Sabretooth evolution

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What stages did the evolution of Sabretooth Cats (Smilodon) go through?

I assume that the teeth acquired their distinctive size over a series of generations (as expected in Evolution). In other words, the first fulll blown sabretooth wasn't born to a non-sabretooth parent. Instead, there must have been slight genetic changes over generations which led to an increase in tooth size (guided by selection).

But for this to happen it implies that a series of very similar random mutations must have occurred over generations to create a larger and larger canine tooth (amidst all the other dentitition of the mouth which was unaffected by any similar mutation, or selected against).

(I have a rather subtle question to ask here with regard to evolutionary process and genetics, and it's going to be tough to explain, but I'll give it a shot)

As most of you know from my posts, I'm a firm believer in the scientific method and in the validity of evolution as a general concept for describing the biology of our world. However, I have always believed that there is more to the process than we currently know (or something in the existing theory which I don't understand), and I'm trying to find that thing (and no, I don't think it's anything supernatural).

The implication of my question above, and the conjecture I would like to pose in this thread is that mutations can occur in the genome (we already know this), but that they can occur in such a way as to result in a propensity for morphological change in successive generations.

For this to happen, the change we see at the surface of an organism (such as increased tooth size), would be only the tip of the mutational iceberg.

I'm thinking that the real mutation would actually be something which affects the way the Gene replicates, such that successive replication would tend to carry a pattern of change.

So, to summarize, my conjecture is that some mutations do not directly affect physical morphology, but instead affect genetic replication such that successive changes of a particular type accumulate over generations.

I don't know enough about genetics to evaluate this myself. Is this possible?

It should also be noted that my conjecture here is not limited to Sabretooth cats, but to the evolutionary process in general. I'm just using Sabretooth evolution as a focal point to describe the process I'm trying to understand.

Thanks,

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Reference material: http://www.mun.ca/biology/scarr/3900_Tooth_evolution.htm

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Without following the links, and if I understand your thesis, I don't believe it's possible, but hey, I'm a bookkeeper.

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roger wrote:

Without following the links, and if I understand your thesis, I don't believe it's possible, but hey, I'm a bookkeeper.

The links don't say much in reference to my conjecture anyway. They are just background information.

Unfortunately, I'll probably have to defend my conjecture on my own (without web reference), which means I'll probably get killed on this one, but hey, nothing ventured... Wink

Regards,

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there is a working hypothesis forwarded by EO Wilson and E Mayr that the original genetic diversity built into the animals genome is enough to account for major evolutionary change. , even without mutations.

In other words, in the overall population of sabre tooths, there already was a variability of tooth length that varied from moderate to large size canines.9This can be seen in the population of African lions today. If you were to go to the Field Museum in Chicago, they have the stuffed lions that wreaked months of havoc during the building of the trans-vaal RR in south Africa,these 2 lions were, I believe, from the same litter, but one has much longer canines than the other)
i believe (not sure though) that a behavior pattern to uSE these sabre teeth effectively as daggers had to develop . AND , since evolution first occurs to an individual, the first individual that successfully adapted the use of these teeth, passed on this trait in as simple a fashion as a Mendelian pea flower.

The interesting feature about sabre teeth is that Smilodons were not the only ones to carry these types of teeth. By convergent evolution, animals from totally different orders have shown sabre-toothness in their forms. So there must have been a good reason for its successful adoption. The genetic expression for certain dentition was already established in the Paleocene, so , while many orders and families showed sabret teeth, the genetic expression, once established in a genome, doesnt need reinventing.
Thats where I usually depart from the "mutation driven" evolutionary theorists. Mutations are a mechanism, but not the only one. Most mutations are lethal.

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If characteristics of the population (here in the sense of statistics) are functions of time variable, then mean values around which actual values of variables distribute with a steady variance (stat.) are also functions of time.

And I am always impressed with the power of time.

Deformation

http://www.earth.rochester.edu/structure/matty/photos/field_camp/deformation.jpg

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Sure. I've already seen Scrat, the Sabre toothed squirrel.

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good shot satt. Its obviously a glacial deposit overlying a good batch well compressed(ptygmatic folds).
Need some scale and it would make a good candidate for a cover photo for GEOLOGY (GSA mag)

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I'm not an expert on the subject, but you don't need a new "random" mutation for every change in tooth size. The genes for building teeth are already in place; all it takes is a modification to the regulatory mechanism which tells the tooth when to stop growing.

Something as simple as the number of repetitions of a certain DNA sequence in the gene can accomplish this. Individuals in the population may have varying numbers of repetitions due to copying errors or the particular alleles they inherited.

"The Beak of the Finch" by Jonathan Weiner documents the changes in the finch population in the Galapagos Islands when drought which affected the food supply. There were already variations in beak size, but birds with larger, stronger beaks that were able to crack larger seeds had a survival edge and produced more surviving offspring, thus increasing average beak size.

If mates with large teeth are preferred, average tooth size may increase even if it does not confer any hunting advantage. Being able to survive in spite of a costly feature (such as a peacock's tail) is an indication of fitness and good genes. See "The Red Queen" by Matt Ridley.

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yep.

, but Ill bet there was some advantage to killing and behavior. Maybe the sabre toothness , as a 'lifestyle' favored quick kills by an ambush hunter than a runner,
the red queen argument stated that a prey animal became better at escaping and the predator kept evolutionary pace by catching up in the killing success mode. the lion became a better stalker and the gazelles became quicker and then the lions became even better at stalking .

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Re: Sabretooth evolution

rosborne979 wrote:

I assume that the teeth acquired their distinctive size over a series of generations (as expected in Evolution). In other words, the first fulll blown sabretooth wasn't born to a non-sabretooth parent. Instead, there must have been slight genetic changes over generations which led to an increase in tooth size (guided by selection).

Or perhaps the ancestors of the sabertooth had a mouthful of entirely large teeth and through evolution progressed through stages where all of the other teeth in it mouth reduced in size??

I'm not suggesting this was the case but it seems that you are starting with an assumption here that seems to be totally random.

The human thumb is typically shorter than all of the other fingers on our hands. Was the thumb always that way? Did the thumb get reduced in size over the ages through genetic changes? Or did all of the other fingers increase in length due to genetic changes? Any one of the 3 possibilities get you to the end result...

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Re: Sabretooth evolution

rosborne979 wrote:

However, I have always believed that there is more to the process than we currently know (or something in the existing theory which I don't understand), and I'm trying to find that thing (and no, I don't think it's anything supernatural).

The implication of my question above, and the conjecture I would like to pose in this thread is that mutations can occur in the genome (we already know this), but that they can occur in such a way as to result in a propensity for morphological change in successive generations.

I believe that you're wrong. I think that this is a very simple process involving natural selection and random mutation. In order for an evolutionary change to take place, two things are necessary. First of all, the change must in some way improve the creature's chances of having more surviving children. Generally, a change that results in the creature itself surviving has this effect. The second thing that's necessary is that there must be a viable evolutionary path to the change - what I mean by this is that every additional degree of change must confer an advantage. For example, it might be beneficial for an animal to have treads like a tractor for locomotion, but there is no viable path to this, since the first small fraction of a tread is probably not functional. In the case of the tiger, if its tooth were slightly bigger and sharper, I can see this conferring some small statistical advantage, so that the tooth would begin to grow, and slowly expand to the point of maximum benefit.

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First I want to thank everyone for their responses. I'm thinking about them to see where I'm losing this.

In general, I have a pretty good understanding of evolutionary process, so I'm not suggesting that the normal processes are not in play, only that there may be something else here as well. It's also quite possible that I don't understand things as well as I think I do. If this is the case, then I hope to discover it through this thread.

I'll try to respond to several posts as time permits...

Thanks again,

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farmerman wrote:

there is a working hypothesis forwarded by EO Wilson and E Mayr that the original genetic diversity built into the animals genome is enough to account for major evolutionary change. , even without mutations.

Ok, so you're saying that the genetic diversity in a population is sufficient to have individuals with long or short teeth. But aren't the changes in tooth size within a population traceable back to a genetic mutation somewhere in the parentage which is simply moving through the population?

If this were the case, then to get form normal teeth to sabre teeth, you would still have to have another (several other) mutations of the tooth length gene re-injected into the population. I'm not saying this is impossible, but it seems unlikely to me.

Or as Satt suggest, maybe I'm simply not giving enough credence to the depth of time here (though I'm fully aware of the scale).

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Terry wrote:

I'm not an expert on the subject, but you don't need a new "random" mutation for every change in tooth size. The genes for building teeth are already in place; all it takes is a modification to the regulatory mechanism which tells the tooth when to stop growing.

But isn't a "modification to the regulatory mechanism which tells the tooth to stop growing" also a genetic change. Isn't that also something which has to originate from a mutation or copying error in the DNA?

Terry wrote:

Something as simple as the number of repetitions of a certain DNA sequence in the gene can accomplish this. Individuals in the population may have varying numbers of repetitions due to copying errors or the particular alleles they inherited.

But since a "normal" tooth doesn't go from being normal to sabre within a single generation, don't even these numbers of repetitions have to be altered through random mutations several times in the ancestry of a population?

Terry wrote:

"The Beak of the Finch" by Jonathan Weiner documents the changes in the finch population in the Galapagos Islands when drought which affected the food supply. There were already variations in beak size, but birds with larger, stronger beaks that were able to crack larger seeds had a survival edge and produced more surviving offspring, thus increasing average beak size.

Yes, I understand how natural selection works. And I agree that it does what it must do; it is inevitible. But it can only select for things that it has available to it, so my question is focused more on the variation aspect of the whole theory.

Ok, so let me try to describe this step by step and see where things don't make sense to me.

* Within a population there is diversity (Lions teeth and finches beaks).

* But within a population at any given time, there is not the extremity of diversity which we see when we compare a given animal to its distant ancestor (Lion's teeth range from small to large, but not to huge).

* A selective pressure develops in favor of large teeth, and those individuals breed more effectively, so the population eventually has larger teeth, but still not huge teeth.

* Within this new population of large tooth cats, there is still diversity (large to small), but there will not be any with still larger teeth than the limits of the original range, right? To get to the "next" level of tooth size, don't you need "another" mutation in the tooth size gene to occur?

Maybe this is where I'm losing it.

Let's say that the original population had a tooth range from 2" min to 5" max, but that there were none over 5". Over the generations, selection favors the 5"ers, so more of them are around now. Within that new population of 5"ers, are there now some with 10" teeth (the new Max), and if so, how did those 10"ers develop without an additional mutation in the same gene which led to the 5"ers?

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Re: Sabretooth evolution

Brandon9000 wrote:

...so that the tooth would begin to grow, and slowly expand to the point of maximum benefit.

Hi Brandon,

But how would a tooth grow to the point of "maximum benefit" (I'm assuming that you mean over generations through normal evolutionary process, not an individual), without successive mutations of the same tooth length gene to drive it along?

Natural selection can only select from the pool of diversity it has to work with. The diversity itself comes from mutation and copying errors.

Even the added layer of diversity which comes from sexual reproduction does not introduce new body plans, it only mixes and matches existing ones, right?

I'm suggesting that maybe there's something in the gene itself which causes it to error in a particular way when it divides. We already know that some genes control other genes, making them active or inactive. Could it be that there are some genes which control the division process such that repeating errors flow from one generation to another, leading to the process you described in which "a tooth would begin to grow, and slowly expand to the point of maximum benefit"... only to stop when selection began to prune the genes which had gone too far.

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Okay, just shooting in the dark here. (I'm given to speculation and painting myself into difficult corners, but... whatever. Ain't writing a thesis here.)

Here's, perhaps, a (grossly simpole) model. There are, let's say, five genes that regulate the size of the upper canines. Let's say there is some variety in each of these alleles, so that variation in each, with all the others held constant, causes an increase in tooth size, but these alleles are intially rare enough that the occasion of an individual being homozygous for them at two different loci is very low.

Nonetheless, there occasionally is an individual who is homozygous for big teeth at two of these loci, and it's got 6-inch teeth instead of the mean 3-inch teeth. Now, let's say this is really advantageous, and this individual is in a small, isolated population, so that after some number of generations most of the individuals are homozygous for both of these alleles.

Now, being homozygous for a big-tooth allele at a third locus bumps the tooth-length from 6 to 8 inches...

And homozygotes for big teeth at a fourth locus have 10 inch teeth.

And homozygotes at the fifth locus have 12 inch teeth.

Just a very crude model. Not a lot of morphological traits are influenced by one or two genes.

As to the real question -- could there be mutations which increase the susceptibility of a genome to mutation -- it's conceivable, I suppose, but I don't know of any examples. Perhaps something that regulates meiosis could have an effect. I just read in a text about regions of DNA that appear to have a propensity for moving around the genome, but like any mutagen they're likely to cause a great deal more harm than good.

There is apparently a mutation (perhaps a common one?) in DNA polymerase that increases the overall mutation rate. However...

Quote:

What they found is that the higher mutation rate (the mutator strains) does not necessarily accelerate the pace of fitness evolution. Only small population coupled with high mutation rate will speed up the fitness evolution. Figure 1 & 2. This is because in a small population, the clonal interference is reduced within the population. Thus, the population spends most of its time waiting for beneficial mutations. In addition, mutators are very common in asexual populations, since they are most likely to hitchhike to high frequency by introducing mutations at a higher rate and thereby create beneficial mutations more often. It is very likely that to have both mutators and beneficial mutations coupled in one E. coli. They will not accelerate fitness evolution due to clonal interference. Within the population, there will be more beneficial mutations as a result of the increased mutation rate by the mutator mutation. However within the population, all beneficial mutation are competing against each other to be the fittest. Thus the process still remains the same. Clonal interference also imposes a speed limit on adaptive evolution in asexual populations, because two or more beneficial mutations that arise in different lineages cannot be combined into the same lineage. (That what sex is good for) Thus the rate of evolution will be capped, or independent of the mutation supply rate.

Not sure how this would apply to animals (since it appears here to be a bacterial thing). A researcher I worked for claimed to have found mutator-like strains of Plasmodium falciparum (malaria), which is a eukaryote but still unicellular and, so far as I know, primarily an asexual reproducer.

This is something I've mulled over, but I don't know enough to posit any mechanism. I mean, you could have mutations in the enzymes responsible for correcting mutations, but that's not likely to be a cell that lives very long, let alone happening in the germline and getting passed on.

(Mayhaps I'll be able to say more when I'm done with genetics, animal development, and bacteriology, but I very much doubt it.)

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Just realized I lifted that quote from someone's term paper (here) so it may not be any more reliable than my own general gibberish.

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Quote:

The implication of my question above, and the conjecture I would like to pose in this thread is that mutations can occur in the genome (we already know this), but that they can occur in such a way as to result in a propensity for morphological change in successive generations.

I don't think you have quite grasped that the driving force of natural selection is that the species is always looking for a 'best fit' to the conditions that are currently present, not anticipating events that will occur in geological time. There can't be anything gained from changing the genome now to cope with an environmental change that may be millions of years ahead of us.

From a population of finches that were genetically identical, there will arise separate populations on islands such as the Galapagos. Local conditions shape what is the 'best fit', not the other way around (ie this sub-group of finches will have a long, sharp beak so they arrange to make sure that they are on the island where the insects live in long, narrow tunnels).

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Hmmm. I took it to be a question as to whether some genomes may be inherently more prone to random mutation than others, but looking back I'm not so sure...

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Sabretooth evolution

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