Debates on “gradualism” in evolutionary biology address the size distribution of evolutionary changes. The classical Darwinian position, better described as “infinitesimalism”, holds that evolutionary change is smooth in the sense of being composed of an abundance of infinitesimals (not one infinitesimal at a time, but a blending flow of infinitesimals). The alternative is that evolution sometimes involves “saltations” or jumps, i.e., distinctive and discrete steps. The dispute between these two positions has been a subject of acrimony at various times in the 20th century, with several minor skirmishes, and a larger battle with at least one genuine casualty (image).
Walter Frank Rafael Weldon (public domain image from wikipedia). Weldon ignored an advancing illness and worked himself to death (1906) poring over breeding records in an attempt to cast doubt on discrete inheritance. Along with Pearson and other “biometricians”, Weldon held to Darwin’s non-Mendelian view combining gradual hereditary fluctuations with blending inheritance.
Today, over a decade into the 21st century, we have abundant evidence for saltations, yet the term is virtually unknown, and we still seem to invoke selection under the assumption of gradualism. Are we saltationists, or not? I’m going to offer 3 answers below.
But first, we need to review why the issue is important for evolutionary theory.
This is the 5th in a series of 2010 blogs entitled “The Mutationism Myth” (a more scholarly version of this material ended being published in J. Hist. Biol. by Stoltzfus and Cable, 2014)
The Mutationism Myth, part 5. The Restoration
In the Mutationism Myth (see part 1), the Modern Synthesis (MS) rescues evolutionary biology from the Mendelian heresy, by showing that genetics is consistent with selection. In reality, the Mendelians had already synthesized genetics and selection (part 3), but rejected Darwin’s errant views of heredity (part 2) and rejected, to varying degrees, the Darwinian doctrines that subordinated the role of variation so as to render selection the ruling principle in evolution. How, then, did the Modern Synthesis restore Darwinism?
This is the 4th in a series of 2010 blogs entitled “The Mutationism Myth” (a more scholarly version of this material ended being published in J. Hist. Biol. by Stoltzfus and Cable, 2014)
In this oft-told story (see part 1), the discovery of genetics in 1900 leads to rejection of Darwin’s theory and the rise of “mutationism”, a laughable theory that imagines evolution by mutation alone, without selection. “Mutationism” prevails for a generation, until Fisher, Haldane and Wright show that genetics is the missing key to Darwinism. In the conclusion to the story, the world is set right again when the “Modern Synthesis”, combining selection with Mendelian genetics, shoulders aside the mutationist heresy, which ends up in the dustbin of history with the other “doomed rivals” of Darwin’s great theory. As we found out in part 2, the Mendelians rejected Darwin’s errant principles of heredity, not his principle of selection. In part 3, we considered aspects of the Mendelian view that became the foundations of mainstream 20th-century thinking.
But this begs another question: if the Mendelians invented the 20th-century consensus, and just left it to others to work out the math, why aren’t they lauded as “founders” of modern neo-Darwinism, instead of being derided as fools? We’ll find out in part 4 (below), and part 5. (more…)