The arc of evolutionary genetics may be irreversible

One of the banes of modern life is the stack of papers in one's "to-read" list. I guess that goes to show how cushy modern life is, as what sort of complaint is that? In any case, I began to consider this after reading Joe Thornton's magisterial response to Michael Behe's giddy excitement over his most recent paper, An epistatic ratchet constrains the direction of glucocorticoid receptor evolution. Thornton dispatches Behe's muddled misconceptions with economy and precision, but after reading the paper, as opposed to cogent summaries such as Carl Zimmer's in The New York Times I'm even more at a loss as to how Behe arrived at the conclusions he did as to the paper's significance (please read the paper, available on Thornton's lab website, and then try and make sense of what Behe is asserting) . But at least Michael Behe prompted me to push the paper to the top of my stack. To understand why this paper is important, one has to know a bit of the history of evolutionary theory (ironic in that this paper points to the importance of contingency, that is, historical sequence of events, in evolution). The arguments about whether evolutionary process is contingent or not go back a century. One can see glimmers of it in the debates between R. A. Fisher and Sewall Wright, the two preeminent figures of 20th century population genetics (Fisher arguably invented the field). Fisher and Wright are relevant to the discussion around this paper because of their views about the genetic architecture of evolutionary process. The former tended to elaborate a simple perspective whereby the focus was on genetic variants of independent and additive effect against an averaged genetic background. As an example, presumably the thousands of genes which effect normal variation in height, and are selected en mass when a shift in height away from the mean value results in higher reproductive fitness. In a Fisherian world the adaptive landscape is smooth and without discontinuity, with a simple symmetrical peak around which the species' total population flows. The world may not be flat, but its geometry is elegant. In contrast, Sewall Wright conceived of the adaptive landscape as far more rugged, subject to the complex interlocking effects of epistasis as well as stochastic random walks, and fragmented into numerous populations with some barriers to gene flow. Wright's ideas were not entirely coherent, as his biographer Will Provine lays out in Sewall Wright and Evolutionary Biology, but the metaphors and generally more empirical methodology left an imprint (while Fisher's contributions tended to be theoretical, Wright was an experimental geneticist with a more intuitive feel for the shape of reality). Though Stephen Jay Gould gave a hearty nod to Sewall Wright as a primary influence (though he also argued that Wright watered-down his own theories to conform to Neo-Darwinian orthodoxy), Gould's nemesis William D. Hamilton declared that Wright was the strongest influence upon his later work in his collection of papers. The combination of Wright's extremely long career (he was 1 year older than Fisher, but outlasted him by 26 years) and his occasional tendency toward lack of clarity may go some way to explaining how and why such disparate intellectuals could look to him as an influence. The tension between Fisher's elegant formalism which led one toward an ahistorical sensibility, and Wright's more sloppy empirically informed conjectures, has to a great extent echoed down the decades. In the 1960s Ernst Mayr attacked "beanbag genetics," in other words, Fisher's genetics. Fisher's British colleague, J. B. S. Haldane, rose to the challenge (Fisher had died by this point), publishing A Defense of Beanbag Genetics. In more recent years Stephen Jay Gould laid out his vision of historical contingency in Structure of Evolutionary Theory. As the inevitable counterpoint Richard Dawkins took Simon Conway Morris' side in The Ancestor's Tale, arguing for the inevitability of particular morphological types due to the deterministic character of natural selection. And so it goes. Thornton's paper does not resolve this general argument, but it does illustrate starkly the constraints which natural selection faces in a universe of finite time: