Evolution and development: explanation and causation

In Uncategorized on April 17, 2010 by Lynn Chiu

I have a question about the explanatory aspect of projects that integrate evolutionary and developmental biology.

Mayr claimed that explanations of traits can be divided into the ultimate and proximate. The ultimate explanation deals with the “why” question, which is answered by evolutionary biology; while the proximate explanation deals with the “how” question, which is answered by developmental biology.

For example, why is there a certain trait X? The ultimate answer would appeal to its survival value in the past, also taking into consideration historical contingencies such as the effects of random events, drift, etc. The proximate answer would show how the trait came to be from an embryo or other initial conditions, it would specify the causal stages that lead to the final form of the trait.

The distinction seems nicely contained until evo-devo or devo-evo theses come into the scene.

Traditionally conceived, natural selection deals with end-products. Regardless of how a product is produced, reproductive variation in the products result in evolution by natural selection. On the other hand, development deals with the building process of these end-products. Regardless of whether the end-product ultimately survives, the building up of the end-product from its initial stage is development.

However, some synthetic theories argue that natural selection acts not only on the end-product, but also the developmental process, “while” it is developing. Therefore, not only is the end selected, the different inter-mediate stages might also be selected for their own properties. This is a case in which development is not black-boxed from evolution. On the other hand, the developmental process of a trait itself becomes part of the selection forces on itself and other traits, this is how the evolutionary process is not black-boxed from development.

If this is the case, the ultimate/proximate explanatory distinction would break down. Every moment is simultaneously an evolutionary and developmental process, the explanation of a trait then is just a causal one that points out how traits change. One can only arbitrarily cut the causal link into long term and short term durations to make the evolution/development distinction meaningful.



One Response to “Evolution and development: explanation and causation”

  1. I’m afraid I don’t know enough about evo-devo to fully comment. How does natural selection work during the development process? Via the Baldwin Effect?

    The Baldwin Effect works like this (unfortunately, I’m going to use classical evolutionary language): the genetic “program” doesn’t specify a given Cool Trait. However, the organism can achieve this Cool Trait through phenotypic/developmental plasticity. Organisms whose genotypes produce a ‘naive’ phenotype that is closer to the Cool Trait have less of a learning curve. So there is a gradual fitness differential between genotypes, depending on how close the ‘naive’ phenotypes they produce are to the Cool Trait. Thanks to this gradual fitness differential, natural selection pushes towards a genotype that will produce this Cool Trait in its ‘naive’ phenotype. A trait that can, at first, only be achieved through phenotypic plasticity will eventually become genetically inheritable.

    If that is what is going on, I think you might be able to maintain the ultimate/proximate distinction, although the proximate explanation will be doing a lot more work. Why does *this* organism have the Cool Trait? Proximate: Phenotypic plasticity. Ultimate: the genetic program that enables this plasticity and establishes the ‘naive’ phenotypic state. Why does *that* organism have the Cool Trait? Proximate: Because of its genetic program. Ultimate: evolutionary history, including Baldwin Effect. However, this might be an artifact of me still using classical evolutionary language. I’m not familiar enough with the language (and therefore *ontology*) that evo-devo uses to know how to fully respond.

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