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A review of evo-devo (Jenner, R.A., Wills, M.A. (2007) The choice of model organisms in evo-devo. Nat Rev Genet. 8:311-314. Epub 2007 Mar 6.) is starting to make rounds on the blogs. I cannot access the paper (I’d like to have it if someone wants to e-mail me the PDF), but the press release (also found here) is very vague, so I had to wait for some blogger to at least post a summary.

This is what the press release says (there is more so click on the link):

The subject of evo-devo, which became established almost a decade ago, is particularly dependent on the six main model organisms that have been inherited from developmental biology (fruit fly, nematode worm, frog, zebrafish, chick and mouse).

To help understand how developmental change underpins evolution, evo-devo researchers have, over recent years, selected dozens of new model organisms, ranging from sea anemones to dung beetles, to study.

One of the selection criteria deemed most crucial is the phylogenetic position of prospective model organisms, which reflects their evolutionary relationships.

Phylogenetic position is employed in two common, but problematic, ways, either as a guide to plug holes in unexplored regions of the phylogenetic tree, or as a pointer to species with presumed primitive (ancestral) characteristics.

Drs Ronald Jenner and Matthew Wills from the Department of Biology & Biochemistry at the University of Bath (UK), call for a more judicious approach to selecting organisms, based on the evo-devo themes that the organism can shed light on.

Larry Moran and PZ Myers went into a completely different direction which I find quite uninteresting: evo-devo was and currently is a study of animals and if people who study other organisms want to make their own equivalents, good for them, more the merrier, hi-ho-hi-ho, etc.

I have no problem with the idea that Earth is a planet dominated by bacteria and that the animals are a recent afterthought. I sympathize with those who lament the lack of interest, funding and teaching in the ares of plant, protist and fungal biology. But evo-devo is currently an area of Zoology, so the search for new animal models, as opposed to plant models, is a perfectly appropriate question. We want to know how animals develop and evolve and evo-devo tries to put those two questions together. I am sure botanists, mycologists, microbiologists are working on their own version within their own domains – and hopefully the groups will read each other and learn – but that is outside the realm of this particular review paper.

What bothers me about the press release is its vagueness. Different people have different definitions of the terms “development”, “evolution” and “evo-devo”. Different people have different evo-devo questions they deem important and the review appears to reflect the biases of the authors (and so do posts by Larry and PZ).

Development

Some people focus on the early embryos and things like pattern formation, determination of dorso-ventral axis, or limb development. Others consider the entire life-cycle, including growth, maturation and senescence, to be parts of development. Some focus on patterns of expression of developmental genes. Others are more interested in phenotypes. Some focus entirely on the development of anatomical structures, while others are more interested in the development of biochemical, physiological and behavioral traits and how they evolved. Obviously, people with different focus in development will ask evo-devo to pursue different questions.

Evolution

Again, some people are interested in genotypic evolution. They use the population-genetic definition of evolution as “change in frequency of alleles in a population over time”. Their models can detect some things (e.g, type, strength and direction of selection), but not others (levels/units of selection, effects of population structure, etc.), so they focus on the former and the latter is ignored, or given lip-service, or even deemed unimportant (or even non-existent!).

Others are interested in phenotypic evolution. After all, genes are invisible to selection – it is organisms that get selected and the changes in gene frequences are a downstream result of that process. They have different aims and goals for evo-devo as a discipline.

Using the broadest definitions of both development and evolution, the classical studies of imprinting, developmental ‘windows’ for learning birdsong, and organizing vs. activating effects of hormones are smack in the middle of evo-devo research – the mainstream onto which some genetic stuff has been added lately.

Evo-devo

Evo-devo is short for “evolution of development”. But, it actually asks three distinct questions:

How animal development evolved

Trying to trace and document how various developmental mechanisms evolved over time, in essence building a phylogenetic tree of developmental changes in animals on this here planet Earth since the apperance of first animals until today.

How animal development evolves

Figuring out generalizations, hopefully rules, and perhaps even laws, about the ways different evolutionary mechanisms affect different developmental mechanisms.

How animal development affects animal evolution

Figuring out the way different developmental mechanisms affect the way evolution can proceed, i.e., developmental constraints in the positive sense of ‘funneling’ evolutionary direction by making some directions more likely than others. From the very inception of the field, fueled by the publication of Stephen Jay Gould’s “Ontogeny and Phylogeny” (his by far the most influential book, though ALL the others are more popular), the focus has been on things like allometry, heterochrony, heterotopy, etc. This paper appears to be focused on this goal as all the suggestions appear to have such processes in mind:

Developmental programming. Allometry of horns in the beetle Onthophagus nigriventris.

Developmental bias. Variation in body size in C. elegans.

Developmental constraint. Shell morphology in the gastropod Cerion.

Redundancy. Anterior-posterior axis development in Drosophila melanogaster.

Modularity. Sense organs in the cavefish Astyanax mexicanus.

Evolvability. In silico cell-lineage evolution.

Origin of evolutionary novelties. The sea anemone Nematostella vectensis (bilateral symmetry, triploblasty).

Relationship between micro- and macroevolution. The three-spined stickleback and Heliconius butterfly wing patterns.

Canalization and cryptic genetic variation. D. melanogaster phenotypic variation increase during HSP90 impairment.

Developmental and phenotypic plasticity, polyphenism. Ant caste polyphenism and caste determination by primordial germ cells in the parastic wasp Copidosoma floridanum.

Frankly, ALL of these topics I find immensely exciting and, sure, I’d love to see these ideas implemented and these models adopted, and this research done. But what bothers me is that this list just enlarges the Big Six list into a Big Many list. It does not do what it is purported to do – move from separate studies of devo and evo to an evo-devo research program.

You can study development in an organism, but to study evolution of development you HAVE to do comparative work. This means that choices of single species miss the mark completely. If I have written this paper I would have suggested pairs and groups of species, not single species.

For some questions, one wants to compare closely related species, perhaps all in the same genus, e.g., Drosophila (D. melanogaster, D. pseudoobscura, D. yakuba, etc.). Rudolf Raff made great strides early on in the field of evo-devo by comparative studies of two closely related species of sea-urchins, one of which undergoes metamorphosis (i.e., goes through a larval stage) and the other one skips it and develops directly from an egg to an adult.

For other questions, one may want to look at somewhat less related species that cover a greater spread of evolutionary relationships. Perhaps a bunch of different insects: fruitlies, house flies, mosquitoes, cockroaches, termites, beetles, butterflies, moths, sandflies, wasps, honeybees, etc. (like this paper does, for instance), or a bunch of different fish, e.g., zebrafish, medaka and fugu, or comparing chicken to quail to turkey to ostrich.

For yet other questions, looking at the philogenetic depth is quite fine. It is exciting what we are learning about the origin, evolution and development from the studies of Cnidaria (see this, this and this for an example), or about the origin of Vertebrates from the comparative studies of echinoderms, hemichordates, urochordates, cephalochordates, agnathans and fish (check out this and this).

So, if you had unlimited space, time, manpower, money and freedom, tell me what pairs or groups of animals you’d choose as new evo-devo models, not individual species, and what would you study with them? What for? Which of the defintions of development and evolution you ascribe to? Which of the three evo-devo questions excite you personally?

Comments

  1. #1 Jim Collins
    April 2, 2007

    Don’t waste your brain power.
    If evolutionists want to end the arguments all they need to do is, get their brilliant heads together and assemble a ‘simple’ living cell. ‘Surely’ this is possible, since they certainly have a very great amount of knowledge about what is inside the ‘simple’ cell.

    After all, shouldn’t all the combined Intelligence of all the worlds scientist be able the do what chance encounters with random chemicals, without a set of instructions, accomplished about 4 billion years ago,according to the evolutionists, having no intelligence at all available to help them along in their quest to become a living entity. Surely then the evolutionists scientists of today should be able to make us a ‘simple’ cell.

    If it weren’t so pitiful it would be humorous, that intelligent people have swallowed the evolution mythology.

    Beyond doubt, the main reason people believe in evolution is that sources they admire, say it is so. It would pay for these people to do a thorough examination of all the evidence CONTRARY to evolution that is readily available: Try answersingenesis.org. The evolutionists should honestly examine the SUPPOSED evidence ‘FOR’ evolution for THEMSELVES.

    Build us a cell, from scratch, with the required raw material, that is with NO cell material, just the ‘raw’ stuff, and the argument is over. But if the scientists are unsuccessful, perhaps they should try Mother Earth’s recipe, you know, the one they claim worked the first time about 4 billion years ago, so they say. All they need to do is to gather all the chemicals that we know are essential for life, pour them into a large clay pot and stir vigorously for a few billion years, and Walla, LIFE!

    Oh, you don’t believe the ‘original’ Mother Earth recipe will work? You are NOT alone, Neither do I, and MILLIONS of others!

  2. #2 coturnix
    April 2, 2007

    Bwahaaha! You know, this is supposed to be a serious post on a serious topic, but I get silly creationist posts so rarely, I think I will not delete yours just to keep it as a souvenir.

  3. #3 Pedro Beltrao
    April 14, 2007

    He is right on one thing at least, we should build a minimal cell, although this is not really not related to your discussion. I know 3 groups that are working on minimal cell projects and there are probably more.

    Anyway, more on topic, one nice outcome of the development of the ‘omics methods is that we are no longer bound to study only a few species (model organism) at a time. We can just point the battery of high throughput methods to a large group of species and learn more from the comparison than from the in-depth analysis of single species. I am still more interested by the cell level than evo-devo so my choice for comparative studies would go for yeasts. Evolvability is one of things I am interested in. One of the cool things about studying evolution is that we can then apply these concepts we learn from natural evolution to engineered evolution.

  4. Thank you for your interesting post!
    I thought perhaps you may also find this related post interesting to you:
    Longevity Science: Evolution of Aging
    http://longevity-science.blogspot.com/2007/03/evolution-of-aging.html

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