II. Evolutionary-Developmental Biology: An Internalist View of Evolution

Evolutionary-Developmental Biology (Evo-Devo): An Internalist View of Evolution

Evo-Devo addresses (1) the specific changes we see over time in individual development (in embryology as well as cross species comparisons) and (2) the paradox of metazoans' (multicellular critters) retaining the cell metabolism of bacteria but attaching zillions of adaptations to it (Kirschner & Gerhart, 1998; Gerhart & Kirschner, 1997; Gould, 2002; Raff, 1996; Carroll et al., 2001) (Humans, in a similar manner, retain their nature but make tools that extend the range of niches they occupy.) It further asks: is the capacity for change, "evolvability," itself evolving (human science is an extension of that very process)?

Relevance: It's something of a stretch but Evo-Devo is often more careful to view changes in forms and behavior as first emerging from internal sources. The organism then attempts to match its new equipment to its current niche or uses that new equipment to define a new one. While environment still "selects," so does the organism!

Evo-Devo reminds us of our kinship not only with mice but also with flies and barnacles: all share a "Hox" structure that defines anterior-posterior organization and that, when duplicated from a single set in flies to a quartet in mammals, allowed an every greater specialization and variation in the duplicated systems through the processes of dissociation (the new structures can vary independently of the older ones that maintain stability in older functions), divergence (separation over generations), and co-option (the taking on of new functions by an existing feature) (Raff, 1996). For example, a gene, "distal-less" (dll), is a necessary factor for legs whether in flies, humans, or birds. Similarly, the 7-8 segments (rhombomeres in vertebrates, metameres in flies) of the hindbrain emerge from "labial" in flies and from its homologs, Hox1-4 in vertebrates. The rhombomeres appear to correlate with organization of the cranial nerves (Gould, 2002) and pharyngeal arches. The mouse forebrain emerges from Otx and Emx, homologs to empty spiracle" (Ems) and "orthodenticle" (Otd) in Drosophila (Raff, 1996). (Our forebrain probably originated before the vertebrate line was established!)

Evolution can proceed by variations in the Hox architecture (the number of Hox assemblies varies from 1-4, each Hox assembly contains 7-13 genes in a fixed order) and by variations in the downstream genes that are organized by Hox (Carroll et al, 2001). It can also proceed by "heterochrony," changes in the onset and duration of influence from Hox and other regulatory genes.

Exploratory systems match an organization with its setting, as if the posts on an engagement ring and their diamond each conform to the other. Microtubules, immune systems, the layout of blood vessels, and neural organizations are all exploratory systems: structures are randomly offered and retained if they elicit a response from wherever they end. The model has wide application: parents are exploratory systems for infants and therapists for clients. Likewise, many of our imprinted and conditioned behaviors are solidified outcomes from exploratory experiments.

Evo-Devo gives structure to how evolution proceeds, it cushions our surprise that whales and hippos are closely related and that camels and llamas will produce viable, fertile offspring despite 13 my separation of the parent species, and it lends shelter to those of us who see birds executing human behaviors and even to those of us who are convinced that we really could fly if only we had wings. Did Hox direct the Wright brothers?

Notes & Refs:

Carroll, Sean, Grenier, J., & Weatherbee, S. (2001) From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Malden, MA: Blackwell.
Gould, S. (2002) The Structure of Evolutionary Theory. Cambridge, MA: Harvard Belknap (esp. Chapter 10).
Gerhart, John & Kirschner, Marc (1997) Cells, Embryos, and Evolution. Malden, MA: Blackwell.
Kirschner, M & Gerhart, J (1998) Perspective: Evolvability. Proceedings National Academy of Science, 95(15), 8420-8427.
Raff, Rudolf (1996) The Shape of Life. Chicago, IL: University of Chicago Press.

Copyright, James Brody, 2002, all rights reserved.

Replies:

Evo-Devo: Credo, by James Brody, 12/12/02 Evo-Devo Utility for Clinical Work, by James Brody, 12/12/02 Evo-Devo: Characteristic Beliefs, by James Brody, 12/12/02 The Vole's Hox and I: Exploratory Systems (or, "Am I a Psychopath?"), by James Brody, 04/24/03 Re:The Vole's Hox and I: Exploratory Systems (or, , by Todd, 04/30/03 Hox, Curtis, & Kuby: Their 3rd Anniversary, by James Brody, 05/03/03 Flocking and C&W, by James Brody, 11/07/03

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