Monthly Archives: April 2009

Mother Nature as a Sculptor

Aha! I had a couple of essays saved from a few weeks back……

Most arguments about the origin of life are sucked into the domain of the Traditional Template. In other words, we are once again treated to the old arguments whereby people dispute whether or not it was possible for geochemical processes to spawn biological processes. But I think it is more interesting to approach this topic while looking for clues – facts about the world we might expect to find if a given hypothesis is true.

The Seeding Story and Spawning Story have a different story to tell. One begins with a consortium of sophisticated, complex cells while the other begins with a simple self-replicating molecule able to co-opt from a huge assortment of potentially useful chemicals in the thick prebiotic broth. Does it really make sense to think such two radically different starting points cannot leave any traces that would help us distinguish between the two?

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Tiarina

I should be getting some precious time back toward the end of this week.  In the meantime…

Scenedesmus

Two Types of Selection

Let’s consider another excerpt from Eugene Koonin’s recent paper, “Darwinian evolution in the light of genomics” (Nucleic Acids Research, 2009, 1–24). I’m going to soon highlight some very significant, yet subtle points, that would seem to follow from this excerpt that outlines the difference between two types of selection. So check it out (emphasis is added):

According to the neutral theory, a substantial majority of the mutations that are fixed in the course of evolution are selectively neutral so that fixation occurs via random drift. A corollary of this theory is that gene sequences evolve in an approximately clock-like manner (in support of the original molecular clock hypothesis of Zuckerkandl and Pauling) whereas episodic beneficial mutations subject to natural selection are sufficiently rare to be safely disregarded for a quantitative description of the evolutionary process. Of course, the neutral theory should not be taken to mean that selection is unimportant for evolution. What the theory actually maintains is that the dominant mode of selection is not the Darwinian positive selection of adaptive mutations, but stabilizing, or purifying selection that eliminates deleterious mutations while allowing fixation of neutral mutations by drift (17).

Subsequent studies refined the theory and made it more realistic in that, to be fixed, a mutation needs not to be literally neutral but only needs to exert a deleterious effect that is small enough to escape efficient elimination by purifying selection—the modern ‘nearly neutral’ theory (38). Which mutations are ‘seen’ by purifying selection as deleterious critically depends on the effective populations’ size: in small populations, drift can fix even mutations with a significant deleterious effect (16).

[…]

Importantly, in the later elaborations of the neutral theory, Kimura and others realized that mutations that were (nearly) neutral at the time of fixation were not indifferent to evolution. On the contrary, such mutations comprised the pool of variation that can be tapped into by natural selection under changed conditions, a phenomenon that could be potentially important for macroevolution (17,41).

Tetrahymena

A dividing Tetrahymena cell.

A dividing Tetrahymena cell.

Choanflagellate

Strict Gradualism

Earlier, I quoted from Eugene Koonin’s recent paper, “Darwinian evolution in the light of genomics” (Nucleic Acids Research, 2009, 1–24), as he outlines two of the excerpts from the principal concepts of the Modern Synthesis:

Evolution proceeds by fixation of the rare beneficial variations and elimination of deleterious variations: this is the process of natural selection that, along with random variation, is the principal driving force of evolution according to Darwin and the Modern Synthesis. Natural selection which is, obviously, akin to and inspired by the ‘invisible hand’ (of the market) that ruled economy according to Adam Smith, was the first mechanism of evolution ever proposed that was simple, plausible, and did not require any mysterious innate trends. As such, this was Darwin’s second key insight. The founders of population genetics, in particular, Sewall Wright, emphasized that chance could play a substantial role in the fixation of changes during evolution not only in their emergence, via the phenomenon of genetic drift that entails random fixation of neutral or even deleterious changes. Population-genetic theory indicates that drift is particularly important in small populations that go through bottlenecks (6,16). However, the Modern Synthesis, in its ‘hardened’ form (13), effectively, rejected drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution.

And

The beneficial changes that are fixed by natural selection are ‘infinitesimally’ small, so that evolution proceeds via the gradual accumulation of these tiny modifications. Darwin insisted on strict gradualism as an essential staple of his theory: ‘Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being . . . If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.’ [(1), chapter 6]. Even some contemporaries of Darwin believed that was an unnecessary stricture on the theory. In particular, the early objections of Thomas Huxley are well known: even before the publication of the Origin Huxley wrote to Darwin ‘‘You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly’”

I then asked two questions.

Why did Darwin insist on “strict gradualism as an essential staple of his theory”?

Why did most proponents of the Modern synthesis “reject drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution”?

Let me take a stab at these questions.

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