The reality or otherwise of Common Descent

[Didymus]

As per the request in this thread, I am starting this thread to debate the question of common descent. I'm perfectly happy to kick off the discussion with a presentation of a tiny slice of the evidence biologists consider established common descent.

Common descent is a historical claim about the evolutionary past of organisms. It refers to the idea that all organisms are related to one another like a gigantic extended family. For example: five million years ago, chimpanzees and humans did not exist separately, but were represented by a single species. Approximately 4.5 million years ago that population underwent speciation and each new species (at that point very similar to one another), gradually diverged. According to all of mainstream biology, this principle holds true for all life, so that every species shares ancestry with every other at some point in the distant past.

To make this claim, biologists have a large amount of evidence that they think establishes common descent as a very robust scientific claim. The famous talkorigins '20+ evidences for macroevolution' article is actually a list of evidence for common descent, and doesn't actually have much to do with macroevolution as modern biology uses the term.

Rather than use that article here, I will instead go strait to the most authoritative sources I have access to, and give a quick summary of the overview of the evidence from Douglas J. Futuymas Evolutionary Biology 3rd edition, the most respected general textbook in the field.

Futuyma lists eight categories of evidence, each of which would require a fair amount of elaboration. I'll give the full list before expanding on one or two.
1. The hierarchical organisation of life.
2. Homology
3. Embryological similarities
4. Vestigial characters
5. Convergence
6. Suboptimal design
7. Geographical distributions
and 8. Intermediate forms.

I'll expand a little on 1 and 8 before throwing open the floor.

Life on earth exists in a nested hierarchy. This means that if you divide organisms into groups according to their features, you can then divide each of those groups into more groups without overlapping. So, if one divides the tetrapods from the arthropods, one can then pick out smaller groups; say, the mammals out of the tetrapods, and the insects from the arthropods, and those groups will not overlap in any new features that are not present in their parent groups. Meanwhile, if you try to group cars first by engine size, and then by colour, you'll find that there is a lot of overlap of features in groups that aren't present in those groups parent categories. So both red cars and blue cars can have both V6 and V12 engines, but triple segmentation with six legs is found only in arthropod groups (namely insects), while milk is found only in tetrapod groups (namely mammals).

This is evidence for common descent, because this pattern of nested hierarchy is produced by a historic pattern of descent and divergence, and by hardly any other types of processes. For example, tracing the surnames in a family tree yields a nested hierarchy: if one branch of the plantaginet family changes its name to plantaginet-smith, then you'll find plantaginet smith only in the larger plantaginet extended family. Only objects that have been produced by descent and branching fall into true nested hierarchies.

This fact about living things allows us to do all sorts of things in the exciting world of molecular biology. Nested hierarchies are what enable us to produce phylogenetic trees of life, which are essentially hypotheses about a particular pattern of descent. Using the same methods we now use to establish paternity from blood samples in court, we establish the relationships of species. The most striking thing is that we can yield the same phylogenetic tree by analysing just about any genetic feature: be it a gene, or mitochondrial DNA, or a particular protein sequence like hemoglobin, or most tellingly, endogenous retroviral insertions (I can expand on this last one if it's unfamiliar), the same tree keeps appearing again and again. This would be inexplicable if all species had separate origins, but is exactly what common descent predicts.

Now, to intermediate forms. Here, I will list the fossilised forms of three major evolutionary lineages, from three of my favourite books. I can't supply the diagrams, obviously, but I will give page numbers for anyone who wants to check up on me.

First, it should be established that according to the scenario of independent creation of organisms, intermediate sequences should not exist at all. Given that, the following three gradual evolutionary sequences establish the evolutionary history of these organisms: lizards to mammals, four-legged artiodoctyls to whales, and hyracotherium to modern horses.

From Ridleys Evolution text: The following fossils form a transitional lineage: Reptilian pelycosaurs to Ophlacodontids (which also gave rise to the famous Dimetrodon ), to the broadly varying Therapsids, from amongst whos number came the Cynodots, representing whom there are many individual specimens, such as(in approximate evolutionary order):

Procynosuchus
Thrinaxodon (here's another thrinaxodon)
Diademodon
Probainognathus

These specimens become increasingly less reptilian and begin to accumulate mammilian features over time. The latest cynodonts in this series blend neatly into the early mammals.

I'm running short on time, so I'll finish whales and horses quickly.

Refer to At the Waters Edge by Carl Zimmer for more information on this graduation from four legged pakicetus to modern whales:

Pakicetus, Ambulocetus, Dalanistes, Rodhocetus, Takracetus, Gaviocetus, Basilisaurus, Dorudon, Mysticetes (modern baleen whales). Googling these names might yield more information as well.

Refer to Strickberger and Monroes' Evolution textbook for more information on the transition from the small 55 million year old Hyracotherium, through the Oligocene Miohippus and Miocene Merychippus to modern Equus. This is a famously well documented transition, so it should be easy to find online resources.

But right now, I've made myself late. I suppose the topic of this debate should be: 'Do these observations, which are commonly advanced by mainstream biology, provide evidence for common descent?'

Good night and enjoy the debate.

[otseng]

Let's take the mutated growth of extra legs as an example. In the lab, having an extra pair of legs is not an advantage nor a disadvantage.

I assume you are talking about the antennapedia mutation in which the antennas are replaced by leg-like structures growing out of its head. From my understanding, the legs are not functional. And even if they were functional, the antennas, which are essential to the life of the fly, were sacrificed.

Also, this is an example of things that we do not find in the fossil record. If mutations such as these are supposed to be common, shouldn't we find evidence of antennapedia mutations (or other strange mutations) in the fossil record?

Furthermore, the studies of fruit fly mutations certainly didn't introduce any new beneficial morphological features. What we see are existing morphological features being placed somewhere else, smaller/curly wings, the removal of the eyes, etc. (Some pictures of Mutant Flies)

[ST88]

Let's take the mutated growth of extra legs as an example. In the lab, having an extra pair of legs is not an advantage nor a disadvantage.

I assume you are talking about the antennapedia mutation in which the antennas are replaced by leg-like structures growing out of its head. From my understanding, the legs are not functional. And even if they were functional, the antennas, which are essential to the life of the fly, were sacrificed.

Point taken. But this part of my discussion was purely speculative. My point was that we do not know if any of these mutations could be adaptations in the real world. There are birds with wings that are not used for flying, snakes with legs that are not used for locomotion, and bears with tails that are not used for balance. What I mean is, based on a previous definition of what a structure does, it is possible that we may not recognize how a familiar structure may operate in a different way.

Furthermore, there are plenty of examples of animals that have certain missing or de-emphasized morphological features in favor of others. Bats are a prime example of this, having little need for their limited vision since their hearing is so well-developed. Or, as a better example, wings instead of arms, so that their ground locomotion is different from other mammals and their grasping ability is severely limited, if present at all. Granted, an extra pair of legs in place of vital antennae in the insect world might not be a good trade. But I hope you see my point.

Also, this is an example of things that we do not find in the fossil record. If mutations such as these are supposed to be common, shouldn't we find evidence of antennapedia mutations (or other strange mutations) in the fossil record?

Firstly, these mutations are not common. The fruit fly experiments are done in conditions that are not found in nature (to my knowledge). The amount of radiation that is actually found in nature causes very few mutations in a generation.

Secondly, the fossil record is by no means a complete research library. Many areas of the world, such as the vast majority of the Asian continent, and parts of Africa and South America have been in political turmoil or have been politically hostile to outsiders since scientists began studying the fossil record, impairing fossil research. These huge areas have not been adequately surveyed for fossils.

Thirdly, there are transitional fossils in the record. Though I don't know of any insect fossils that meet this definition, there are other families which have fossil transitional forms. This website lists numerous transitional forms in the fossil record. Naturally, some of these will be in dispute. Some of the transitional forms include rabbit-like rodents that are not quite rabbits in a full form, but do have some rabbit characteristics. Other examples are present, like bony fish and amphibians, but perhaps the most spectacular example is the land whale section. Almost all of these discoveries are recent (on the Asian continent). Some of them - like Basilosaurus - were thought to be other animals when they were found because they share certain traits with other genera. These "land whale" fossils have transitional forms such as Ambulocetus Natans which has large hind legs that are unsuitable for efficient land locomotion, but are suitable for water, and yet has a tympanic ear that is not useful for underwater hearing.

Furthermore, the studies of fruit fly mutations certainly didn't introduce any new beneficial morphological features. What we see are existing morphological features being placed somewhere else, smaller/curly wings, the removal of the eyes, etc. (Some pictures of Mutant Flies)

Naturally, it is highly likely that these studies will never produce an individual that is not a fruit fly. But this is not because it isn't possible. It is because natural selection is a process that doesn't necessarily involve just radiation exposure in a lab. If the goal was to produce an individual who could be said to be "not a fruit fly", then the experiment would have been very different.

[otseng]

But this part of my discussion was purely speculative.

So, my question then still stands, what evidence do we see of mutations that produce any new morphological features? (I will drop the beneficial adjective for now, but I will say that it should at least be functional.)

Furthermore, there are plenty of examples of animals that have certain missing or de-emphasized morphological features in favor of others.

I would argue that missing or de-emphasized morphological features actually argues against common descent. What common descent requires are new morphological features.

The fruit fly experiments are done in conditions that are not found in nature (to my knowledge). The amount of radiation that is actually found in nature causes very few mutations in a generation.

If "fruit fly experiments are done in conditions that are not found in nature", then it cannot be used to support common descent.

Thirdly, there are transitional fossils in the record. Though I don't know of any insect fossils that meet this definition, there are other families which have fossil transitional forms. This website lists numerous transitional forms in the fossil record. Naturally, some of these will be in dispute. Some of the transitional forms include rabbit-like rodents that are not quite rabbits in a full form, but do have some rabbit characteristics.

The general problem with examples such as those is "how do we know they are transitional forms"? How can one objectively conclude that one animal is a "transition" between two other animals? Because they have morphological features common with other animals doesn't by itself prove that it is a transitional form.

Naturally, it is highly likely that these studies will never produce an individual that is not a fruit fly. But this is not because it isn't possible. It is because natural selection is a process that doesn't necessarily involve just radiation exposure in a lab. If the goal was to produce an individual who could be said to be "not a fruit fly", then the experiment would have been very different.

It might be probable, but it is purely conjecture if there are no evidence to support such a claim.

[ST88]

Furthermore, there are plenty of examples of animals that have certain missing or de-emphasized morphological features in favor of others.

I would argue that missing or de-emphasized morphological features actually argues against common descent. What common descent requires are new morphological features.

Please state your argument for this.

The fruit fly experiments are done in conditions that are not found in nature (to my knowledge). The amount of radiation that is actually found in nature causes very few mutations in a generation.

If "fruit fly experiments are done in conditions that are not found in nature", then it cannot be used to support common descent.

This part of my argument was in response to the claim that:

I understand that natural selection would weed out the features that are not beneficial. But, shouldn't we see a preponderance of features that are not beneficial? Half-developed organs, malformed features, 3 legged spiders,etc? Of course, these animals would die rather quickly. But, the fossil record does not have much evidence of animals (or plants) with these such features.

My point here was that we don't need fossils to show that this is true, because we have live evidence that this sort of thing does happen. The fact that it doesn't happen on the same scale as the experiment is irrelevant to the point. The lab studies merely sped up the population's mutation rate.

Thirdly, there are transitional fossils in the record. Though I don't know of any insect fossils that meet this definition, there are other families which have fossil transitional forms. This website lists numerous transitional forms in the fossil record. Naturally, some of these will be in dispute. Some of the transitional forms include rabbit-like rodents that are not quite rabbits in a full form, but do have some rabbit characteristics.

The general problem with examples such as those is "how do we know they are transitional forms"? How can one objectively conclude that one animal is a "transition" between two other animals? Because they have morphological features common with other animals doesn't by itself prove that it is a transitional form.

If it is unclear whether or not any of these fossils are transitional forms, then what would be an acceptable example of such?

I gather from other posts on this forum that microevolution is a perfectly acceptable theory for many Creationists. That small mutations in the DNA make changes in the cat line, but do not change the essential "cat-ness", if you like. Fair enough. This is a thread on common descent and not C vs. E, after all.

However, common descent is not incompatible with microevolution either. The point I was getting at in my cat example is that there are traits that are not presently in the DNA of organisms, but that can show up later anyway. These traits are completely unknown. The commonality of DNA in cats implies that all the variations in cats came from an original proto-cat; whether that was an original evolution-based ancestor, or a Creationist model cat (or groups of cats - i.e, one original leopard gave rise to jaguar, jaguarundi, ocelot, etc.), this fact should not be in dispute.

This also means that all the DNA of members in those groups should be possible variations of the groups (with microevolutionary alterations "here and there"). How these groups are grouped is much in dispute for many individuals. But I have noticed after doing some research on the problem that for every individual that could possibly be an exception to this, i.e., is not specifically in one group or another, either physically speaking or genetically speaking, the Creationist model has an answer: It is its own separate group.

That is, the Creationist argument points out that there are no missing links that will help prove the common descent component of evolution. But when evidence for a missing link is presented, the Creationist model states that it is not, in fact, a link at all, but a separate group that just happens to have some of the features of one species and some of the features of another species. In other words, there are no possible proofs using this argument. Not that there isn't any evidence, but the standard of proof gets moved every time evidence is offered.

I can see this in the Giant Panda. Recent DNA studies have shown that it can tentatively be placed in the bear family, though it lack some key bear features and has some interesting differences. Bears produce a gastrointestinal fluid called ursodeoxycholic acid (other mammals do also, but bears do in very large amounts -- so large that they are farmed for it in Asia). This acid helps break down waste materials so that bears can hibernate or enter long periods of inactivity. However, the Giant Panda does not produce this acid. Further, among other differences, Giant Pandas are not carnivores (though they have carnivore traits), they have a different birth cycle, and thay have pseudo-thumbs - sixth digits on their hands.

Common descent says that Giant Pandas diverged from the current bear line instead of being a step in that line. If this is true, then there must be some earlier proto-bear that most likely resembles one of the other existing seven species of bears. But in Creationism, it cannot be true that this kind fo multi-mutational organism has microevolutionarily changed from an earlier proto-bear. The natural solution here is to place the Giant Panda into a family of its own. (Some evolutionary biologists are pushing for the Giant Panda to be in a family of its own, similar to the way the aardwolf is distinct from hyenas; though this does not change its lineage.)

What I am saying is that this line of thinking seems to be an exercise in futility. Am I wrong? Is it probable that any transitional form presented as such would be rejected as a co-incidence?

[otseng]

I would argue that missing or de-emphasized morphological features actually argues against common descent. What common descent requires are new morphological features.

Please state your argument for this.

What common descent requires are new morphological features arising from one generation to the next (occurring over a long span of time). Yet we see no evidence of this happening through mutation studies. We do however see missing features through mutation studies (fruit flies with no eyes). The Creation Model fits more with the data than the Common Descent model. CM doesn't require any new morphological features to come about through mutations and, in fact, it says that it cannot happen. The CM also predicts that the only morphological changes that can happen are the losing of features, which is exactly what we see in our observations. Therefore, the Creation Model fits more with the evidence than the Common Descent model.

I gather from other posts on this forum that microevolution is a perfectly acceptable theory for many Creationists. That small mutations in the DNA make changes in the cat line, but do not change the essential "cat-ness", if you like. Fair enough. This is a thread on common descent and not C vs. E, after all.

Microevolution is accepted by Creationists. There is no dispute over that.

However, common descent is not incompatible with microevolution either.

Yet common descent is quite an extrapolation from microevolution. One can easily jump over a 2 foot hole. But it doesn't mean you can jump over a 100 foot hole.

The commonality of DNA in cats implies that all the variations in cats came from an original proto-cat; whether that was an original evolution-based ancestor, or a Creationist model cat (or groups of cats - i.e, one original leopard gave rise to jaguar, jaguarundi, ocelot, etc.), this fact should not be in dispute.

One proto-cat giving rise to all the different cats we see today is not in dispute.

What I am saying is that this line of thinking seems to be an exercise in futility. Am I wrong? Is it probable that any transitional form presented as such would be rejected as a co-incidence?

Simply relying on similar features to argue common descent is not sufficient. There has to be more evidence. For example, if two animals are determined morphologically to be direct ancestors and the molecular analysis shows that they are genetically 99.999 similar, that is a more convincing argument. If a study showed that accelerated mutations of a single cell organism turned into a multicellular organism, that would also be a strong argument.

[ST88]

What common descent requires are new morphological features arising from one generation to the next (occurring over a long span of time). Yet we see no evidence of this happening through mutation studies. We do however see missing features through mutation studies (fruit flies with no eyes). The Creation Model fits more with the data than the Common Descent model. CM doesn't require any new morphological features to come about through mutations and, in fact, it says that it cannot happen. The CM also predicts that the only morphological changes that can happen are the losing of features, which is exactly what we see in our observations. Therefore, the Creation Model fits more with the evidence than the Common Descent model.

But what is the difference between gain and loss? In terms of evolution, I do not see a distinction between the addition of features and the loss of features. Traits are gained and lost as the environment changes. Speciation is as haphazard as natural disasters.

Now that I know where you're coming from, I have to say, I think that this version of Creationism is untenable. Science has yet to truly crack the genome of the vast majority of species. Assuming that this position is true, and that the existing evidence points to this CM, this allows for no exceptions. It should be noted that these theories, on either side, cannot be proven with the evidence we have, only either supported or disproved. If the two can be said to be pitted against one another (a view I don't share), common descent has the advantage here, if only because it is not a dogma, but is itself a theory that evolves as we learn more about the evidence.

The fact that this CM fits current evidence better is a matter of opinion over the word "better". The fit may stem from the fact that this version of Creationism is a recent creation and is therefore structured to fit the current evidence. This isn't to say that current theories of common dscent and evolution have not done this either, but the latter two allow for theoretical model alterations if the evidence warrants.

common descent is not incompatible with microevolution either.

Yet common descent is quite an extrapolation from microevolution. One can easily jump over a 2 foot hole. But it doesn't mean you can jump over a 100 foot hole.

This assumes that we know nothing else about speciation except morphology. There is nothing in the way we currently understand how DNA works that says the alterations of phenotypes can't be beneficial, like how a roulette wheel will occassionally land on the green 00.

Just as there is no reason to assume that current experiments will yield an answer to the formation of multi-cellular life from single-celled life, there is also no reason to assume that they never will.

I am comfortable with the notion that science has not yet answered every question posed by its theories. I am curious, though, about what other model of Creationism you would choose if common descent was supported beyond a reasonable doubt.

[otseng]

But what is the difference between gain and loss? In terms of evolution, I do not see a distinction between the addition of features and the loss of features. Traits are gained and lost as the environment changes. Speciation is as haphazard as natural disasters.

In terms of explaining common descent, there is a big difference between a gain and a loss. Common descent theorizes that life originated from a single cell. We have today complex multicellular organisms. Numerous steps in between would have required a gain in features. Along the way, features can be lost, but without a gain in features, common descent cannot be supported.

Science has yet to truly crack the genome of the vast majority of species. Assuming that this position is true, and that the existing evidence points to this CM, this allows for no exceptions. It should be noted that these theories, on either side, cannot be proven with the evidence we have, only either supported or disproved.

Neither theory can or ever will be proven. They are both models. However, to support a model, we can only use evidence that we currently have. We cannot assume that a model is tenable simply because we believe evidence to be out there, but we just haven't found them yet.

If the two can be said to be pitted against one another (a view I don't share), common descent has the advantage here, if only because it is not a dogma, but is itself a theory that evolves as we learn more about the evidence.

The theory of common descent has been around for over 150 years. In this time, we have gained a remarkable understanding in the world of physics, paleontology, molecular biology, biochemistry, and a host of other areas of science. We now have understanding of biology of certain things down to the molecular level. And such studies have actually led to more uncertainty of evolution. (I'll get more into this when I broach the topic of intelligent design.)

The fact that this CM fits current evidence better is a matter of opinion over the word "better".

Simply this, from the evidence that we currently have, does it support or not support the model. Like I mentioned in the evidence (or rather a lack of evidence) of the formation of new morphological features, this supports the CM and does not support the EM.

This isn't to say that current theories of common dscent and evolution have not done this either, but the latter two allow for theoretical model alterations if the evidence warrants.

The alterability of a model doesn't make a model more tenable. Just as if I said the CM doesn't have to change, therefore it is a more tenable model. The nature of the model itself doesn't support if the model is true or not.

common descent is not incompatible with microevolution either.

Yet common descent is quite an extrapolation from microevolution. One can easily jump over a 2 foot hole. But it doesn't mean you can jump over a 100 foot hole.

This assumes that we know nothing else about speciation except morphology. There is nothing in the way we currently understand how DNA works that says the alterations of phenotypes can't be beneficial, like how a roulette wheel will occassionally land on the green 00.

About speciation, I believe science knows very little about this. That is, on the genetic level, how do species arise? Why do we have species in the first place?

But I don't think speciation is the main point of common descent. I believe the main point is the successive introduction of new features. Certainly this is the case with the first cell. Speciation has nothing to do with a cell. But in order for common descent to be true, it must have somehow gained new features (like turning into a multicellular organism).

I am curious, though, about what other model of Creationism you would choose if common descent was supported beyond a reasonable doubt.

If common descent was unquestionably supported without a single doubt, then the next thing to tackle would be abiogenesis. If these two things are solved, then the CM would not have anything meaningful thing to say in regards to life origins.

[fried beef sandwich]

Another question about the phylogenetic tree, how do you respond to allegations that morphology and molecular systematics produce differing results? Bones, molecules, ... or both?

My response: The article is talking about the relative merits of two techniques and systems of evolutionary organization. Common descent itself is not in debate.

[fried beef sandwich]

In terms of explaining common descent, there is a big difference between a gain and a loss. Common descent theorizes that life originated from a single cell. We have today complex multicellular organisms. Numerous steps in between would have required a gain in features. Along the way, features can be lost, but without a gain in features, common descent cannot be supported.

Bacteria can gain antibiotic resistance. Sure, it's still a single-celled organism, but I'm just putting it there to show you that gaining features can be done.

If you want to investigate a transition between single=celled and multicellular organisms, the best example I can think of is Volvox.

The theory of common descent has been around for over 150 years. In this time, we have gained a remarkable understanding in the world of physics, paleontology, molecular biology, biochemistry, and a host of other areas of science. We now have understanding of biology of certain things down to the molecular level. And such studies have actually led to more uncertainty of evolution. (I'll get more into this when I broach the topic of intelligent design.)

I disagree, but I'd like to hear what you have to say first.

The alterability of a model doesn't make a model more tenable. Just as if I said the CM doesn't have to change, therefore it is a more tenable model. The nature of the model itself doesn't support if the model is true or not.

Actually, you're wrong. The ability of a model to be open to the possibility of change in light of new evidence is a strength, not a weakness. Even the entire intelligent design movement is a response to change creationism from the hard-line Young Earth flavor to something more science-based.

[ST88]

Neither theory can or ever will be proven. They are both models. However, to support a model, we can only use evidence that we currently have. We cannot assume that a model is tenable simply because we believe evidence to be out there, but we just haven't found them yet.

I didn't mean for this to sound like a game of, "heads I win, tails we flip again". What I meant was, the common descent model can accommodate more change if evidence points in different directions. The fact that we don't completely understand or have figured everything out is what gives it strength.

The theory of common descent has been around for over 150 years. In this time, we have gained a remarkable understanding in the world of physics, paleontology, molecular biology, biochemistry, and a host of other areas of science. We now have understanding of biology of certain things down to the molecular level. And such studies have actually led to more uncertainty of evolution. (I'll get more into this when I broach the topic of intelligent design.)

Not quite. The phrase "uncertainty of evolution" implies a static theory of evolution that is as monolithic as Creationism. This is precisely my point. Of all the discoveries that have been made, nothing points away from evolution or common descent. You would think that we would have found evidence against it by now, but this is not the case. On the other hand, there is a basketload of evidence against the Creationist model. (Forgive me for my ignorance of the history of this version of Creationism that incorporates microevolution.) The discovery of DNA is an example - what is the purpose of DNA in the Creationist model? If there is no speciation process, then I would expect that RNA would be enough to allow for the degrading of species. The nucleus would be a useless appendage that wastes transcription time, only to encourage the appearance of error. These errors can occur just fine on their own in RNA, but the transcription adds another layer of information transfer, like a game of "telephone".

Common descent says that DNA transcription is vital because the structure of the cell outside the nucleus makes little difference. DNA doesn't "know" what kind of creature it is helping to create. It is, instead, merely perpetuating itself the best way it knows how. We can see this in the way viruses hijack the DNA of cells to turn them into virus factories.

The fact that this CM fits current evidence better is a matter of opinion over the word "better".

Simply this, from the evidence that we currently have, does it support or not support the model. Like I mentioned in the evidence (or rather a lack of evidence) of the formation of new morphological features, this supports the CM and does not support the EM.

We should be able to agree that current fossil evidence supports both models, regardless of "degree of support."

This isn't to say that current theories of common dscent and evolution have not done this either, but the latter two allow for theoretical model alterations if the evidence warrants.

The alterability of a model doesn't make a model more tenable. Just as if I said the CM doesn't have to change, therefore it is a more tenable model. The nature of the model itself doesn't support if the model is true or not.

I don't think this is true. Whether or not a model is capable of incorporating new discoveries would seem to be vital to its success. See the recent example of Hawking's refutation of his eariler theory about black holes. The evidence did not support his hypothesis, and so he was willing to abandon it.

But even if this is true, that the tenability (?) of the model is not based on its alterability, the basic underlying principles of each model are arrived at differently. This is what I was getting at. In the God model, the assumption of God requires an explanation for all observed phenomenae in terms of God's story. That is, evidence is always seen as part of the P/plan.

In the secular model, no assumptions are made about anything. We observe and extrapolate based on the observations.

If the God model says that the number 13 can never exist and we see a set of data that looks like: 8, 9, 10, 11, 12, 14, 15, 16, 17. Then we must assume that there is a reason that 13 is not present, so we come up with a mathematical calculation about what may have happened before 8 and after 17 so that 13 can be left out of the plan. However, a secular model that says that the number 13 can never exist, faced with the same set of data, has the option of saying there is either incomplete or inaccurate observation.

About speciation, I believe science knows very little about this. That is, on the genetic level, how do species arise? Why do we have species in the first place?

In the common descent model, speciation is the direct result of being able to exploit new areas of an environment, allowing for the unfettered population of these environments. Take the example of air-breathing fish. For some reason I won't get into here, fish filter oxygen out of the water in order to "breathe." It is not a big step to take to assume that some fish would have adapted themselves to take oxygen out of the air in order to breathe. Once this happened, there were entirely new out-of water environments to explore and populate without competition.