evolution - Definitions of robustness and canalization

The concepts of robustness and canalization are fashionable today in the biology literature. However, I am not sure of their definitions and I am not sure either that all authors actually use the same definition.

I am therefore asking here for a the definitions of these terms or for a short review of the common definitions of robustness and canalization. How do different authors define these words? I am particularly interested in...

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  The differences between robustness and canalization (canalization seems to apply more in developmental biology than robustness who seems to be a more general concepts)

  
  


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  At what level do these concepts apply? To populations, to genotypes, to both, ..?

  
  



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  How does the concept of adaptation link to these definitions.

  
  


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  How does the concept of plasticity fits in opposition of the concepts of robustness and canalization?

  
  

Answer

As canalization is defined in your question (also in wikipedia) it means robustness.

Semantically it is possible to differentiate the two.

Robustness of a system refers to its sensitivity to perturbations. In other words small differences in parameters would not affect the steady state of the system (parameter changes in a physically plausible range would not lead to bifurcations).

By system I mean a set of entities connected to each other in a defined fashion. For example an ecosystem in which different species are connected to each other by interactions of the food chain. At steady state a system configuration (for instance- 10 foxes, 100 rabbits, 10000 grasses, 1000 honeybees etc) remains constant over time (that, however, doesn't mean it is static). Or in case of development a system may be the body defined by a set of certain phenotypes.

Canalization/channelization refers to mechanisms that make the system follow a certain trajectory. So if you see it mathematically canalization keeps the trajectory dictated by a given set of eigenvectors. In case of development you may call mechanisms such as dosage compensation, functional compensation by isoforms, parallel metabolic flux channels etc as canalizing mechanisms.

So in contemporary usage we can call a system robust (a property of a system) whereas we should address mechanisms that help to preserve trajectories as canalizations. The final aim of canalization is to render robustness to the system but it is not the only mechanism that is responsible for robustness (for e.g. the steady state may just be independent of certain parameters).

Robustness is a property of the system and is dependent on how the different entities of the system are coupled and what their interaction parameters are (rates etc).

You should note that these are parlances that are adopted by different groups in the scientific community and may perhaps mean the same thing- this is merely my attempt to remove redundancy of terms by the means of semantics.

Now, plasticity is a different concept altogether. Plasticity is in a way opposite to robustness. This means that the qualitative nature of the system can change if certain parameters are tweaked. However, the system should still maintain its stability. In a stable system mild perturbations from steady state will result in system going back to that steady state - like a ball in a well. However, a system may inherently allow for multiple steady states (like switches), and depending on certain parameters the system can be in either of the states (but if pushed too far then the system can go to another stable steady state). This is called multistability.

NOTE: some people also use the term plastic to mean "responsive". The above definition may not be exhaustive or the most perfect.