|"When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first bed of the Cambrian system was deposited, they seem to me to become ennobled."|
|- Charles Darwin|
The Origin of Species, p. 647
volution, the overarching concept which 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 challenged, however, is macroevolution. Macroevolution, as used here, is the theory of common descent with gradual modification.
Common descent is a general descriptive theory that proposes to explain the origins of living organisms. Because it is so well supported scientifically, macroevolution is often called the "fact of evolution" by evolutionary biologists. The theory specifically postulates that all of the earth's biota are genealogically related, much in the same way that siblings or cousins are related to one another. Thus, macroevolutionary processes necessarily entail the transformation of one species into another and, consequently, the origin of higher taxa. For these reasons, proponents of special creation are especially hostile to macroevolutionary aspects of evolutionary science.
This article directly addresses the scientific evidences in favor of macroevolutionary theory and common descent. It 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, or cannot be falsified.
In this essay, macroevolution alone is specifically considered and weighed against the scientific evidence; in general, separate "microevolutionary" theories are left unaddressed. Microevolutionary theories, which biologists use as mechanistic theories to explain macroevolution, include such concepts as natural selection, genetic drift, sexual selection, neutral evolution, theories of speciation, etc. Whether microevolutionary theories are sufficient to account for macroevolutionary adaptations is a question that is left open. However, the fundamentals of genetics, developmental biology, molecular biology, biochemistry, and geology are assumed to be correct in general - especially those that do not directly purport to explain adaptation.
Neither is abiogenesis considered in this discussion of macroevolution and common descent. Abiogenesis is not part of evolutionary theory; it 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.
As stated earlier, for the purposes of this article macroevolution and universal common descent are treated as virtual synonyms. Common descent is the hypothesis that all living organisms are the lineal descendants of one original living species. All the diversity of life, both past and present, was originated by normal reproductive processes observable today. Thus, all extant species are related in a strict genealogical sense. More specifically, macroevolution is proposed to occur on a geological timescale and in a gradual manner. "Gradualness" has little to do with the rate or tempo of evolution; it is a mode of change that is dependent on population phenomena. Gradualness concerns genetically probable organismic changes between two consecutive generations, i.e. those changes that are within the range of normal variation observed within modern populations. Morphological change may appear fast geologically speaking, yet still be gradual (Darwin 1872, pp. 312-317; Dawkins 1996, p.241; Mayr 1991, pp. 42-47; Rhodes 1983). Note that though "gradualness" is not formally a mechanism of evolutionary change, it does indeed significantly constrain possible macroevolutionary events, and likewise the requirement of gradualness necessarily restricts the possible mechanisms of common descent and adaptation (which are not explicitly considered here).
In the following list of evidences, 29 major predictions of the hypothesis of common descent are enumerated and discussed. Since one fundamental concept generates all 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. Under each point is a demonstration of how the prediction fairs against actual biological observations, each giving a few examples of evolutionary explanations and confirmations followed by potential falsifications. There are many in-text references given for each point. For those interested, here is a summary of the scientific method and scientific philosophy, including what is meant by "scientific evidence" and "falsification."
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 hypothesis of 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 hypothesis fairs in light of the observable evidence. In every example, it is quite possible that the predictions could fail to match the empirical evidence. In fact, without assuming the truth of universal common descent, it is highly probable that the hypothesis will indeed fail for most of these predictions - and this is exactly why many of these predictions are such strong evidence for common descent. The few examples given for each prediction are meant to be representative of 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.
Darwin, C. (1872). The Origin of Species. Sixth Edition. The Modern Library, New York.
Dawkins, R. (1996). The Blind Watchmaker. New York, Norton.
Mayr, E. (1991). One Long Argument. Cambridge, Harvard University Press.
Rhodes, F. H. T. (1983). "Gradualism, punctuated equilibria, and the origin of species." Nature 305: 269-272.
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