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Evolution - Evolutionary Development
Expert: Dana Krempels - 10/28/2009
Question
Hello Dana,
It's been months since I asked any questions. I've been pretty busy this semester.
My latest question is about evolutionary development. I recently finished reading "Why Evolution is True" by Jerry Coyne after reading good reviews of the book. I thought it was very informative and I apparently have the basics down, which is good. My favorite section may have been on biogeography. After thinking about my trips to Australia and asking myself why there were no kangaroos anywhere else in the world, species distribution really started to interest me! His discussion of development was fascinating.
I know Ernst Haeckel's "ontogeny follows phylogeny" has pretty much been discarded. As I understand, this is because he thought looking at the progressive embryological stages of various organisms sort of played back their evolutionary history as to what they looked like. Dr. Coyne mentions this but also says recapitulation is, in a sense, true. This made me scratch my head. Apparently, Haeckel thought the embryological stages represented past ADULT forms of it's ancestors and this is the idea that has been discarded. However, as Dr. Coyne mentions that development is a conservative process (which makes sense - a lot of energy would be wasted trying to make a whole new blueprint for a body plan from scratch, and the "tools" might not even be available), the progression of embryological stages does represent their ancestors...in the EMBRYOLOGICAL form. In other words, a mammal embryo goes through a stage where it resembles a reptile EMBRYO, not a reptile adult.
Despite all the good reviews of whatever book I might read, anytime I read something that profound to me I always want to verify it with other sources. I asked my biology teacher about that and she agrees that embryological forms of an organism's ancestors are conserved during development. So, in that sense, recapitulation is in fact true. Do you agree? What insight can you give me about this topic?
Tom
P.S. Dr. Coyne also mentioned lanugo. He said the appearance of this trait in our embryological development may be an ancestral trait from the time our ancestors were covered in hair, like modern apes are today. I've never heard of lanugo and I thought that was fascinating! What can you tell me about "hairy fetuses?"
Answer
Dear Tom,
Dr. Coyne is absolutely correct. Ernst Haeckle was on the right track, but his terminology wasn't quite consistent with what we now know to be true: More complex adult organisms are the result of more complex ontogenies, NOT the result of more complex adult forms. The homology we see in embryo development is just that: embryo homology.
So yes. In a sense, recapitulation is true. But not in the way Ernst Haeckle envisioned it.
I'd never heard of lunago until you mentioned it, so I did a quick search on it. Very cool! Just goes to show that we retain genes that are inactivated in our own species due to a more complex ontogeny--but that they can be reactivated under the right circumstances. Apparently, anorexics and people who are starving for other reasons *also* develop lunago, ostensibly the body's response to a lack of body fat, and an emergency measure to try and retain body heat (or so goes the hypothesis).
In our own embryos, lunago is shed, and the true hair replaces it. But there it stays, waiting for the genes to be reactivated under the right conditions.
Double cool! :)
Dana
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