Tuesday, August 12, 2008

Loricifera, Cycliophora, Micrognathia: New Phyla Get Me Wondering About Twisty Metazoan Evolution

pic of loriciferan and life cycle

pic of cycliophora and life cycle

pic of micrognathia and phylogeny

get a look at that cycliophora life cycle. what insanity! where's the individual? these are whole new phyla, on the level of mollusks or segmented worms or arthropods, discovered within the past few decades.

after reading this article i woke this morning thinking about where these critters fit so i reviewed all these phyla...

loriciferans, cycliophorans, and micrognathozoa. So i review the phyla: priapulid, kinorhynch, loricifera, nematode, sipuncula, bryozoa, phoronida, brachiopoda, gastrotrich, rotifer, acanthocephala, entoproct, cycliophora, gnathostomulid...

in my invertebrate biology text. What chaos of lifestyles and phylogeny.

well actually the phylogeny is confused! we do not know when the phyla differentiated from each other, nor how long it took? 70million years int the precambrian? 10million years right at the beginning? faster?

a lot is going on. many levels of organizition are evolving in parallel with and against each other:

organismal anatomy
genetic networks.

In each clade one can find instances of radical switches in ecology, which select for radical difference in morphology. lineages can go back and forth between these ecological strategies multiple times. see the chart of switches between planktotropy and lecithotrophy in annelid larvae quoted on page 491 of Valentine, from:

Rouse, G. W. 2000. "the epitome of hand waving? Larval feeeding and hypotheses of metazoan phylogeny". Evol. Dev. 2: 222-233

so it is hard to infer phylogeny from looking at aparent homologies in the anatomy/behavior.

the next level of messiness is in the genetics. the genes and the regulatory circuits guiding development can evolve independantly. the same gene can be coopted again and again for a different developmental task. and we know that genetics can be subject to recombination, so that whole level doesn't form a clean phylogenetic tree...

Sean Carroll on Evo Devo

I'm struck by what a rich fragrant MESS evolutionary biology is.

check out a nice summary in

James W. Valentine, "on the origins of phyla"

chapter 3 discussing the fragrant chaos of developmental genetic regulatory networks. the book has chaps describing the phyla, the molecular phylogenies, the fossil record and speculations about there mysterious evolution at the end.

one day i will get to read it.

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