The Theory of Biological Evolution is Evolving

 For this post, I am going to share a fascinating reflection by Dr. William Lane Craig about the evolution of the Theory of Biological Evolution. I found it to be insightful.

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I think one of the most important take-aways for laymen from my study of this subject is the realization that the theory of biological evolution has itself evolved. If we leave aside the theories of Charles Darwin’s predecessors, there are three major stages in evolutionary theory:

Stage I: Darwinism. This was Darwin’s original theory of evolution laid out in his book On the Origin of Species (1859). Darwin’s theory of evolution comprised two fundamental theses: (i) descent with modification of all living organisms from one or a few common ancestors, and (ii) natural selection as the explanatory mechanism for evolutionary change. Darwin’s theory was dead almost upon arrival. While his thesis of common ancestry quickly won the day, for seventy years following the publication of Origin of Species Darwin’s second thesis was widely regarded as explanatorily deficient. Ignorant of Mendel’s genetics, Darwin could provide no account of the sources of the variability of hereditary traits nor how such traits were inherited. It has been justifiably quipped that Darwin’s theory explained the survival of the fittest, but not the arrival of the fittest.

Stage II: The Modern Synthesis. Formulated during the 1930s and 40s, the Modern Synthesis represented the marriage of Darwin’s natural selection and Mendel’s genetics. Its chief contribution was the thesis that hereditary variability arises by random genetic mutations, which, when acted upon by natural selection, can be the source of new and advantageous traits over time. It thereby supplemented Darwin’s theory with a genetic explanation of the source of heritable variations. On this theory new species originated by rather small steps that accumulated over many generations. This theory, sometimes called “Neo-Darwinism,” rapidly became orthodoxy among evolutionary biologists and prevailed almost till the close of the twentieth century.

Stage III: The Extended Evolutionary Synthesis. Proponents of this theory indict the Modern Synthesis for its myopic focus on genetic inheritance as the source of evolutionary change. They contend that new data from adjacent fields such as developmental biology, genomics, epigenetics, ecology, and social science now demand a wider theory. The following diagram illustrates the relation between the Extended Evolutionary Synthesis, the Modern Synthesis, and Darwin’s theory:

The fact is that the Modern Synthesis did not really offer much by way of explanation of the causes of how organisms change over time. The Modern Synthesis postulated correlations between an organism’s genotype (its genetic makeup) and its phenotype (its observable traits) but not causal mechanisms connecting them. As a result, the Modern Synthesis treated all mechanistic aspects of evolutionary change as a “black-box” and so was unable to explain how organismal change is actualized.

According to Pigliucci and Müller, the Extended Evolutionary Synthesis thus prompts several reforms to the Modern Synthesis:

First is gradualism. Because the Modern Synthesis assumed that evolutionary change proceeds via incremental genetic variation, all non-gradualist forms of evolutionary change were excluded. But various new approaches show that non-gradual change is a property of evolutionary processes. Second is externalism. Under the Modern Synthesis the direction of the evolutionary process results exclusively from natural selection. In the Extended Evolutionary Synthesis, organisms themselves are determinants of selectable variation and innovation. Thus, in sharp contrast to claims of the Modern Synthesis, mutations may not be random but actually biased toward the benefit of the host organism in which they occur. Third is “gene-centrism.” The Modern Synthesis’ focus on the gene as the sole agent of variation and unit of inheritance suppressed all calls for more comprehensive attitudes. In the Extended Evolutionary Synthesis, extra-genetic (epigenetic) influences on developing embryos is increasingly emphasized, in contrast to genetic mutations.

J. B. S. Haldane Creationists and proponents of Intelligent Design have long complained about the explanatory deficits of the Modern Synthesis but were uniformly ignored, probably because they were able only to poke holes in the theory without offering a credible alternative. J. B. S. Haldane once remarked that “Theories pass through four stages of acceptance: (i) this is worthless nonsense; (ii) this is an interesting, but perverse, point of view; (iii) this is true, but quite unimportant; (iv) I have always said so.” Today contemporary textbooks already incorporate many of the new insights of the Extended Evolutionary Synthesis without noting the explanatory deficiencies of the Modern Synthesis thereby exposed.

It should not be thought that with the Extended Evolutionary Synthesis, the evolution of the theory of evolution has come to an end, and we can breathe a sigh of relief that all is well. No, while the Extended Evolutionary Synthesis served to expose explanatory weaknesses in the heretofore prevailing evolutionary paradigm and so to open new avenues of research, many of the ideas of the Extended Evolutionary Synthesis remain unproven, poorly understood, and controversial, so that the quest for a final theory must continue. The evolutionary biologist Eugene Koonin observes that what follows in a “post-Modern” era is not a post-Modern Synthesis but a post-Modern state “characterized by a pluralism of processes and patterns in evolution that defies any straightforward generalization.” He opines that whether the directions currently being pursued in post-Modern research “can be combined in a new evolutionary synthesis in the foreseeable future, is too early to tell. I will venture one confident prediction, though: those celebrating the 200th anniversary of the Origin will see a vastly different landscape of evolutionary biology.”