Doubting Didymus

Okay, I can agree with you about non-genetic heritable factors, but I do not see that factors that can not be inherited can effect evolution, except by effecting heritable factors. Could you give some examples, just to clear up this matter a little.

I also want to hearken back to the earlier parts of this thread, where you suggested that phenotypes are heritable. What did you presicely mean, and where do you draw the line (if any) between genotype and phenotype?

lpetrich

Wolbachia making its insect hosts reproduce asexually may be something that benefits that bacterium, because its host will always produce eggs and never sperm, which may be difficult for these bacteria to travel in.

Parasites modifying their hosts and inducing different host behavior is a very common phenomenon. Many disease symptoms are exactly that effect -- cold viruses make their hosts cough, an act which spreads these viruses. Diarrhea-causing organisms spread themselves in a similar fashion. And as for sheer sadism, it is hard to compete with the rabies virus, which makes its hosts irritable and aggressive and willing to bite -- and unable to swallow. Thus getting its host to inject it into another host with a bite.

Some changes in behavior help the host get eaten, but allow the parasite to change hosts. A fluke that infests a certain ant species changes that ant's phototropism, so the ant moves toward light rather than away from it, making it easy to eat by some grazing animal. The protozoan Toxoplasma makes its small-rodent hosts less suspicious of possible predators, like cats. Meaning that they become easier to catch and eat.

And some parasites castrate their hosts, meaning that their hosts keep growing continuously.

Here's a <a href="http://www.zoo.ufl.edu/bolker/eep-2000/syllab1.html" target="_blank">nice collection of pages on parasite evolution</a>, courtesy of a course on that subject.

Doubting Didymus

Thank you very much for those links, lpetrich. In most of these cases, including wolbachia, it looks like we are dealing with extended phenotype effects that have their origins in genetic mutations.

From the page on complex life cycles:

All three of these are compatible with a gene (or heredity) centric perspective. Note also that, while only the first example is referred to as 'selectionist', natural selection applies equally to the second example, constraint hypothesis. It would also apply to the third explanation unless it happens to be a neutral trait.

RufusAtticus

DD,

Wolbachia are a class of bacterial parasites. They infect the cytoplasm of invertebrates, especially insects. Because they infect the cytoplasm, they benifit from manipulating their hosts reproduction. In many instances "simply" causing their hosts to produce only daughters. There are many examples of populations and species being generated simply by wolbachia infection. Wolbachia infection can change a the traits of a population without affecting the hosts' nuclear genomes.

Browse this site fro more info: <a href="http://www.bacteriamuseum.org/species/Wolbachia.shtml" target="_blank">http://www.bacteriamuseum.org/species/Wolbachia.shtml</a>

Peez

Yup. This just illustrates the difference between the definition of something (species or evolution) and how we recognize it (reproduction vs morphometry or genetics vs morphological change). That may be so, but it is certainly not my impression. ??? You are saying that it is flawed just because it doesn't incude things that you wish to be included? I could just as easily argue that your definition is flawed because it includes things that I don't think should be included. Just because you use a different definition of "evolution" than I do does not make yours ‘correct' and mine "flawed". I am not familiar with the example, but if the phenotype is simply modified by the presence of a symbiont then I would not consider it evolution. That is one example of a phenomenon that you apparently consider "evolution" and I do not. Neither definition is automatically correct, and this is really a semantic argument. However, I believe that the "allele-centric" definition is more widely accepted, at the very least.

Peez

P.S.: I should add that I am using "allele" rather loosely here, so as to include any sequence of DNA (not just those that code for RNA).

[ October 22, 2002: Message edited by: Peez ]</p>

Peez

Could you please provide a few examples of non-nucleotide inheritance that "make substantial contributions to the form and function of organisms"?

Peez

pz

I presume that you've read D'Arcy Thompson's On Growth and Form? There's a whole book full of 'em, and Thompson doesn't mention genetics or genes except to disparage them (not that I agree with that bit of his attitude, of course).

RufusAtticus

Peez,

The difference here is that because of my linguistic backgroud I am a descriptionist. In that sense I am attempting to describe how the term "evolution" is actually used in the life sciences. A perscriptionist, like you, tries to define how it should be used. From my perspective, because the allele-centric defination doesn't cover some forms of evolution that are recognized by the scientific community it is incomplete. As scientists we should look to "describe" and not "perscribe."

Why not? The modification of phenotype is passed from generation to generation. If you observed the change of a population from sexual to asexual would you consider it evolution? Why should it matter what the underlying mechanism of this change was?

lpetrich

???

What DAT had described was lots of patterns of growth; what is interesting is that in many cases, only relatively simple algorithms are necessary to generate growth patterns. This, I'm sure, is ultimately tied into development-control genes, though the gene-&gt;shape cause-and-effect sequence is still far from clear.

pz

Yes, and one of the reasons they are far from clear is the persistent attempt to explain it entirely in terms of genes. Thompson had part of the story right: a substantial part of the patterns we see are independent of genes, and are 1) a consequence of the physico-chemical structure of the organism and its environment. Another part is due to 2) the epigenetic history of the tissue, which also cannot be described in terms of genes. And yet another part is 3) specifically shaped by gene action. Thompson ascribed almost all of it to (1), with a vague nod towards (2). Much of the recent history of biology ties it all to (3), denies that (1) is even interesting, and dismisses (2) as an inconsequential side effect of gene activity...as we are seeing on this thread.

I'm saying that (1), (2), and (3) are all indispensible components of the recipe for biology and the history of life on this planet.