The tree of life, what is it and what does it tell us?

[Tomk80]

There are many misconceptions about the twin nested hierarchy, what it shows and how it is arrived at. This thread is an attempt to clear some of these up. The main point of this thread will be to show that we can draw up such a tree without using evolution as reason for it. Rather, evolution is a conclusion from the tree instead of vice versa. I've created this thread because of the comments made by Edial in this thread, so I do hope he will participate himself.

So what is the tree of life? Basically, it is a way of grouping things. We want to divide creatures into groups to meaningfully categorize them. So the first thing to do is to see what is a meaningful way of grouping organisms, when we are talking about biology.

Now, the way we categorize organisms is essentially thought up by the creationist Linnaeus (1707-1778. Before Linneaus, there were many different ways of classification, giving rise to many different names, some more then 10 words long. Linnaeus got the idea of grouping organisms in larger groups, based on characteristics intrinsic to the organism itself. The groups are drawn up according to the similarities. This results in a hierarchy, where only two words were necessary to name an organism. This largely simplified name-giving. His system was refined by many others, for example John Ray and Richard Owen, but the basic idea stayed the same.

So the first question to ask is what we should use as characteristics to group animals meaningfully. Many characteristics can, and have been used in the past. For example, animals were grouped according to whether they could fly, swim or walk, or based on whether they can be eaten or not. But these characteristics do not tell us much. Both chickens, austriches and hawks are grouped in the same group, and all would agree. But autriches cannot fly and hawks aren't used as food. So we need something better, more specific.

Using a combination of all morphological and genetic traits of an organism to group organisms gives the most meaningful results, it is the most justified. This has two main reasons:
1. They are intrinsic to the organism; whether an organism can be used by us, for example, tells us more about us and what we can use (or can think of to use) than it tells us about the organism itself. Using the morphology and genetics of an organism, gives us meaninful groupings when talking about them in a biological context.

2. It is detailed; Referring to whether an organism can fly or not, tells us something about the organism, but not much. Looking at the wings is preferable, because it is more precise, more detailed.

So we have determined which characteristics to use. After comments have been given regarding this first post, I will move on to how we group things using these characteristics.

 

[JohnR7]

So what is the tree of life? Basically, it is a way of grouping things.

It does not work for evolution. They tried and tried and tried to get it to work, but it does not work.

What they ended up using was some of the charts and software for networking. That is a lot better and a lot easier then the old methoids they were using.

 

[Tomk80]

It does not work for evolution. They tried and tried and tried to get it to work, but it does not work.

What they ended up using was some of the charts and software for networking. That is a lot better and a lot easier then the old methoids they were using.

Two things:
1. The above is a tree. If you take the root (given in the middle), you can place that below and the three lines to Eucarya, Bacteria and Archaea above it and presto, tree.

2. I would ask you to stay on the current topic in this thread. We are not at the tree yet, although we'll get there in time. At this point, we are the part where we decide how we would classify things, without looking at what this results in yet. That'll come. For clarity, I ask you to keep on the current topic of discussion, so it will not drift into nothingness. If you have troubling following that line, please do not reply anymore.

 

[Nooj]

Tom, this is a great topic that you're trying to address here, one that is confused by many creationists. I appreciate you trying to simplify it so it's easy to understand,

If I may offer a tiny suggestion, when you say this:

We want to divide creatures into groups to meaningfully categorize them.

You could explain why we want to categorise animals. It seems pretty simple at first, but to make things as simple as they can be, you could say that without distinct groupings, we'd have a tough time differentiating between different animals.

Also, I believe the term is ostriches, not austriches.

This is a minor quibble. Good luck.

 

[Edial]

...The main point of this thread will be to show that we can draw up such a tree without using evolution as reason for it. Rather, evolution is a conclusion from the tree instead of vice versa. .

Interesting statement.

Why would you conclude with that last sentence when you are saying this below ...

......
So what is the tree of life? Basically, it is a way of grouping things. We want to divide creatures into groups to meaningfully categorize them. So the first thing to do is to see what is a meaningful way of grouping organisms, when we are talking about biology..

Meaningful, meaning what?

.........
For example, animals were grouped according to whether they could fly, swim or walk, or based on whether they can be eaten or not.

But these characteristics do not tell us much. Both chickens, austriches and hawks are grouped in the same group, and all would agree. But autriches cannot fly and hawks aren't used as food. So we need something better, more specific. .

If hawks are not used for food and are excluded for that reason, why is that?
Isn't being used for food a methodology that determines longevity of a kind? What is the purpose of a tree?
And why flightless ostriches are placed elsewhere?
I really would like to know what is the meaning of a tree if not supporting the theory of evolution?

.........Using a combination of all morphological and genetic traits of an organism to group organisms gives the most meaningful results, it is the most justified. This has two main reasons:
1. They are intrinsic to the organism; whether an organism can be used by us, for example, tells us more about us and what we can use (or can think of to use) than it tells us about the organism itself. Using the morphology and genetics of an organism, gives us meaninful groupings when talking about them in a biological context.

2. It is detailed; Referring to whether an organism can fly or not, tells us something about the organism, but not much. Looking at the wings is preferable, because it is more precise, more detailed.

So we have determined which characteristics to use. After comments have been given regarding this first post, I will move on to how we group things using these characteristics.

OK.
A term "meaningful" is used again.

OK. Go on.

Ed

 

[Tomk80]

Interesting statement.

Why would you conclude with that last sentence when you are saying this below ...

I have no idea what you mean here. I am going to try to show that grouping animals in the tree of life will be done consistently, without using the theory of evolution as a basis for it. So next, the first thing I described is what our criteria for grouping should be.

Meaningful, meaning what?

Meaningful is in properly reflecting nature. Meaning that we group animals together that really, objectively should fit in the same category. For example, we want to create a category named 'dogs' that contains all dogs, so that every time we encounter a new dog, we will know it is a dog and not a cat or a bird.

If hawks are not used for food and are excluded for that reason, why is that?

Because, if we group things according to whether they are food or not, and hawks are not food, we cannot group them in a group of animals that are food. So if we are grouping animals according to whether they are useful as food, hawks will not be excluded from the category of animals serving as food, but included in the category of animals not serving as food.

Isn't being used for food a methodology that determines longevity of a kind?

No, whether something is food or not, has little to do with longevity, but more with what we find tasty and what gives us enough energy, and whether we tried it before. In Europe, insects wouldn't quickly be categorized as food. Go to India, and they would. This has no other reason than that people from India did think up recipes for them, and we didn't. A maggot here has as much longevity as a maggot there, but there they eat them, here we don't.

What is the purpose of a tree?
And why flightless ostriches are placed elsewhere?

Let's not go to much into this. Would you agree that, if we want to categorize animals into groupings that accurately reflect nature, whether it be through evolution or through created kinds with a hierarchy, we have to use physical characteristics of the animal and not how we use them?

Some chickens are used as food, others as show animals, bred purely for their beauty. Do you agree that, if we want to define a chicken in a way that reflects nature (created or evolved), we will have to look at their characteristics and not how we use them?

I really would like to know what is the meaning of a tree if not supporting the theory of evolution?


OK.
A term "meaningful" is used again.

Creationists like Linneaus, who started categorizing animals, did this to try and reflect the hierarchy in nature. They were convinced that some created kinds were higher than others, with man on top. They wanted to group animals according to their 'kinds', so they could get insight in God's creation.

Maybe I should have been clearer in my wording, but you seem to be jumping ahead of the line of reasoning. You jump to the tree, but we're not their yet. We're now only at the point where we decide how we should categorize animals, regardless of whether this will give a tree or not. The way to group them should reflect nature, regardless of whether nature consists of seperate, independant creations or one great lineage from a common ancestor. That is the objective. We want to do this, so that everytime we encounter a certain animal, we will be able to say what kind of animal this is. If we encounter a new kind of wolf, we want to be able to tell it is a wolf, and not a cat, hyena or whale.

So we'll need to find a way to group organisms objectively, so that any time we see a chicken, we will also be able to determine that it is a chicken, and not a hawk, ostrich or pandabear. Would you agree that the way to do this, is by looking at their physical attributes and not at other things, like their usefulness to us or other ways of grouping that we can come up with?

This is an important point in understanding, and I do not want to carry on before it is understood.

 

[Edial]

... I am going to try to show that grouping animals in the tree of life will be done consistently, without using the theory of evolution as a basis for it.

OK.

... Meaningful is in properly reflecting nature. Meaning that we group animals together that really, objectively should fit in the same category. For example, we want to create a category named 'dogs' that contains all dogs, so that every time we encounter a new dog, we will know it is a dog and not a cat or a bird..

OK.

... Because, if we group things according to whether they are food or not, and hawks are not food, we cannot group them in a group of animals that are food. So if we are grouping animals according to whether they are useful as food, hawks will not be excluded from the category of animals serving as food, but included in the category of animals not serving as food.

OK. I found this. I mean that, correct?

... No, whether something is food or not, has little to do with longevity, but more with what we find tasty and what gives us enough energy, and whether we tried it before. In Europe, insects wouldn't quickly be categorized as food. Go to India, and they would. This has no other reason than that people from India did think up recipes for them, and we didn't. A maggot here has as much longevity as a maggot there, but there they eat them, here we don't. .

Actually I misstated, I did not mean longevity, but availability (like in being in a danger of extinction).

... Let's not go to much into this. Would you agree that, if we want to categorize animals into groupings that accurately reflect nature, whether it be through evolution or through created kinds with a hierarchy, we have to use physical characteristics of the animal and not how we use them? .

Fine.

... Some chickens are used as food, others as show animals, bred purely for their beauty. Do you agree that, if we want to define a chicken in a way that reflects nature (created or evolved), we will have to look at their characteristics and not how we use them?.

Fine.

... Creationists like Linneaus, who started categorizing animals, did this to try and reflect the hierarchy in nature. They were convinced that some created kinds were higher than others, with man on top. They wanted to group animals according to their 'kinds', so they could get insight in God's creation.

Fine.

... Maybe I should have been clearer in my wording, but you seem to be jumping ahead of the line of reasoning. You jump to the tree, but we're not their yet. We're now only at the point where we decide how we should categorize animals, regardless of whether this will give a tree or not. The way to group them should reflect nature, regardless of whether nature consists of seperate, independant creations or one great lineage from a common ancestor. That is the objective. We want to do this, so that everytime we encounter a certain animal, we will be able to say what kind of animal this is. If we encounter a new kind of wolf, we want to be able to tell it is a wolf, and not a cat, hyena or whale.

Fine.

... So we'll need to find a way to group organisms objectively, so that any time we see a chicken, we will also be able to determine that it is a chicken, and not a hawk, ostrich or pandabear. Would you agree that the way to do this, is by looking at their physical attributes and not at other things, like their usefulness to us or other ways of grouping that we can come up with?.

Primarily, yes.

... This is an important point in understanding, and I do not want to carry on before it is understood.

Fair enough.

Go on.

Ed

 

[mark kennedy]

With a seemingly endless string of transmutations Darwin thought you could trace lineages all the way back to primordial soup. We now know that the blending of characterisitcs Darwin based his conjecture on is completly mistaken. All the tree of life represents is an a priori assumption of a single common ancestor with limitless genetic transpostion. Natural selection is rethoric, all it ever did was reject special creation without considering it as a valid possibility.

 

[Apos]

No, special creation became increasingly impossible to maintain given the evidence: at the same time evidence was mounting for common descent. Today, that evidence for common descent is overwhelming, and the only way to sustain special creation is by reference to near daily ad hoc miracles (all of which merely work towards the end of making special creation look as much like common descent as possible), or a matrix-like denial of reality.

We knew that the blending theory was wrong even before Darwin died: he agreed that it wouldn't make sense pretty quickly after he proposed it: it was basically his best guess as to how heredity might work, and it was wrong.

And in fact, that was a positive development for common descent, not a blow. If blending had been true, then the rates of mtuation necessary to maintain variety would have had to be implausibly high.

However, blending wasn't true: Mendelian genetics was true instead. And it was far far more in line with exactly what common descent required to make sense. Furthermore, ironically, genetic transposition is only "limitless" in the case of special creation. Common descent and genetics, on the other hand, impose all sorts of very veyr specific limits on the way genes can change and move. The fact is, all known observed evidence of the natural world follows these very perculiar limits.

Why would it do that, unless the account of common descent is true

 

[mark kennedy]

No, special creation became increasingly impossible to maintain given the evidence:

That is simply not the case, common ancestory has never had any serious basis beyond the level of genus. Special creation was the object of Darwin's argument and he only refuted immutabilty of species.

at the same time evidence was mounting for common descent.

That is a sweeping generality, the assumption of a common ancestor for apes and humans predates the advent of genetics by at least half a century. Mounting evidence? Try ad hoc conjecture.

Today, that evidence for common descent is overwhelming, and the only way to sustain special creation is by reference to near daily ad hoc miracles (all of which merely work towards the end of making special creation look as much like common descent as possible), or a matrix-like denial of reality.

Baloney, everything in biology is viewed through the lens of common ancestory. Special creation is never considered an option this refracts and distorts the evidence beyond recognition. The evidence itself is unchanged by your being either a creationists are a Darwinian materialist.

We knew that the blending theory was wrong even before Darwin died: he agreed that it wouldn't make sense pretty quickly after he proposed it: it was basically his best guess as to how heredity might work, and it was wrong.

Darwin was wrong about a whole lot of things, the only thing to survive him is the term natural selection. Survival of the fittest is not a reliable adaptive mechanism, recombination of genes is how diversity transposes living systems. There are also prions that turn genes on and off and a number of regulatory genes that also facilitate adaptation. Blending of characteristics is still the heart of the problem with Darwinian evolution and the convoluted assumption of the Single Common Ancestor Model.

And in fact, that was a positive development for common descent, not a blow. If blending had been true, then the rates of mtuation necessary to maintain variety would have had to be implausibly high.

Now that is an extraordinary leap in logic, common descent has no explanation for the level of divergance between transitions from kingdom level (bacteria to plantea and animalia cells) to human evolution and the expansion of the Homo sapien brain. It does not happen in the natural world and yet we are supposed to assume it happened in natural history. I'm not buying it.

However, blending wasn't true: Mendelian genetics was true instead. And it was far far more in line with exactly what common descent required to make sense. Furthermore, ironically, genetic transposition is only "limitless" in the case of special creation. Common descent and genetics, on the other hand, impose all sorts of very veyr specific limits on the way genes can change and move. The fact is, all known observed evidence of the natural world follows these very perculiar limits.

That is the biggest rationalization yet, transpostion of specially created kinds is an impossiblity both in creationism and Mendelian genetics. It is only in Darwinian rationalizations of the evidence that transpostions on a macro scale is even concievable.

Why would it do that, unless the account of common descent is true

There are measurable and obvious limits beyond which species cannot change. That is a scientific fact that no self-respecting scientist would deny. Yet, in the course of accumulating actual evidence common descent is never seriously question because the alternative (special creation) is objectionable to peoples naturalistic assumptions. Common descent for humans is perfectly scientific while a common ancestor for chimpanzees and humans have no genetic basis other then an a priori assumption.

 

[Tomk80]

Could you people please limit this to the point at hand. I'll be happy to respond to all your points, just as to John's points, at the appropriate time in the thread. Now is not it.

Discussion at hand: When we want to categorize animals, regardless of the outcome of this, we should use the physical characteristics of the animals to do this.

We'll get to whether that categorization results in a tree and what can be concluded from in time, and then we can happily discuss that. What I don't want is jumps in the flow of reasoning.

 

[Loudmouth]

That is simply not the case, common ancestory has never had any serious basis beyond the level of genus.

Baloney. Vestigial pelvises and atavistic legs in whales is strong evidence that whales share common ancestory with terrestrial mammals. Shared ERV's solidly evidence common ancestory between whale and ungulates.

Special creation was the object of Darwin's argument and he only refuted immutabilty of species.

More evidence has surfaced since then, including the evidence I listed for whale evolution. I guess special creation only works if you ignore the last 150 years of scientific research.

That is a sweeping generality, the assumption of a common ancestor for apes and humans predates the advent of genetics by at least half a century. Mounting evidence? Try ad hoc conjecture.

How are shared ERV's, intermediate fossil species, and a well understood chromosomal fusion ad hoc conjectures? Looks like evidence to me.

Baloney, everything in biology is viewed through the lens of common ancestory.

Everything in astronomy is viewed through the lens of teh Earth orbitting the Sun. So what? You need to show that common ancestory is wrong.

Special creation is never considered an option this refracts and distorts the evidence beyond recognition.

Why should science go with an idea that has been shown to be false?

The evidence itself is unchanged by your being either a creationists are a Darwinian materialist.

What evidence shows that divergent species are unrelated?

Darwin was wrong about a whole lot of things, the only thing to survive him is the term natural selection. Survival of the fittest is not a reliable adaptive mechanism, recombination of genes is how diversity transposes living systems.

The genes that recombine the most belong to the fittest in the population.

There are also prions that turn genes on and off and a number of regulatory genes that also facilitate adaptation.

The effects of those prions and the ability to adapt are hereditary. Natural selection controls heredity from generation to generation.

Blending of characteristics is still the heart of the problem with Darwinian evolution and the convoluted assumption of the Single Common Ancestor Model.

Genes are passed on as units, not as melting pot. The issue was settled long ago.

Now that is an extraordinary leap in logic, common descent has no explanation for the level of divergance between transitions from kingdom level (bacteria to plantea and animalia cells) to human evolution and the expansion of the Homo sapien brain.

The cause of change in the human lineage is seen in the difference between chimp and human genomes. Or we can go with special creation and just say "God Did It." Which do you think is more useful from a scientific and research driven perspective?

It does not happen in the natural world and yet we are supposed to assume it happened in natural history. I'm not buying it.

Actually, the evidence tells us that it did happen.

There are measurable and obvious limits beyond which species cannot change. That is a scientific fact that no self-respecting scientist would deny.

Then evidence these limits with reference to fossil and DNA data.

Yet, in the course of accumulating actual evidence common descent is never seriously question because the alternative (special creation) is objectionable to peoples naturalistic assumptions.

If special creation were true we would not expect to see a nested hierarchy. If evolution were true we would expect to see a nested hierarchy. We see a nested hierarchy. Evolution is accepted and special creation is rejected. No assumptions necessary.

Common descent for humans is perfectly scientific while a common ancestor for chimpanzees and humans have no genetic basis other then an a priori assumption.

Common ancestory of all apes is based on evidence, such as fossils and shared ERV's.

 

[Tomk80]

OK. I found this. I mean that, correct?

That isn't exactly what I meant. Maybe another way to illustrate what I meant, although I have the feeling it was already better explained in my last post.

When categorizing animals for the point of naming them, the categories need to be objective, so that they are the same to everyone, regardless of who looks at them. That's the problem with the categorization of animals by using whether they are food to us or not.

Suppose I am going to categorize organisms and my friend Abdul from India is doing the same. We categorize them according to "purpose". Now we encounter a cow. For me, a cow is food, so I place it in the "food"-category. Abdul however is not satisfied over this, because cows are holy and because of this cannot be used as food. So he places it in the "worship"-category.

A different problem arises when we want to categorize chickens. Now, we encounter a D'Uccle.

But d'Uccles are not used as "Food", they are mainly show animals. So we put a d'Uccle in a different category then our more usual chicken, like the Catalan.

We want a standard that puts every chicken in the same category, and let's Abdul and me put cows in the same category. That would be an objective standard reflecting nature, instead of our subjective uses of the animal. Using physiological characteristics allows for this. If we do that, cows will be placed in the same category "Cows" by Abdul and me, and all chickens will be placed in the same category "Chicken" by us.

 

[rmwilliamsll]

There are also prions that turn genes on and off and a number of regulatory genes that also facilitate adaptation.


i have never seen this claim for prions before.


via googling i found:
The function of the prion protein gene (PRNP) and its normal product PrP(C) is elusive.
from: http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=15777726&dopt=Abstract

but no evidence that the prion is ever a regulatory protein.


found some very nice pictures of protein-dna interactions at
http://www.cs.stedwards.edu/chem/Chemistry/CHEM43/CHEM43/motifs/motifcover.html

does anyone know where this claim comes from?
or is it just a simple error?

 

[Tomk80]

There are also prions that turn genes on and off and a number of regulatory genes that also facilitate adaptation.


i have never seen this claim for prions before.


via googling i found:
The function of the prion protein gene (PRNP) and its normal product PrP(C) is elusive.
from: http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=15777726&dopt=Abstract

but no evidence that the prion is ever a regulatory protein.


found some very nice pictures of protein-dna interactions at
http://www.cs.stedwards.edu/chem/Chemistry/CHEM43/CHEM43/motifs/motifcover.html

does anyone know where this claim comes from?
or is it just a simple error?

I have the feeling that it is an error. From wikipedia on Prions:
"Some prion diseases (TSEs) can be inherited, and in all inherited cases there is a mutation in the Prnp gene. Many different Prnp mutations have been identified and it is thought that the mutations somehow make PrPC more likely to spontaneously change into the PrPSc (disease) form. TSEs are the only known diseases that can be sporadic, genetic or infectious; for more information see the article on TSEs.
Although the identity and general properties of prions are now well-understood, the mechanism of prion infection and propagation remains mysterious. It is generally assumed that PrPSc directly interacts with PrPC to cause the normal form of the protein to rearrange its structure (enlarge the diagram above for an illustration of this mechanism). One idea, the "Protein X" hypothesis, is that an as-yet unidentified cellular protein (Protein X) enables the conversion of PrPC to PrPSc by bringing a molecule of each of the two together into a complex.[9]"

 

[Edial]

That isn't exactly what I meant. Maybe another way to illustrate what I meant, although I have the feeling it was already better explained in my last post.

When categorizing animals for the point of naming them, the categories need to be objective, so that they are the same to everyone, regardless of who looks at them. That's the problem with the categorization of animals by using whether they are food to us or not.

Suppose I am going to categorize organisms and my friend Abdul from India is doing the same. We categorize them according to "purpose". Now we encounter a cow. For me, a cow is food, so I place it in the "food"-category. Abdul however is not satisfied over this, because cows are holy and because of this cannot be used as food. So he places it in the "worship"-category.

A different problem arises when we want to categorize chickens. Now, we encounter a D'Uccle.

But d'Uccles are not used as "Food", they are mainly show animals. So we put a d'Uccle in a different category then our more usual chicken, like the Catalan.

We want a standard that puts every chicken in the same category, and let's Abdul and me put cows in the same category. That would be an objective standard reflecting nature, instead of our subjective uses of the animal. Using physiological characteristics allows for this. If we do that, cows will be placed in the same category "Cows" by Abdul and me, and all chickens will be placed in the same category "Chicken" by us.

OK.

 

[Tomk80]

Okay, with the characteristics that we are going to use decided, we can now go on to look at the method of categorization. For that, we will start at the absolute basics, because this is another thing that often is misunderstood. How do we group things or organisms? How do we create such groups? Now, this is an easy concept once you realize how it works, but I've noticed it is hard to understand if you don't and I find it difficult to explain. Nevertheless, I'll try.

The important thing to realize here is that we look at all the things that all members in a group have, to determine which members belong in that group. The unique characteristics of one single member do not decide whether it is inside or outside the group. What decide this, is whether it has all the characteristics the other members of the group also have.

To take an example, let's take airplanes. Suppose we group an F16, a Harrier and a Cessna. We'll also take a Ferrari F40 with us, as comparison.

Now, we could first put all aircraft in a group. We would do that by determining that they have wings, which is a determining characteristic of airplanes. They are also wings creating an uplift, meaning they'll go up if they get a high speed.

So, now we can first group the F16, Harrier and Cessna in one group, based on the wings with uplift. Because a Ferrari does not have this characteristic, a Ferrari is not an airplane. That is clear so far? Pretty simple, right?

But now comes the important part. Suppose someone wants to make the argument that a Harrier is not an airplane, because the exhausts can be turned in such a way that it can lift off vertically. Does this suddenly make the Harrier not a plane? Of course not! The Harrier is a plane, because it still has wings that create an uplift. No matter what other characteristics the Harrier has, whether it can jump, fly radiocontrolled or play the violin doesn't matter. It has all the defining characteristics of an airplane and because of that, it is an airplane.

That is how we create groups. We look at all the characteristics that an organism or any other thing (because it works outside the biological world exactly the same) and describe all the things that those organisms or things have in common. If we then want to determine whether some new organism or thing belongs in that category, we look whether it shares those same characteristics. If it does, it belongs in the same categor, no matter which other characteristics it might have.

 

[Edial]

OK.

Planes and cars we know. And it is a good analogy.

But there are some organisms that we do not know and would have great difficulty grouping them, since some characteristics simply could go either way ... but let's go on.

Agreed.

Ed

 

[Tomk80]

OK.

Planes and cars we know. And it is a good analogy.

But there are some organisms that we do not know and would have great difficulty grouping them, since some characteristics simply could go either way ... but let's go on.

Agreed.

Ed

Well, that's the whole point in taxonomy. To describe a group in such a way that they cannot go either way. That means that the description has to be clear. Of course, the more detailed we go (ie, the smaller the group becomes) the harder it gets to create a clear group.

Also, what should be stretched is that above was an example based on one characteristic. In reality, we do not use one characteristic, but many. We do not create groups based on a single characteristic, but on a combination of many, which makes the groupings clearer.

Also, what should be stretched is that my example of cars and airplanes is not exactly an analogy, because the exact same method as we use on them is used on organisms. Even the characteristics (physical characteristics of the car/aeroplane) are the same. The only reason we won't group animals with cars or airplanes, is because cars and airplanes are not organisms and thus have none of their characteristics. The method of grouping I used above (if a bit simplistic for illustration) is completely the same.


Now, at this point I'm a bit in doubt on how to proceed. I'm still deciding whether I'll now go straight to the categorizing itself, or discuss the exact methods a bit further. I want to discuss both yet, but I'm still thinking about the order, as the exact methods don't matter too much for the general basics I can use in discussing the categories. I'm also a bit torn on whether I want to begin at the most general categorization (ie, the 'bottom' of the tree) or the most specific (ie, the top). So I might take a couple of days to think that out.

 

[Tomk80]

Moving on, one more concept that is important to learn is that of homologie and analogie. It ties right in with using characteristics of the animal itself as a basis for classification. The creationist sir Richard Owen came up with these terms. Owen described this as: "the same organ in different animals under every variety of form and function." The opposite is analogy, which is the same function, but stemming from a different organ.

What did he mean by this? Let's first take homology. Examine the skeletal structures of the limbs below (klick the link for a larger picture):

The flipper of a whale, the wing of a bat, the arm of a human and the paw of a cat all have the same basic structure. Sure, the different bones have a different length, but all the bones it is made up of are the same bones, only modified. This is what we call homology.

Analogy is the opposite. Take a look at this picture. The wings of an insect, pterodactyl (flying reptile), bird and bat all have approximately the same shape. However, with insects, no skeleton is present. In birds, bats and pterodactyles, the same homologuous structure becomes apparant again, although all modified in different ways.