Friday, 11 May 2007

Teach the controversy

Evolutionary biology is a vibrant field of science, and full of active dispute and uncertainty and new discovery. Science education ideally gives students the basic background so that can find their way around popular descriptions of what is going on at the bleeding edge of research.

Here are some of the controversies that could easily be part of a lesson even in high schools.

1. The Cambrian Explosion. About 530 million years ago, and over a period of some 10 million years or so, there was a dramatic diversification of animal life, the like of which has never been seen before or since. There are a host of unanswered questions and hypotheses about what is involved. When it occurred, how long it lasted, what are the antecedents, what changes took place, what mechanisms were involved. It is seen as an artefact of sampling, giving limited insight to a much more spread out period of change, or as a comparatively abrupt phase shift in the biosphere. It's seen as driven by evolution of exoskeletons, or of vision, or of predation, or of HOX genes. It is seen as driven by oxygenation, or warmer temperature, or changes in the sea floor. On going investigations continue to shed light on this fascinating period, but it's still wide open for new research and ideas.

2. Horizontal transfer verses variation through descent. For the most part, we carry within our bodies the genetic heritage of our ancestors; but there are also possibilities for genetic information to become incorporated into the genome from other organisms or a virus. This is particularly common in bacteria, but it can also occur for more complex organisms. The extent and importance of this is an open research question.

3. Particulars of lineage and relationships. There are many cases in which scientists are uncertain as to the relationships of different organisms. A classic example is the whole dinosaurs to birds issues, though this one is now all but resolved. There are plenty more that remain wide open. One example would be relationships between insectivores (insect eaters -- shrews, hedgehogs, moles, tenrecs, etc). There are many other examples.

4. Drift verses positive selection. It's a common misconception to think of evolution as a kind of continuous progress in improved fitness. Recent decades have given strong support for a notion of neutral drift, and the development of characteristics that are not adaptive, but simply a consequence of accumulated neutral change. What features of an organism actually are adaptive, or a result of positive selection? There are ways to measure this when we have access to a DNA sequence. With fossils and paleospecies it gets more difficult.

5. Target of selection. People often think they understand selection; but what is selected? Is it genes? Individuals? Populations? Species? All of these have been proposed as targets for selection, and the debate continues as to their relative importance.

6. Evolution and behaviour. We are more than our physical forms; we also have tendencies and predilections for different kinds of behaviour. To what extent is behaviour subject to evolution? This is a very difficult and contentious dispute!

7. Physical details of microevolution. There's still a lot we don't know about how development works, and how variation accumulates from generation to generation. What kinds of change can arise in the genome? Genes are far more than a coded protein sequence. They come with promoter regions and splicing points, and ways of encoding multiple proteins depending on how exons are combined. Evolution can duplicate whole genes, or combine them, or alter the regulatory promoters. Some parts of the genome are much more subject to variation than others. What kinds of variation are possible? What is their relativity frequency and importance? What proportion of variation is deleterious? How important is the piggyback effect where selection for one variation carries along other consequences? I'm only touching on the huge range of active research questions here.

I'm not a biologist. If you can educate me further in any of these or add some more, please leave me a comment!

(Disclaimer: this post is a repeat with minor revisions of something I wrote sometime ago in a discussion forum.)



Update: I omitted to say why I wrote this. It was originally given in the context of creationist notions about teaching the controversy. I intended to contrast what genuine scientific controversy looks like, as opposed to the screwy notions of creationists.

3 comments:

  1. A few comments -
    1. The Cambrian Explosion.
    But not on this - outside my area!

    2. Horizontal transfer verses variation through descent.
    I think this could only be done at an advanced level: I think the problem is that it is rare in eukaryotes, so you would have to make sure the students understood the differences between prokaryotes and eukaryotes in terms of their genetics and ecology.

    3. Particulars of lineage and relationships.
    I think this would be a great idea! It would show where there is lack of knowledge, and could be used to motivate the understanding of the techniques (morphological comparison, paleontology, molecular evolution etc.). The only problem is that the moment an example is written up in a textbook, someone will solve it.

    4. Drift verses positive selection.
    Another good idea! We don't have a good idea about the relative importance over a short time scale: this is actually something that's related to a lot of my research.

    5. Target of selection.
    I think this has more or less been solved, in the sense that we now understand the difference between levels of selection and levels of heredity, and that they need not be the same.

    The problem is one of deciding how important different levels are, and without a systematic survey of nature this is difficult. Because of this, the debates now seem to be producing more heat than light.

    There's a book called Levels of Selection where, in the introduction, the authors get marvellously grumpy about the debates, and end up saying (in effect) "just shut up about them, and look at the interesting stuff. Like the rest of this book".

    6. Evolution and behaviour.
    Yes, also a good idea, although it might be difficult to teach it without getting into political controversies.

    7. Physical details of microevolution.
    I think this is an area where we're starting to get a good handle on what's going on, but we might need a few years to consolidate our knowledge. So, I'm not sure it's a controversy as such: there are debates, but that's just part of the process. Mind you, I think it would be great to teach some of this.

    There. That's my kaksi pennia.

    Bob

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  2. Bob -- thanks for the comments. Your comment on evolution and behavior was one that stuck out to me. You're quite right; this needs to be handled well if it is mentioned in schools. It's also probably an important one precisely because it is open to abuse!

    And congratulations; you've been singled out for mention in a recent post by Afarensis: Pav and Frequency Dependence: Once Again D Fails to Understand the Issues. Good work! I am honoured to have to drop by here as well. Kiitos.

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  3. I'm not a biologist. If you can educate me further in any of these or add some more, please leave me a comment!

    I am a biologist (at least under my own working definition), and this is the best summary of real controversies within evolutionary biology that I've seen. This is cool, I'll definately be visiting here again.

    2. Horizontal transfer verses variation through descent.
    I think this could only be done at an advanced level: I think the problem is that it is rare in eukaryotes, so you would have to make sure the students understood the differences between prokaryotes and eukaryotes in terms of their genetics and ecology.


    While I agree that students of this topic would need some decent background in cellular anatomy, top-level taxonomy, and molecular genetics, as well as a smattering of cellular physiology and basic evolutionary theory (especially population genetics), that doesn't to me imply necessarily "an advanced level". I can (optimistically) imagine a second-year university course that would cover horizontal transfer in about a week or two, focusing on the evolutionary effects more than on the mechanisms. That could be very interesting.

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