volution, the overarching concept that unifies the biological sciences, in fact embraces a plurality of theories and hypotheses. In evolutionary debates one is apt to hear evolution roughly parceled between the terms “microevolution” and “macroevolution”. Microevolution, or change beneath the species level, may be thought of as relatively small scale change in the functional and genetic constituencies of populations of organisms. That this occurs and has been observed is generally undisputed by critics of evolution. What is vigorously challenged, however, is macroevolution. Macroevolution is evolution on the “grand scale” resulting in the origin of higher taxa. In evolutionary theory, macroevolution involves common ancestry, descent with modification, speciation, the genealogical relatedness of all life, transformation of species, and large scale functional and structural changes of populations through time, all at or above the species level (Freeman and Herron 2004; Futuyma 1998; Ridley 1993).
Universal common descent is a general descriptive theory concerning the genetic origins of living organisms (though not the ultimate origin of life). The theory specifically postulates that all of the earth’s known biota are genealogically related, much in the same way that siblings or cousins are related to one another. Thus, universal common ancestry entails the transformation of one species into another and, consequently, macroevolutionary history and processes involving the origin of higher taxa. Because it is so well supported scientifically, common descent is often called the “fact of evolution” by biologists. For these reasons, proponents of special creation are especially hostile to the macroevolutionary foundation of the biological sciences.
This article directly addresses the scientific evidence in favor of common descent and macroevolution. This article is specifically intended for those who are scientifically minded but, for one reason or another, have come to believe that macroevolutionary theory explains little, makes few or no testable predictions, is unfalsifiable, or has not been scientifically demonstrated.
What is Universal Common Descent?
niversal common descent is the hypothesis that all known living, terrestrial organisms are genealogically related. All existing species originated gradually by biological, reproductive processes on a geological timescale. Modern organisms are the genetic descendants of one ancient, original species (broadly defined as a communal population of organisms exchanging genetic material). Genetical “gradualness”, a much misunderstood term, is a mode of biological change that is dependent on population phenomena; it is not a statement about the rate or tempo of evolution. Truly genetically gradual events are changes within the range of biological variation expected between two consecutive generations. Morphological change may appear fast, geologically speaking, yet still be genetically gradual (Darwin 1872, pp. 312-317; Dawkins 1996, p.241; Gould 2002, pp. 150-152; Mayr 1991, pp. 42-47; Rhodes 1983). Though gradualness is not a mechanism of evolutionary change, it imposes severe constraints on possible macroevolutionary events. Likewise, the requirement of gradualness necessarily restricts the possible mechanisms of common descent and adaptation, briefly discussed below.
Common Descent Can Be Tested Independently of Mechanistic Theories
In this essay, universal common descent alone is specifically considered and weighed against the scientific evidence. In general, separate “microevolutionary” theories are left unaddressed. Microevolutionary theories are gradualistic explanatory mechanisms that biologists use to account for the origin and evolution of macroevolutionary adaptations and variation. These mechanisms include such concepts as natural selection, genetic drift, sexual selection, neutral evolution, and theories of speciation. The fundamentals of genetics, developmental biology, molecular biology, biochemistry, and geology are assumed to be fundamentally correct—especially those that do not directly purport to explain adaptation. However, whether microevolutionary theories are sufficient to account for macroevolutionary adaptations is a question that is left open.
Therefore, the evidence for common descent discussed here is independent of specific gradualistic explanatory mechanisms. None of the dozens of predictions directly address how macroevolution has occurred, how fins were able to develop into limbs, how the leopard got its spots, or how the vertebrate eye evolved. None of the evidence recounted here assumes that natural selection is valid. None of the evidence assumes that natural selection is sufficient for generating adaptations or the differences between species and other taxa. Because of this evidentiary independence, the validity of the macroevolutionary conclusion does not depend on whether natural selection, or the inheritance of acquired characaters, or a force vitale, or something else is the true mechanism of adaptive evolutionary change. The scientific case for common descent stands, regardless.
Furthermore, because it is not part of evolutionary theory, abiogenesis also is not considered in this discussion of macroevolution: abiogenesis is an independent hypothesis. In evolutionary theory it is taken as axiomatic that an original self-replicating life form existed in the distant past, regardless of its origin. All scientific theories have their respective, specific explanatory domains; no scientific theory proposes to explain everything. Quantum mechanics does not explain the ultimate origin of particles and energy, even though nothing in that theory could work without particles and energy. Neither Newton’s theory of universal gravitation nor the general theory of relativity attempt to explain the origin of matter or gravity, even though both theories would be meaningless without the a priori existence of gravity and matter. Similarly, universal common descent is restricted to the biological patterns found in the Earth’s biota; it does not attempt to explain the ultimate origin of life.
What is Meant by “Scientific Evidence” for Common Descent?
Scientific theories are validated by empirical testing against physical observations. Theories are not judged simply by their logical compatibility with the available data. Independent empirical testability is the hallmark of science—in science, an explanation must not only be compatible with the observed data, it must also be testable. By “testable” we mean that the hypothesis makes predictions about what observable evidence would be consistent and what would be incompatible with the hypothesis. Simple compatibility, in itself, is insufficient as scientific evidence, because all physical observations are consistent with an infinite number of unscientific conjectures. Furthermore, a scientific explanation must make risky predictions— the predictions should be necessary if the theory is correct, and few other theories should make the same necessary predictions.
As a clear example of an untestable, unscientific, hypothesis that is perfectly consistent with empirical observations, consider solipsism. The so-called hypothesis of solipsism holds that all of reality is the product of your mind. What experiments could be performed, what observations could be made, that could demonstrate that solipsism is wrong? Even though it is logically consistent with the data, solipsism cannot be tested by independent researchers. Any and all evidence is consistent with solipsism. Solipsism is unscientific precisely because no possible evidence could stand in contradiction to its predictions. For those interested, a brief explication of the scientific method and scientific philosophy has been included, such as what is meant by “scientific evidence”, “falsification”, and “testability”.
In the following list of evidences, 30 major predictions of the hypothesis of common descent are enumerated and discussed. Under each point is a demonstration of how the prediction fares against actual biological testing. Each point lists a few examples of evolutionary confirmations followed by potential falsifications. Since one fundamental concept generates all of these predictions, most of them are interrelated. So that the logic will be easy to follow, related predictions are grouped into five separate subdivisions. Each subdivision has a paragraph or two introducing the main idea that unites the various predictions in that section. There are many in-text references given for each point. As will be seen, universal common descent makes many specific predictions about what should and what should not be observed in the biological world, and it has fared very well against empirically-obtained observations from the past 150+ years of intense scientific investigation.
It must be stressed that this approach to demonstrating the scientific support for macroevolution is not a circular argument: the truth of macroevolution is not assumed a priori in this discussion. Simply put, the theory of universal common descent, combined with modern biological knowledge, is used to deduce predictions. These predictions are then compared to the real world in order see how the theory fares in light of the observable evidence. In every example, it is quite possible that the predictions could be contradicted by the empirical evidence. In fact, if universal common descent were not accurrate, it is highly probable that these predictions would fail. These empirically validated predictions present such strong evidence for common descent for precisely this reason. The few examples given for each prediction are meant to represent general trends. By no means do I purport to state all predictions or potential falsifications; there are many more out there for the inquiring soul to uncover.
Are There Other Scientifically Valid Explanations?
The worldwide scientific research community from over the past 150 years has discovered that no known hypothesis other than universal common descent can account scientifically for the unity, diversity, and patterns of terrestrial life. This hypothesis has been verified and corroborated so extensively that it is currently accepted as fact by the overwhelming majority of professional researchers in the biological and geological sciences (AAAS 1990; AAAS 2006; GSA 2009; NAS 2005; NCSE 2012; Working Group 2001). No alternate explanations compete scientifically with common descent, primarily for four main reasons: (1) so many of the predictions of common descent have been confirmed from independent areas of science, (2) no significant contradictory evidence has yet been found, (3) competing possibilities have been contradicted by enormous amounts of scientific data, and (4) many other explanations are untestable, though they may be trivially consistent with biological data.
When evaluating the scientific evidence provided in the following pages, please consider alternate explanations. Most importantly, for each piece of evidence, critically consider what potential observations, if found, would be incompatible with a given alternate explanation. If none exist, that alternate explanation is not scientific. As explained above, a hypothesis that is simply compatible with certain empirical observations cannot use those observations as supporting scientific evidence.
How to Cite This Document
Many people have asked how to cite this work in formal research papers and academic articles. This work is an online publication, published by the TalkOrigins Archive. There are standard academic procedures for citing online publications. For example, if you last accessed this page on March 12, 2012, and used version 2.89, here is a reference in formal MLA style:
Theobald, Douglas L. “29+ Evidences for Macroevolution: The Scientific Case for Common Descent.” The Talk.Origins Archive. Vers. 2.89. 2012. Web. 12 Mar. 2012 <http://www.talkorigins.org/faqs/comdesc/>
For more information about citing online sources, see the formal style guidelines given in the book Research and Documentation in the Information Age: Online.