Pursuing praxis

HB wrote: "Small gradual changes can accumulate over huge periods of time (an idea Darwin got from Lyell’s Principles of Geography* and from observations he made on his journeys, particularly in South America)"

An interesting side-note on Darwin’s usage of the word "gradual": while in South America, there was an earthquake, and upon surveying a section of rock that had dropped many feet (I can’t recall the exact distance), exposing new strata, Darwin referred to this change in his notebook as "gradual", in the sense that a graduated cylinder is graduated - by steps. What Darwin comfortably folded under the term "gradual" is rather different from what we mean today.

Although Darwin convinced many people of the idea of evolution with his Origin, the mechanism of natural selection, which he labored so long and hard to explicate, did not have an immediate or lasting hold among biologists. By the turn of the century, natural selection was perhaps the least favored mechanism for evolution. Above it was saltation (often favored by those with an embryological bent), orthogenetics (often associated with paleontologists), the neo-Lamarkians, and combinations of these.

(Saltation is the idea that discontinuous change is an important source of novelty and variation. Orthogenetics was based on observations of long-term, apparently directional trends in both the fossil record and living organisms, and concluded that organisms are more or less pushed in a similar direction evolutionarily, and you see more and more exaggeration of distinctive traits until it becomes too much and the critter goes belly-up. The giant Irish elk (which was neither Irish nor an elk, and died out about 10,000 years ago), is frequently cited as an example of this. Neo-Lamarckianism centers on the inheritance of acquired traits - that, for example, if you work out regularly and cultivate giant pecs, your offspring will have larger (though perhaps not immediately as large) pecs as well.)

Thanks to the huge advances in genetics, by the 1930s evolutionary biologists finally had a way to track heritable changes. Combined with a R. A. Fisher’s mathematical prowess, natural selection could be demonstrated, tested, and simulated. Darwin’s mechanism was back. As the evidence and rigor increased, the competing (though not always contradictory) lines of thought and evidence were first set aside, then criticized, then denounced, then stigmatized and banned from the profession. While much dirty bathwater was tossed out, a few babies went with it.

The view that evolution is gradual, in the sense that it is *only* slow, seamless, and practically imperceptible was a view held (more or less willingly) by mid-20th century evolutionary biologists. Interest in step-wise changes,  was derided as distinctly un-Darwinian, unevolutionary, and unscientific, and specifically directed at the saltationists. "Hopeful monsters"[2] was apparently the catch-phrase for discontinuous change, the signal for rolling your eyes as proof that you were part of the in-crowd that knew better.

It wasn’t until the 1970s that the notion of apparently discontinuous change became a viable research question again. In paleontology, the application of a population genetics model of speciation to observations from the fossil record resulted in the now-famous concept of punctuated equilibria. And, even more important in my view, a few years later there was a merging of embryology, classical and molecular genetics, evolutionary biology and paleontology into what is now called evolutionary developmental biology - "evo-devo," though it didn’t really take off until the 1990s. Now we are tracking the origin, control and deployment of changes in general (including step-wise ones) at multiple levels of biological organization - from genes, proteins, cells and gross morphology, to making predictions about ancestors, and testing them with old and new fossil finds.

The linking of the concept of natural selection with smooth, seamless, imperceptible change is not a logical necessity, but simply a question of scale and frequency - within and among lineages - and this is an active area of research.

Every little bit does count in biology and evolution - and sometimes a little bit of change (at the genetic level) turns out to be a whole lot of change (at the developmental level). And a whole lot of change in development, can be one of the "little bits" of accumulated change leading to speciation. Or not. A lot of things don’t play out. There are far better odds in Vegas.

*Corrected: Charles Lyell, Principles of Geology, 1830

 
[1] Cool examples of step-wise changes in development (not evolution): the bithorax mutant in the fruit fly has two thoraxes, with a pair of wings each, instead of the normal single pair of large wings.  In the antennapedia mutant, the fly grows mutant legs where antenna should have been. Or you can grow an extra set of fly eyes instead of antennae, albeit rather smaller than normal. Or you can turn fly mouthparts into legs with the proboscipedia mutation. Or eye tissue (lenses and all) on a fly leg by tweaking the activity of the eyeless gene. [The associated news article on eye evolution is an indication of what evo-devo is about; very, very cool stuff.] I’d dig up non-fly examples, but it’s late.

[2] "Hopeful monster" was a one-time, unfortunate phrase used by Richard Goldschmidt, a displaced German Jew interested in butterflies, development, and evolution. Metamorphosis is a kind of double-development for a single individual, where a larval butterfly becomes a fully functional caterpillar, then at metamorphosis, basically dissolves its first body and builds a second from a collection of cells set aside in the first round of development. A highly reasonable 1940s view of this situation is that an individual houses the ability to produce two radically different bodies. He also knew that if you tinker with development a little bit, you can get big, big change. Should even a minor beneficial variant of this sort arise in nature, it would still be a very significant departure from the norm. And given that even closely related species differ in important ways, he reasoned this might be a way to eventually bridge the gap to speciation. This put him in both the orthogeneticist camp (since development is a highly directional and programmed process) and the saltationist camp (evolution can happen in steps or jumps) at precisely the wrong time, and he and his "hopeful monsters" became the whipping boy of the Modern Synthesis.

PS:  I’m planning a follow-up post on the role of gradualism in Darwin’s formulation of natural selection, and in current understanding of rates and frequencies of processes in evolution, in terms of the law of cause and effect.