of data in support of what he called “descent with modification”.

Over the past 150 years, this initial list has been supplemented by countless observations in paleontology, comparative anatomy, developmental biology, molecular biology, and (most recently) comparative genomics, and through direct observations of evolutionary change in both natural and experimental populations. Each of thousands of peer-reviewed articles published every year in scientific journals provides further confirmation (though, as Futuyma (1998) notes, “no biologist today would think of publishing a paper on ‘new evidence for evolution’ ... it simply hasn’t been an issue in scientific circles for more than a century”). Conversely, no reliable observation has ever been found to contradict the general notion of common descent. It should come as no surprise, then, that the scientific community at large has accepted evolutionary descent as a historical reality since Darwin’s time and considers it among the most reliably established and fundamentally important facts in all of science.

Establishing the fact of evolution was only half of Darwin’s objective. He also sought to explain this fact by proposing a mechanism: his Theory of Evolution by Natural Selection. As he stated in 1871, “I had two distinct objects in view; firstly, to show that species had not been separately created, and secondly, that natural selection had been the chief agent of change.”

Natural selection was neither the first nor the last theory proposed to explain the fact of evolution. Lamarck’s theory, in particular, had been based on two key ideas: “use and disuse” and the inheritance of acquired characteristics. In combination, these suggested that the traits acquired through use by organisms during their lifetimes would be passed on to offspring (for example, that the children of individuals who exercise vigorously would be born with greater musculature) and conversely that features that went unused would be lost. (The notion that evolutionary change occurs in response to need in combination with an internal striving toward greater perfection is a common misconception that is often attributed to Lamarck; see Kampourakis and Zogza 2007 for criticism of this practice). Lamarck’s proposed mechanism is not compatible with the modern understanding of genetics and has therefore been abandoned. However, it seems that it and the notion of striving to fulfill needs are more intuitive than Darwinian natural selection,¹ which probably explains why they were proposed first and why so many students and others continue to conceive of evolution in these inaccurate terms (Bishop and Anderson 1990; Demastes et al. 1995; Alters and Nelson 2002). A similar phenomenon has been observed in physics education, in which Newtonian or Einsteinian ideas taught to students must compete with incorrect but evidently more intuitive Aristotelian preconceptions (Halloun and Hestenes 1985a,b).

Although he succeeded in establishing the fact of evolution in short order, Darwin did not live to see natural selection adopted as a central mechanism in evolutionary theory. In fact, by the dawn of the 20th century, natural selection had been nearly eclipsed as a favored mechanism of evolutionary change. Theories involving instantaneous, rather than gradual, changes (mutationism), internal forces creating a sort of unavoidable evolutionary inertia even to the point of extinction (orthogenesis), and renewed appeals to use and disuse (neo-Lamarckism) had moved into the spotlight (Bowler 1992). It was not until the “Modern Synthesis” of the 1930s and 1940s that natural selection returned to the fore when it was shown to be compatible with Mendel’s laws of inheritance² (Mayr and Provine 1980; Bowler 2003). Darwin himself had no knowledge of genetics, making this revised version sufficiently distinct from the original to qualify as “neo-Darwinian theory”.

Modern evolutionary theory represents a multifaceted set of explanations for patterns observed both in contemporary populations and in deep time as revealed by the fossil record. Natural selection is considered by many to be the prime component of evolutionary theory and is the only workable mechanism ever proposed that is capable of accounting for the adaptive features of organisms. At the molecular level, nonadaptive mechanisms are recognized as highly significant, and there is also an increasing emphasis on changes due to processes such as genetic drift that differ from natural selection by being due to chance.³

Because of this complexity, biologists rarely make reference to “the theory of evolution,” referring instead simply to “evolution” (i.e., the fact of descent with modification) or “evolutionary theory” (i.e., the increasingly sophisticated body of explanations for the fact of evolution). That evolution is a theory in the proper scientific sense means that there is both a fact of evolution to be explained and a well-supported mechanistic framework to account for it. To claim that evolution is “just a theory” is to reveal both a profound ignorance of modern biological knowledge and a deep misunderstanding of the basic nature of science.

Some scientific disciplines—geology, archeology, astronomy, and evolutionary biology among them—deal not only with general processes and mechanisms, but also unique historical particulars. In addition to its incarnations as a “fact” and a “theory,” evolution also can be discussed in a third distinct capacity, namely, as a “path” (Ruse 1997). Evolution as path deals with the factual details of life’s history, such as the degree of relatedness of modern species to one another, the timing of splits among lineages, the characteristics of extinct ancestors, and the major events that have occurred over the nearly 4 billion years of life’s saga. As an example, specialists including paleontologists and molecular systematists may investigate whether birds are the descendants of a lineage of dinosaurs (and if so, which one), when flight first evolved and what changes this entailed, and what the patterns of diversification of birds have been since the evolution of flight. Similar questions can be asked about each branch of the tree of life.

As Moran (1993) noted, some details of life’s history are insufficiently established to warrant designation as “facts,” but this can (and probably will) change as more data are brought to bear on particular issues. For example, it is now an accepted fact that dinosaurs were the dominant terrestrial vertebrates for a period of 160 million years and that they disappeared comparatively abruptly about 65 million years ago. It is not yet clear, however, what the implications of this mass extinction event were for the subsequent evolution of the mammals and birds who now fill many of the niches previously occupied by dinosaurs (Bininda-Emonds et al. 2007; Wible et al. 2007).

As with its status as scientific fact and its nature as theory, there are often confusions about the path of evolution among nonspecialists. The erroneous notion that humans descended from chimpanzees or monkeys falls into this category. Chimps and humans are not related as ancestors and descendants, but rather as cousins whose lineages last shared a common ancestor about 6 million years ago. A great deal of change has occurred along both lineages since their split from this common ancestor, and many species have come and gone along both lines of descent.

Is evolution always gradual, or can it follow a more punctuated pattern? Are chance mechanisms such as genetic drift ever as important as the nonrandom process of natural selection? Does natural selection operate only among organisms (or genes) within populations, or can it occur at other levels such as among groups or species? Did mammals diversify as a consequence of the extinction of dinosaurs? Is the primary divide among groups of organisms between those with and those without nuclei, or are there deeper splits? Are wholescale genome duplications common in evolution, and if so, are they associated with major evolutionary changes? Can complex features ever be regained once they have been lost from a lineage? Is a substantial fraction of noncoding DNA functional, or is most of it simply “junk” or “parasitic”? Was Australopithecus afarensis (“Lucy”) a direct ancestor of Homo sapiens or a member of a different hominid lineage? Debate over these questions of theory and path can become quite acrimonious within evolutionary biology, but in no case do they raise doubt about the fact of evolution. As Gould (1981) noted, “facts do not go away when scientists debate rival theories to explain them.”

Unfortunately, conflation of fact and theory in this regard is not limited to opponents of evolution. Some biologists make the inverse mistake of considering clear evidence of common descent as evidence that it occurred by natural selection. Certainly, one can propose that natural selection is responsible for any changes that show evidence of having been adaptive, but change through time (evolution as fact or path) does not, by itself, evince any particular mechanism (evolution as theory). Neither this failure to distinguish between fact or path and theory by scientists, nor that perpetuated by antievolutionists, is compatible with a proper understanding of the scientific definitions of these terms.

It has been noted many times that evolution is both a fact and a theory (Gould 1981; Moran 1993; Futuyma 1998; Lenski 2000). It can also be considered in terms of a historical path (Ruse 1997). The fact of evolution, that organisms alive today are related by descent from common ancestors, is fundamental to an understanding of biology. As Dobzhansky (1973) famously stated, “nothing in biology makes sense except in the light of evolution”. Nevertheless, a great deal remains to be determined regarding the mechanisms that have created (and destroyed) biological diversity since the emergence of life on Earth. Put in another way, modern evolutionary biology rests upon an extraordinarily solid foundation supported by multiple pillars of evidence, while its theoretical framework remains under construction. That the edifice of evolutionary theory is not yet complete is no cause for concern. Indeed, this is what makes evolutionary biology such an exciting and dynamic modern science.