Thursday, September 18, 2008

Open thread: are genes really a guide to homology?

I have been putting this question to some of my colleagues:

What is the value of gene expression data in determining homology of morphological features?

Are genes really important in determining if two structures in two different animals are homologous? If so, why? If not, then what does really matter?



Derek said...

If they aren't responsible for homology then what is???

Derek said...

Also, isn't unification (on a grand scale) one of the overarching goals of the sciences?

Showing that structures are all related would unify... something.

Martin Brazeau said...

You haven't answered the question. I want to know what (if anything) does genetic data tell us about homology, and if so, why?

Derek said...

I'm not a geneticist, but I thought the homeobox genes were responsible for the body plan of everything from fruit flies to people.

Derek said...

For example, check this page out:

It shows how there's a similar gene that controls eye development in both flies and mice.

I guess the implication is that fruit flies and mice are descendants of the same species.

Martin Brazeau said...

Homeobox genes, their distribution, and apparent conservation are an interesting discovery of the last few decades. However, we've known about homology since pre-Darwinian times. Moreover, the importance of homeobox genes and their apparent conservation lies in how their expression maps to features we already know to be homologous (i.e. head, thorax, wings legs, etc.).

Martin Brazeau said...

Hi Derek,
I see what you're saying, but my question is how we arrive at that implication. Is it really true? Or is it something we've been taught for a long time?

Derek said...

I think science has to rely on "circumstantial evidence" from lots of different sources, to build up a theory that is highly probable.

The fact is, we will never witness evolutionary events that happened billions of years ago, so we have to deduce based on the evidence.

Besides, Darwin's theory is only 150 years old, not a long time at all!

Martin Brazeau said...

I agree. But the point is that Darwin made successful use of the concept of homology, as did many scientists before him. Darwin distinguished himself from previous workers in being able to explain homology in terms of common ancestry. He was able to do this long before we knew anything about genes, let alone gene expression patterns in embryos.

Today, gene expression patterns (as well as other developmental criteria) have become "big business" in terms of the resolution of homology. My question remains the same: do they act as a guide and, if so, what is the theoretical justification for this conclusion? If they don't act as a guide, why not and how do we know?

Derek said...

I thought cause and effect has been demonstrated clearly. E.g., removing the "eye" gene produces flies with no eyes, or putting the eye gene in different locations will create flies with eyes on different parts of the body.

Anyways, homology isn't needed for genetics anymore, there are other things, like protein/enzyme structure and mitochondrial DNA that can be used to show hereditary relationships.

Darwin relied on homology, because that's all he had to work with.

UrbanVoltaire said...

I could be wrong, but I was under the impression that it wasn't homology alone that reinforced- or even primarily so- the theory of evolution but verification from independent sources of science.

Sam Harris said...

Science should not be based completely on circumstantial evidence. I always thought Science was something you can definitively prove over and over. Please correct me if I'm wrong.