Gradualism, Contingency, and Punctuated Equilibria

Since Ed is in Vegas again, I offered to put up some guest posts to help alleviate the terrible burden being placed on him.  And today being Darwin Day, I thought I’d put up a post on evolution.

As a consequence of lacking data supporting their own explanations, creationists, like other denialists, have to rely on attacking perceived weaknesses in the mainstream theories.  A popular target arises whenever disagreements between scientists over aspects of the theory crop up.  These disagreements are then exaggerated so as to makeit seem as if the whole edifice is crumbling, when in reality it is just a minor difference in opinions, both of which can be explained by the theory.

In evolution, one such disagreement is between gradualism and punctuated equilibria.  Broadly speaking, gradualism is the idea that changes accumulate over time, while punctuated equilibria is the idea that there are short periods of lots of change interspersed among long periods of little change.  Since these two ideas appear to be contradictory, creationists love to use the debate over the two concepts as evidence that evolution is wrong.  However, the two concepts are not only not contradictory, under the right circumstances gradualism results in punctuated equilibria.

How can this be?  Here’s a mnemonic: gradualism is genotypic, punctuated equilibria is phenoytpic.  Mutations change the genotype.  Kimura’s neutral theory showed that changes to the genotype do not always equate to changes to the phenotype.  Therefore, gradually accumulating mutations can lead to punctuated equilibria if the phenotypic changes are rare and tend to clump.  Investigations into neutral theory show that most mutations are neutral or near-neutral.  Furthermore, many phenotypic changes don’t preserve well, which can make phenotypic changes appear to be more rare than they actually are in the fossil record.  So the first requirement, rarity, is well-established.  But how on Earth do the phenotypic changes clump?

The answer (or at least part of it) is contingency.  Contingency is the idea that some events can’t take place unless another event has taken place.  That some things have pre-requisites.  Several recent papers have confirmed that contingency plays a vital role in evolution, and is a major factor in the history of life.

Here’s how contingency works.  Although the physical features of the environment may appear to be fixed, they do change over time.  So what happens when a feature changes?  First, some organisms that have mutations that were previously neutral find that these mutations are now advantageous (or deleterious).  Phenotypic changes can therefore immediately arise from contingent events without requiring simultaneous genotypic change.  Second, new mutations that would have previously been deleterious may now be neutral or advantageous.  Third, as these phenotypic changes propagate, other organisms can take advantage of these new traits, creating a chain reaction.  In other words, a major change in the environment can spur a spate of new phenotypic change.

But punctuated equilibria also requires long periods of stasis, times of little or no phenotypic change.  Dr. Lenski of Michigan State University has been studying a population of E. coli for the past 20 years.  He has discovered that the rate at which phenotypic changes accumulate slows down as an organism adapts to its environment.  This makes a lot of sense.  First of all, the previously neutral mutations are already present, causing an initial spike.  This is then followed by a series of advantageous changes that are easy to generate.  However, as time passes, it becomes harder and harder for a change to be more advantageous.  Luckily, not all mutations are equally likely to appear, so as time passes, the rarer adaptations begin to accumulate, but significantly slower than the easy changes.  Plotted out, the pattern of changes looks like a thick spike with a quick decline – very much like what is described by punctuated equilibria.

Note that I talked about organisms.  This implies that evolution works at the species level.  However, Gould claimed that punctuated equilibria could be used to explain group selection, the idea that evolution could work at higher taxonomic levels.  Until last month, I didn’t understand what mechanism could explain group selection.  This obviously left me at a disadvantage whenever a creationist brought this up (which was fortunately rare).  But last month, a paper was released that showed that the two major spikes in maximum body size was correlated with changes in oxygen concentration.  Responses to this paper from the blogosphere made me realize that entire clades can have genotypes situated to take immediate advantage of new conditions, that they are pre-adapted as PZ Myers points out in this excellent post.

Yet another creationist talking point debunked.


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