Homology arguments and others

On the Monday holiday, I was working on my lecture for Tuesday.  I’m teaching plant diversity this semester, and in the early lectures of the course I like to discuss concepts and intellectual tools that we will use throughout the semester.   The concepts and tools I introduce are important in comparative biology—that is, they help us to compare one kind of organism to another as we work to explain diversity.

After discussing familiar ideas of classification in the first lecture, I moved on to basic ideas of phylogeny reconstruction—how we infer evolutionary relationships—in the second lecture. My theme for Tuesday was homology, which is one of my favorite topics.   Homology is basically a proposal of equivalence.  If we say that the arm of a human, the arm of a chimpanzee, and the wing of a bird are homologous, then we are hypothesizing that these corresponding structures are somehow equivalent.  The somehow of that equivalence is what a biologist wants to explore and to explain.  In a facile manner, I could say that homology of the arm of a human, the arm of a chimpanzee, and the wing of a bird is explained because they are all modifications of a corresponding structure in their most recent common ancestor, which is to say that we have homology because of evolution.

A less facile explanation would require us to explain how common position within bodies and possible similarities in development, including perhaps similar genes being expressed to control development, of humans, chimpanzees, and birds lead to a hypothesis of homology.  We might also be expected to explain how intermediate structures in the lineages ‘between’ humans and chimpanzees as well as between them and birds help to demonstrate the transformations that could have occurred since their divergence from a common ancestor.  Formulating this set of explanations to propose an hypothesis of homology is sometimes called a ‘homology argument.’

As I wrote my lecture on Monday, I wanted to look for new examples of homology arguments—so I googled ‘homology argument.’  One of the top links that Google returned in the search was for an entry in the Conservapedia, which subtitles itself as “The Trustworthy Encyclopedia.”  The link took me to the Conservapedia entry for homology, which it defined as “. . . the theory that macroevolutionary relationships can be demonstrated by the similarity in the anatomy and physiology of different animals.”  While I don’t regard that definition as accurate, it’s neither egregious nor exactly untruthful.

It was the section below the Conservapedia’s definition of homology that caught my attention—it was labelled “Invalidity of the Homology Argument.”   How could the homology argument be invalid, I wondered?  After all, a homology argument is a method of comparing and reasoning rather than an assertion of truth.

The explanation offered by the Conservapedia for the invalidity of the homology argument is straightforward:  “Creation scientists claim that similarity can just as readily be explained by a common Designer as common ancestry, and that homology is therefore not evidence that can be used to support the evolutionary view.”  

The statement is very interesting, and like most creationist arguments relies more on rhetoric than on knowledge.  It is true that many creationists claim that a ‘designer’ is responsible for the diversity of life; however, that claim is not supported by evidence from any tests of the origins of biological diversity.  All the Conservapedia statement really says is that a ‘claim’ by creation scientists means that homology is not evidence useful to support evolution.  Why would anyone make such an enormous assumption about homology and evolution based on a ‘claim’?

All that seemed to matter to the Conservapedia were dismissals offered by creationists.  The entry suggested that the invalidity of homology arguments was further ‘supported’ by the following quote from J. P. Holding: “To frame our argument against the evolutionists’ misuse of homologous structures requires us to have an understanding of certain values critical to ancient persons. Roman literature of the New Testament period tells us that ‘(t)he primary test of truth in religious matters was custom and tradition, the practices of the ancients.’ In other words, old was good, and innovation was bad. Change or novelty was ‘a means value which serves to innovate or subvert core and secondary values.’”

This was excellent stuff, and I decided to use it in my lecture on homology.  [I note that conservatives complain frequently that their ideas are ignored in academia so I felt especially good about introducing the Conservapedia and its claims in my class.]

We discussed in class a critical difference between a scientist and a creationist.  Creationists think they have THE answer from the beginning, whereas a scientist has only a question in the beginning.  While a creationist may accept absurd dogma and simplistic dismissals of rational ideas, a scientist looks for a way to test ideas.  That willingness to test and to infer from the results of those tests the best explanations distinguishes the scientific method from the creationist method.  [A great untruth of the Conservapedia’s entry on homology was its claim that there are creation ‘scientists’—creationists offer religious explanations and dismiss the results of repeatable scientific studies rather than using a scientific method.]

The Conservapedia quotation from Holding was especially interesting in stating that we are required to “have an understanding of certain values critical to ancient persons” to understand why homology arguments are invalid.  It is unclear to me how knowledge of values invalidates a scientific method.  Indeed, a strength of the scientific method is that it is not contaminated by values (while we can all recognize that individual scientists and others may be influenced by values). How can custom and tradition invalidate homology arguments?

The Conservapedia entry on homology seems more concerned with acceptance of “custom and tradition” as a basis for “truth of religious matters” than with possible comparisons we might make among organisms. Indeed, it seems that the Conservapedia aims to dismiss important scientific approaches through superficial allusions.   Perhaps we should be wary of trusting the Conservapedia, despite its subtitle. 

*  *  *
Quotes are taken from the Conservapedia at http://www.conservapedia.com/Homology.

Comments

[Laurent]

I would love to be a student in your class, it seems to be very entertaining on many grounds! Finally, maybe "culture wars" is the best way to provide students with a good understanding of science(s)...

Moreover, I'd be very curious to hear about "A less facile explanation would require us to explain how common position within bodies and possible similarities in development, including perhaps similar genes being expressed to control development, of humans, chimpanzees, and birds lead to a hypothesis of homology", because I've been wondering for some time how I would teach anything related to this if I had to... (and further acknowledging the complication that shared genes involved into producing non-homological organs during development are also sometimes known to students).

[larry]

Laurent--I try not to become immersed in culture wars in my classes, but those conflicts at times provide important insights on the distinctiveness of scientific reasoning and the particular value of scientific knowledge.

A less facile explanation of the need for homology arguments, the history of reasoning about homology, and the criteria we use to make hypotheses of homology was the stuff of my lecture.

As part of that, I do talk about the application of gene expression in formulating hypotheses of homology and introduce the problems associated with it. For example, I introduce the role of the agamous gene in carpel specification in Arabidopsis and the expectation that an orthologous gene would specify carpels in other flowering plants . . . and yet we find that the agamous orthologue in Antirrhinum does not specify carpel development; instead it is a paralogue of agamous that plays that role.

I think it is important to consider statements of homology to be hypotheses, to evaluate their strength based on supporting data, and to see their value in how they help us to understand diversity.

[Laurent]

Hum... Someday I'll have to dive into developmental genes for flowers. I left my record somewhere during the end of the 90's, i.e. not that far from the beginning... At that time it was all about Arabidopsis, but I've seen there are many things investigating the evolution of these genes now.

[Roger]

Thanks for this - great article. I particularly like "Creationists think they have THE answer from the beginning, whereas a scientist has only a question in the beginning." - I may paraphrase that and put it on my website - it perfectly sums up the opposing stances of religion and science.

Thanks !

Rog

[Tony Sidaway]

Only one thing amazes me than the sheer awfulness of the Conservapedia article: the excellent use you have put it to as a teaching aid.

I wouldn't put it past Andy Schlafly, the owner of Conservapedia, to sue you for copyright infringement. That would be the icing on the cake.

You may not be aware of his recent spat with Richard Lenski, which has reached the point where Schlafly is writing to PNAS alleging, amongst other things, that Lenski is contravening NSF policy. PNAS did not publish the letter.

http://scienceblogs.com/pharyngula/2008/09/andy_schlafly_writes_another_l.php/wiki/Conservapedia#Lenski_dialog

[Martin Brazeau]

Oh balls! That Conservapaedia statement is completely false! Homology is a pre-Darwinian concept.

On a different note, here's a question I've been asking colleagues: why is gene expression so significant in the determination of homology?

[Raynfala]

"Why would anyone make such an enormous assumption about homology and evolution based on a ‘claim’?"

You must be new to Conservapedia. :^}

[Midnight Rambler]

This probably isn't a result of it, but one of the amusing things about Conservapedia is that some of its editors are nuts who genuinely believe this stuff, but a significant portion are trolls who post silly things to make the Schlaflyites look stupid.

The best part is that no one, including the true believers, can tell the difference (c.f. Poe's Law). This leads to endless fights in some of the talk pages and frequent bans of users.

[Metro]

I love Conservapedia. It's vapid, reality-free outlook gave rise to one of my favourite moments on one of the scienceblogs.

Was it "The Questionable Authority"--let me see ...

Ah, yes.

Comment #5:I'm having tremendous fun. I edited the article on Atheism to point out that it leads to pedophilia and bestiality. I checked back an hour later expecting my edit to be gone, but no: they didn't remove it, they added citations.

Posted by: Steevl

[Metro]

Ah, my links got stripped out:

http://scienceblogs.com/authority/2007/02/still_more_conservapedia_humor.php

[stavros]

I really enjoyed the article and I agree with "Metro": Conservapedia is real fun! You get to see how stupid ignorant people are desperately trying to formulate something they think is an argument, but in reality it is not even close...

You can learn a lot from Conservapedia: how NOT to debate, how NOT to construct arguments etc...

[JoJo]

A friend of mine is quite conservative politically. He got banned from Conservapedia because he isn't a Young Earth Creationist. It's an all or nothing deal with the owner of Conservapedia, Andy Schafly.

[D. P. W]

I still am not seeing what your huge point is. You don't think God could create the creatures he created to evolve. It says in the Bible that he created the animals in days. But, it also says in the Bible that a thousand years is a blink of an eye to Him. Therefore, a day in the beginning of Genesis could be hundreds of millions of years. Also, when it comes to bones being similar in bats, horses, humans, etc., couldn't he intend them have the same bones. Also, I notice you are very cruel to Creationists, calling them ignorant, stupid, etc. I think that many of you are being ignorant and prejudice. You probably don't realize that God's creation and Homology CAN exist at the same time. You probably just didn't study the Bible, so you wouldn't know. Also, I can just about predict that many of you will be so upset by these comments that you will immediately rush to the conclusion that I am just a stupid creationist, but I have been Valedictorian many times and am ALWAYS the top of the class. I also happen to be a philosopher, so don't be cruel. I just want you to think.