On BioScience and Life and Such

How can chaperones act in evolution (chapter III, blogging in Just Science 08)

In Uncategorized on February 6, 2008 at 11:15 am

Moving away from the specifics of Hsp90 for a while, this post shall focus on the general principle of chaperones in evolution. What supports such an hypothesis:

To begin lets look at the problem of rapid evolution. Darwinian evolution is based on a constant rate of random mutations in the genome of any evolving organism. This implies that mutations happens constantly, by chance, regardless of the external environment. Adaptations consequently arise by chance. Evolution of new traits one may think, is therefore slow and gradual.

Gradual evolution

Gradual evolution (reproduced with permission from Dr. Dennis O’Neil)

Since such a view, does not fit with the bursts of evolution observed in fossil material, alternative explanations have been put forward (see more below). However, even when using a constant random rate of mutations one would expect “bursts” or rapid transitions. This is elegantly illustrated in this simulation of an evolving clockwork. Since a beneficial mutation can have a profound impact on fitness, then there should be no surprise that the transition between av less fit form and a more fit one, happens quickly. Thus even with a constant mutation rate one would probably not see a slow, gradual evolution of species, – basic math skills on exponential growth should make this clear. Why this notion of gradual evolution is prevailing I cannot understand.

In addition there are those that believe that the mutation rate may not be constant. Thus, with an increased mutation rate and rapid transitions one can start to explain the observed bursts of evolution.

Further explaining bursts of evolution we have the theory of ‘punctuated equilibria’, associated with Niles Eldredge and Stephen Jay Gould, which states that organisms go through short periods of rapid evolution from time to time, against a background of relative stasis (see picture below, and this genomicron post as a starting point for more on punctuated equilibrium ).

Punctuated equilibrium
Punctuated equilibrium (reproduced with permission from Dr. Dennis O’Neil, for his tutorials on evolution go here and here).

This has further led to the theory of hopeful monsters. These theories account for non-linear rapid evolution within the boundaries of Darwinian principles, but they have been heavily criticed. One of the main criticisms of these theories, as far as I can understand, is the improbability of a single mutation to give rise to radical morphological changes, and further that this change, if it happens, is most likely deleterious, and if it against all odds is beneficial, its even more improbable that this individual is able to produce offspring with the same trait(s).

So we are still left with some problems: External environment changes can happen really quickly. Is random mutation events, occurring at a slow rate (even if it’s sped up in larger populations or even if monster are hopeful in times of stress), sufficient to explain the effectiveness of adaptations seen in nature ? Does an organism rely on (slow) random mutations to evolve a trait to help the species adapt to the new environment, or are there additional mechanisms in place to speed up this mutation rate and perhaps guide mutation events towards selected genes that allows rapid changes in phenotypes ?

Enter heat shock proteins…….

The hypothesis is the following: If there is a way to mask (deleterious) changes in proteins under normal conditions, one may accumulate such changes without exposing them.

Hsp90 evolution
Illustration from Sangster TA et al. (more on Waddington will follow in the last post).

Thus, with Hsp90 acting as a buffer: one could potentially get a lot of hopeful monsters, under times of stress, as these traits were exposed. This would drastically increase the chances of a beneficial change to occur at the right time. And since the chance of mating with other monsters with similar traits (there are more than one monster, in fact very many), the chance of keeping the trait(s) in subsequent generations is also increased.

Now missing……..evidence, which will follow in the next post.

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  1. [...] How can chaperones act in evolution (chapter III, blogging in Just Science 08) [...]

  2. [...] First we have the idea of punctuated equilibrium and hopeful monsters discussed in my previous post. To expand on these ideas let’s also include the theory of canalization. Canalization explains [...]

  3. Your concept of punctuated equilibria and your understanding of how it works is completely wrong.

    Punctuated equilibria has nothing to do with macromutations and hopeful monsters.

    Macromutations and Punctuated Equilibria

    The changes seen during the “punctuations” are the perfectly normal kinds of evolutionary changes seen in typical speciation events. In most cases, it takes an expert to recognize that speciation has occurred in the fossil record.

  4. Thank you for your comment and thank you for referring to these posts on your website. Now…, my concept of punctuated equilibria is,.. put bluntly: “evolution in bursts” ….period. The link between Gould and hopeful monsters may be wrong, but that does not change the fact that there is a need to explain rapid non-linear evolution. The reason creationists use these arguments is not mainly because they misunderstand Gould’s theories, but because there is a real lack of credible, solid scientific explanations for evolution in bursts. Explaining evolutionary change as the result of a slow, random and constant mutation rate is just not sufficient. Your statement “In most cases, it takes an expert to recognize that speciation has occurred in the fossil record.” underscores this point, since the diversity and plasticity in nature is obvious to even the unschooled child. The argument is otherwise irrelevant since it is merely a description of skill requirements in paleontology/biology. Canalization through Hsp90 is a credible, scientific hypothesis explaining the world better than the prevailing interpretation of evolution theory. I am looking forward to the canalization hypothesis being challenged, but in order for that to happen it must be accepted as a valid theory.

  5. Look at the diagram you posted from Dr. Dennis O’Neil. That’s not punctuated equilibrium, that’s a form of gradualism where a single species gradually transforms into one species and then another.

    The key concept in punctuated equilibria is that evolution is coupled to speciation by cladogenesis. One species splits into two. One of the species shows phenotypic change while the other one does not.

    This is explained by a founder effect version of speciation where a subset of alleles is fixed in a small isolated population. Nothing special going on there. The alleles were already in the large population. There is nothing mysterious about the process underlying punctuated equilibria. The original paper even referenced Mayr and gave him credit for his work on speciation.

    Let me repeat. Punctuated equilibrium is not about new mutations, it’s about speciation.

    The other thing to keep in mind is that both species continue to exist side-by-side for millions of years. This is difficult to reconcile with your diagram that links phenotypic change to a “rapidly changing environment.” How do you explain the fact that the parent species seems to be very happy in this so-called “rapidly changing environment.”

  6. “…..Punctuated equilibrium is not about new mutations, it’s about speciation……”.

    This is why I feel punctuated equilibrium is relevant to the Hsp90 story. Hsp90 promotes speciation not by creating new mutations, but by exposing existing ones, which in my mind fits perfectly with the punctuation concept. The new twist is that the existing mutations are hidden rather than exposed as a phenotypic subtype in a larger population. With canalization through Hsp90, the new phenotype is indistinguishable from the rest of the population until times of stress. This to me is a better explanation than the one arguing that species rely on the continous presence of an off-chance subtype to meet changing conditions. Wouldn’t evolution just stop if that’s the only mechanism to evolve rapidly ? Are there sufficient examples of such phenotypic subtypes to explain evolution as we know it ? Also if there is “Nothing special going on there”, why is punctuated equilibria controversial ?

    My apologies if the diagram is misleading, but to me there is nothing in this diagram (exept maybe that it is two-dimensional), or in my text, that excludes the co-existence of the parent species and the new one.

  7. This to me is a better explanation than the one arguing that species rely on the continous presence of an off-chance subtype to meet changing conditions. Wouldn’t evolution just stop if that’s the only mechanism to evolve rapidly ?

    These questions reveal two areas where you and I disagree.

    First, I don’t believe that changing conditions drive most of evolution.

    Second, I don’t believe that “rapid evolution” is part of the discussion about punctuated equilibria. At least not the sort of rapid evolution you are trying to account for.

    Are there sufficient examples of such phenotypic subtypes to explain evolution as we know it?

    Of course. Just look at our own species. If the various subgroups had remained isolated for another 100,000 years we might have split into several species. The differences between Asians and Africans are at least as great as the differences between many species.

    Also if there is “Nothing special going on there”, why is punctuated equilibria controversial?

    For four reasons. (1) Some people doubt that it exists. (2) Some people don’t understand it and think that it’s not an important contribution to evolutionary theory. (3) Some people understand it and recognize that it’s a threat to gradualism. (4) It’s the basis of hierarchical theory and macroevolutionary processes like species sorting. Many people aren’t ready to consider such things.

  8. [...] in the  5 post series for JustScience week 08 (Revolution Evolution, Presenting….Hsp90, How can chaperones act in evolution, Evidence for Hsp90 involvement in rapid evolution of new traits and Hsp90 to end controversies in [...]

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