Evolution of Evolutionary Theory
While mountains of evidence prove biological evolution to be a fact, we have different theories to describe how evolution occurs. Many people assume that the theory put forth by Darwin in Origin of Species is the final say on evolutionary theory. Students of biology, however, know that the theory of evolution has evolved over time. For example, Darwin’s original theory evolved when his ideas were merged with ideas from genetics to become the Modern Synthesis. Evolution textbook author, Douglas Futuyma, presents a wonderful slideshow available on Youtube demonstrating how the Modern Synthesis evolved.
Yet what most students don’t realize is that the theory of evolution has evolved way beyond the Modern Synthesis. Failure to recognize this new theory and the many new discoveries in support of it has caused a great deal of confusion among students and non-scientists. This failure is part of the reason that more Americans than ever doubt that evolution even happens (Newport, 2012).
When people understand this new, more integrated theory of evolution, they will see that evolution makes more sense and is thus easier to embrace because of it’s greater power to explain the origins of the great diversity of life on Earth.
Since the Modern Synthesis, scientists have made profound discoveries. They’ve discovered the double helix structure of DNA. They have discovered mobile DNA, transposons, symbiosis, horizontal gene transfer, regulatory elements, gene duplication, and chromosome rearrangements. They have sequenced the human genome and hundreds of other genomes. These new discoveries have changed the theory of evolution yet again. I call this new theory the Integral Model of biological evolution because it integrates Darwin’s ideas with major breakthroughs in biology over the past 50 years.
Here, I give a brief overview of how our understanding of evolution has radically changed over time. I do this by presenting three major theories of evolution: 1) Darwin’s theory 2) the Modern Synthesis and 3) the Integral Model of evolution. The following table summarizes these three theories of evolution.
Table 1. Evolution of Evolutionary Theory
Three theories of evolution have different models for Variation, Inheritance, Selection and Time. Darwin’s theory is represented by the color green. Information that informed the Modern Synthesis is highlighted in blue. Information that informs the “Integral Model” is highlighted in purple.
To make sense of the confusion surrounding the theory of evolution, it helps students to know that the theory of evolution is itself evolving. The table above shows three different models of evolutionary theory. The first is Darwin’s model as described in Origin of Species. The second model (featured in most textbooks and popular web sites) is the Modern Synthesis, a model that combined mid 20th century genetics with Darwinian theory. The most recent model I call the “Integral Model.” I discuss these three theories in more detail below.
Darwin's theory was captured by his sketch of a Tree of Life.
Since before Darwin’s time, people have wondered how life diversified into such a vast array of forms. Most people think Darwin’s greatest contribution was “Natural Selection.” While Natural Selection (co-discovered by Wallace) was indeed a important contribution, Darwin’s greatest and most enduring accomplishment was to create a powerful framework for understanding how species change over time. Darwin suggested that descent with modification (what we today call evolution) involves several “laws.” These include the law of variation, the law of inheritance, and the law of selection (what scientists today call “differential reproductive success”). Darwin also acknowledged that long periods of time were required. Today, scientists use the acronym VIST to describe these necessary ingredients (Variation, Inheritance, Selection and Time) of evolution. Over the years, many details of evolutionary theory have changed, but Darwin’s basic structure for how evolution works remains the same.
Darwin acknowledged that variation, inheritance, selection and time are important for evolution. Yet because Darwin didn’t know about DNA, genomes or genetics, he knew nothing about how variation occurred and he knew nothing about horizontal inheritance. Instead, he focused his attention on what he could observe, which was natural selection. In his famous book On the Origin of Species, Darwin not only outlined the general laws of how species change over time, he also detailed the principles of natural selection.
Before delving into the details of evolutionary theory, it is essential to define our terms. The phrase “Natural Selection” often causes confusion because it has two distinct meanings. For some people, including Darwin, Natural Selection is one element of evolution (the other elements being Variation, Inheritance and Time.) Today, many scientists use the term “differential reproductive success” as a more accurate description that includes natural selection and excludes the processes of variation. For other people, “Natural Selection” is synonymous with the entire theory of evolution and all the processes associated with it, including Variation, Inheritance, Selection, and Time. Because the latter definition includes a reference to itself (natural selection), this circularity tends to confuse. Also, since scientists (including Darwin) have shown that evolution can happen by natural means other than natural selection, (for example via random drift, via endosymbiosis or via hybridization) it is confusing to equate evolution with natural selection. Therefore, when I refer to natural selection, I mean the process that describes one type of differential reproductive success.
This tree of life represents the Modern Synthesis. It does not show horizontal gene transfer.
While Darwin’s theory offered a reasonable explanation for how selection occurred, he had little insight into another essential aspect of evolution, which is variation. In the early 20th century, biologists were beginning to understand how variation occured. Geneticists clearly saw how heritable material contributed to evolution. In 1942, Julian Huxley wrote a book entitled Evolution, The Modern Synthesis. This book summarized and gave a name to the work of hundreds of researchers who had been researching the mechanisms of variation. The Modern Synthesis united the Laws of Selection with the Laws of Variation as they were known at the time.
By coincidence, the mechanisms of variation began to be elucidated in the Nuclear Age. In the 1940s, as nuclear weapons and energy were being explored, scientists exposed flies, molds, bacteria and other organisms to radiation.
They observed that genetic changes happen as a result of exposure to radiation. (Beadle and Tatum, 1941; Muller, 1946.) Later, the growing field of synthetic chemistry led scientists to discover that many chemicals could alter the structure of DNA, and hence introduce variation. Variation by radiation and from certain chemicals produced random changes in the DNA. These changes were called “mutations.”
The term “mutation” originally simply meant “change.” Over time, however, the meaning of “mutation” began to mutate. Instead of simply meaning “change,” mutation also began to mean deleterious change or change due to error or damage. For science geeks, mutation often evokes images of flies with legs sprouting from their heads. In the popular imagination, mutation evokes images of Godzilla, a mythical monster created by radiation from nuclear bombs. (Godzilla was created by Japanese artists after the bombing of Hiroshima and Nagasaki.) According to the Collins Thesaurus of the English Language, the term “mutant” is synonymous with freak, monster, mutation, deviant, oddity, monstrosity, freak of nature.
The Modern Synthesis also added Random Drift as a mechanism for selection. Sometimes, populations undergo changes in gene frequencies, not as a response to the environment, but simply by chance.
Since the Modern Synthesis, people have assumed that variations result from random accidents due to damage or replication errors. In classes on evolution, professors show slides of nuclear bombs and chemical factories to illustrate the causes of variation. For years, people assumed that these random errors were responsible for the variation that gives rise to the tremendous diversity of life. Still others were perplexed as to how errors and damage could cause the tremendous variety of life on Earth. Today, we know that these random errors are only a small part of the story of how useful variations come to be. Thanks to huge breakthroughs in science, we now know that the most important mechanisms for creating variation are not due to random errors or damage. Instead, molecular and cell biology have given us a much more interesting view of how variation is created.
Integral Model of Evolution
Ford Doolittle's Web of Life replaces the Tree of Life by showing horizontal gene transfer. This image illustrates the Integral Model of Evolution.
Since the Modern Synthesis, many new discoveries have shaken the idea that random errors and natural selection alone can explain the magnificent diversity of life on Earth. By not accounting for where useful variation comes from, the Modern Synthesis led some non-scientists to think that science could not explain how life came to be. Paradoxically, while the data was more abundant than ever to explain evolution, the old theory presented as the Modern Synthesis lost it’s explanatory ability. As a result, a growing number of Americans doubt that evolution explains human origins, according to 2012 Gallup Poll (Newport, 2012.)
Click here for more information about the New Integral Theory of Biological Evolution.
For Further Exploration
Ames, Bruce (1979). “Identifying environmental chemicals causing mutations and cancer”. Science 204
Beadle George and Edward Tatum (1941). ”Genetic Control of Biochemical Reactions in Neurospora”. PNAS 27 (11): 499–506.
Doolittle, Ford, W. (February 2000). “Uprooting the Tree of Life”. Scientific American. 282(2): 72–7.
Jacob, François (1977). “Evolution and Tinkering.” Science, New Series, Vol. 196, No. 4295 , pp. 1161-1166
Margulis, Lynn (1970). Origin of Eukaryotic Cells. Yale Univ. Press
Margulis, Lynn (Sagan at the time) (1967). “On the origin of mitosing cells”. J Theor Bio. 14 (3): 255–274.
McGowan, Catherine, Roberta Fulthorpe, Alice Wright and James Tiedje (1998). “Evidence for Interspecies Gene Transfer in the Evolution of 2,4-Dichlorophenoxyacetic Acid Degraders.” Appl Environ Microbiol. 1998 October; 64(10): 4089–4092. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC106609/
Muller, Herman Joseph, (1946) ”For the discovery of the production of mutations by means of X-ray irradiation.” http://www.nobelprize.org/nobel_prizes/medicine/laureates/1946/muller.html?print=1
Newport, Frank (2012). “In U.S., 46% Hold Creationist View of Human Origins: Highly religious Americans most likely to believe in creationism http://www.gallup.com/poll/155003/hold-creationist-view-human-origins.aspx
Olendzenski, Lorraine and J. Peter Gogarten (2009). “Evolution of Genes and Organisms: The Tree/Web of Life in Light of Horizontal Gene Transfer” in Natural Genetic Engineering and Natural Genome Editing: Ann. N.Y. Acad. Sci. 1178: 137–145.
Ohno, Susumu (1970). Evolution by gene duplication. Springer Verlag.
Shapiro, James (2012). Huffington Post Blog, http://www.huffingtonpost.com/james-a-shapiro/
Shapiro, James (2011). Evolution: A View from the 21st Century. FT Press Science.
Woese, Carl (June 2004). “A New Biology for a New Century”. Microbiol. Mol. Biol. Rev. 68 (2): 173–86