Evolution of skull, why did this happened only to primates and not to other animals?

[Myuncle]

Experts in evolution please answer me. I know that every change in evolution happened because of genetic mutations, therefore by accident. Chimps could develop bigger brains only thanks to a genetic mutation in the size of the skull. But I don't understand why this happened only to primates. Why this mutation didn't happen to a zebra or a lion?

[Leader Bee]

What leads you to believe that this has only happened to primates?

Take the horse for example

 

http://en.wikipedia.org/wiki/File:Equine_evolution.jpg

 

The earlier Mesohippus clearly has a longer skull, weaker lower jaw and larger eye sockets much more rodent like in appearance; Compared to Equus, the most recent example we have here which looks more recognisable as todays horses though all are closely related.

 

As for big cats

"The cat family comprises some of the most specialised carnivores in the history of mammals, all exclusively eating flesh. The cat family consists of two major sub-groups: the feline cats (including all modern species) and the sabertoothed cats (which are all extinct). Skeletons from the two groups look broadly similar, but their skulls are often remarkably different, and suggest that members of the two groups underwent radically different adaptations to predation during the course of evolution."

 

Source: http://www.sciencedaily.com/releases/2008/07/080729234258.htm

[John Cuthber]

Why would a zebra want a bigger brain? They are expensive to maintain.

[Myuncle]

What leads you to believe that this has only happened to primates?

Take the horse for example

 

http://en.wikipedia.org/wiki/File:Equine_evolution.jpg

 

The earlier Mesohippus clearly has a longer skull, weaker lower jaw and larger eye sockets much more rodent like in appearance; Compared to Equus, the most recent example we have here which looks more recognisable as todays horses though all are closely related.

 

As for big cats

"The cat family comprises some of the most specialised carnivores in the history of mammals, all exclusively eating flesh. The cat family consists of two major sub-groups: the feline cats (including all modern species) and the sabertoothed cats (which are all extinct). Skeletons from the two groups look broadly similar, but their skulls are often remarkably different, and suggest that members of the two groups underwent radically different adaptations to predation during the course of evolution."

 

Source: http://www.sciencedaily.com/releases/2008/07/080729234258.htm

 

Ok, this happened even to other animals but when we consider the size of the brain we measure it in proportion to the rest of the body, in this case humans have the biggest brain. So why horses or cats didn't develop the same increase in brain size?

 

Why would a zebra want a bigger brain? They are expensive to maintain.

 

It's not something that you want but something that happened to you. Why didn't happen to a zebra? If zebras had a bigger brain they would be as clever as humans and protect themselves better from predators.

Edited August 11, 2011 by Myuncle

[Ringer]

Actually the largest brain to body mass ratio is the shrew IIRC. Rodents and humans also have very similar brain to body ratios, with rodent winning. So although the ratio between body mass and brain mass is alright as a rough estimate of intelligence it is nowhere near the final say.

[swansont]

It's not something that you want but something that happened to you. Why didn't happen to a zebra? If zebras had a bigger brain they would be as clever as humans and protect themselves better from predators.

You didn't address the issue John Cuthber raised: Could they sustain it metabolically? Humans were able to exploit bigger brains — hunting strategy, tool use. How would a bigger brain in a zebra result in being able to get enough more food to sustain it?

[Myuncle]

You didn't address the issue John Cuthber raised: Could they sustain it metabolically? Humans were able to exploit bigger brains — hunting strategy, tool use. How would a bigger brain in a zebra result in being able to get enough more food to sustain it?

 

 

Ok, let's take all the carnivores, or all the animals that need to eat only nuts, they can easily sustain it metabolically. Sorry swansont but I don't buy that John Cuthber theory, and with all respect, I don't even buy the theory according which cooking food and smaller jaws favoured a bigger skull.

Edited August 12, 2011 by Myuncle

[Ringer]

Ok, let's take all the carnivores, or all the animals that need to eat only nuts, they can easily sustain it metabolically. Sorry swansont but I don't buy that John Cuthber theory, and with all respect, I don't even buy the theory according which cooking food and smaller jaws favoured a bigger skull.

 

Any particular reason other than your personal opinion?

 

Cooking food didn't favor bigger skulls, cooking food, probably, allowed us to use the energy that used to be used for metabolizing for other things. One of those things happen to be our brain. Smaller jaws didn't necessarily favor bigger skulls, it was probably the other way around. When our brain and skull grew our jaws happen to become smaller.

[Arete]

Ok, let's take all the carnivores, or all the animals that need to eat only nuts, they can easily sustain it metabolically. Sorry swansont but I don't buy that John Cuthber theory, and with all respect, I don't even buy the theory according which cooking food and smaller jaws favoured a bigger skull.

 

Adaptation of cranial traits can be seen in almost all vertebrates. One of the most reliable, ways to infer adaptation is to measure landmark morphological skull features - often they correlate with diet and environment.

 

There's a few reasons I can think of off the top of my head why enlarged skulls and thus increased analytical intelligence isn't universally selectively advantageous:

1) Enlarging the cranial cavity of a skull comes with several disadvantages. Human skulls are relatively fragile, the area for the attachment of powerful jaw muscles is comparatively reduced, our peripheral vision is reduced my the shape our skulls have taken to allow for a larger brain case, etc. As such, stabilizing selection on a number of other traits can limit the size of the cranial cavity dependent on the ecological niche a vertebrate is exploiting and thus limit optimal size of the brain.

 

2) Increased intellect in humans is accompanied by a number of other complimentary traits - hands optimized for toolmaking, bipedal locomotion, social structure, etc. which offer positive feedbacks in the evolution of problem solving ability. Other animals do not necessarily have these complimentary features. E.g. say you and a zebra are standing in the grassland together being stalked by a lion - Your intellect alone offers you no selective advantage over the zebra in terms of avoiding predation. Your intellect in combination with other complimentary traits could allow you to buy or create a weapon yourself but given a zebra hasn't got hands, increased ability to think about designing a weapon isn't going to help a zebra avoid said lion.

 

3) Increased intellect may not even be selected for. If say, speed, or strength is king in the selective environment you are in, taking on the disadvantages a larger, more resource intensive brain costs at the expense of being faster or stronger places you at a selective disadvantage, in turn selection is against increased brain size. For example, sharks and crocodiles are very sucessful predators and both have relatively small brains with relatively limited function. If you look a the skull morphology of both, you'll see they are both extremely specialized - but the selective push is towards bite power and strength, indicating that it is more selectively advantageous for these organisms to be able to bite harder than it is for them to be smarter.

[Myuncle]

Adaptation of cranial traits can be seen in almost all vertebrates. One of the most reliable, ways to infer adaptation is to measure landmark morphological skull features - often they correlate with diet and environment.

 

There's a few reasons I can think of off the top of my head why enlarged skulls and thus increased analytical intelligence isn't universally selectively advantageous:

1) Enlarging the cranial cavity of a skull comes with several disadvantages. Human skulls are relatively fragile, the area for the attachment of powerful jaw muscles is comparatively reduced, our peripheral vision is reduced my the shape our skulls have taken to allow for a larger brain case, etc. As such, stabilizing selection on a number of other traits can limit the size of the cranial cavity dependent on the ecological niche a vertebrate is exploiting and thus limit optimal size of the brain.

 

2) Increased intellect in humans is accompanied by a number of other complimentary traits - hands optimized for toolmaking, bipedal locomotion, social structure, etc. which offer positive feedbacks in the evolution of problem solving ability. Other animals do not necessarily have these complimentary features. E.g. say you and a zebra are standing in the grassland together being stalked by a lion - Your intellect alone offers you no selective advantage over the zebra in terms of avoiding predation. Your intellect in combination with other complimentary traits could allow you to buy or create a weapon yourself but given a zebra hasn't got hands, increased ability to think about designing a weapon isn't going to help a zebra avoid said lion.

 

3) Increased intellect may not even be selected for. If say, speed, or strength is king in the selective environment you are in, taking on the disadvantages a larger, more resource intensive brain costs at the expense of being faster or stronger places you at a selective disadvantage, in turn selection is against increased brain size. For example, sharks and crocodiles are very sucessful predators and both have relatively small brains with relatively limited function. If you look a the skull morphology of both, you'll see they are both extremely specialized - but the selective push is towards bite power and strength, indicating that it is more selectively advantageous for these organisms to be able to bite harder than it is for them to be smarter.

 

Thanks, very clever response. Basically if you are a predator or a prey, without hands, you don't need to be particularly clever, you just need to run as fast as you can, or be persistent like the wolves. But since these animals don't have fingers to make weapons to kill predators or preys, they don't need a bigger brain. All in all zebras and lions they can survive very well the way they are.

Edited August 12, 2011 by Myuncle

[John Cuthber]

"All in all zebras and lions they can survive very well the way they are. "

So do we, so do chimps, so do earthworms, so do bacteria...

[questionposter]

Why this mutation didn't happen to a zebra or a lion?

 

 

Because the adaptations of evolution are random. Things randomly evolve to have random genetic variations and some times those come in handy. This is not happened to the zebra by chance at a large enough scale to match ours. But, there's also dolphins and ravens which are pretty intelligent and have that sort of "dome-y" part of their head that sort of curves at a lesser degree near the bottom to make that slight egg shape. Our capacity for intelligence as far as science is concerned happened by chance. There happened to be a gene that happened to be mutated as to generate more neurons per square milometer than others, and that gene was passed down since it helped in surviving. I imagine that a skull was also randomly developed to match that type of brain growth so that the brain could grow optimally.

Edited August 13, 2011 by questionposter

[Myuncle]

Because the adaptations of evolution are random. Things randomly evolve to have random genetic variations and some times those come in handy. This is not happened to the zebra by chance at a large enough scale to match ours. But, there's also dolphins and ravens which are pretty intelligent and have that sort of "dome-y" part of their head that sort of curves at a lesser degree near the bottom to make that slight egg shape. Our capacity for intelligence as far as science is concerned happened by chance. There happened to be a gene that happened to be mutated as to generate more neurons per square milometer than others, and that gene was passed down since it helped in surviving. I imagine that a skull was also randomly developed to match that type of brain growth so that the brain could grow optimally.

 

Yes, there is no doubt that all mutations happen accidentally. Our sons can be born with six fingers or two heads, everything can happen. but at the same time exploiting those accidents is essential. Any animals can develop a bigger skull and a bigger brain, but since they don't have fingers (and they don't have a strong muscle in the thumb to manipulate tools like humans), they can't use and exploit this genetic mutation. It's all about building weapons to kill the enemy, that's the competition, you need intelligence to do that, and primates could afford to exploit this mutation thanks to their hands.

[questionposter]

Yes, there is no doubt that all mutations happen accidentally. Our sons can be born with six fingers or two heads, everything can happen. but at the same time exploiting those accidents is essential. Any animals can develop a bigger skull and a bigger brain, but since they don't have fingers (and they don't have a strong muscle in the thumb to manipulate tools like humans), they can't use and exploit this genetic mutation. It's all about building weapons to kill the enemy, that's the competition, you need intelligence to do that, and primates could afford to exploit this mutation thanks to their hands.

 

When did I ever say that animals don't need to develop the right appendages to do everything that we do? I mean I guess they could develop telekinetic powers, but I guess birds of hands and they can use their beaks too but it would still be harder for them to do things.

Also, focus on the goal, not the competition, that's more how things survive, because as far as we know animals on a daily basis aren't thinking "oh, I gotta compete with this guy", they are more like "ok, what do I need to do to survive in the long run? Because there's not a lot of food around?" There's a big difference because in one, animals actually care enough to actually want to wipe out other species, but in the other, they are just trying to survive.

Edited August 14, 2011 by questionposter

[Myuncle]

When did I ever say that animals don't need to develop the right appendages to do everything that we do? I mean I guess they could develop telekinetic powers, but I guess birds of hands and they can use their beaks too but it would still be harder for them to do things.

Also, focus on the goal, not the competition, that's more how things survive, because as far as we know animals on a daily basis aren't thinking "oh, I gotta compete with this guy", they are more like "ok, what do I need to do to survive in the long run? Because there's not a lot of food around?" There's a big difference because in one, animals actually care enough to actually want to wipe out other species, but in the other, they are just trying to survive.

 

By competition I mean both, fight for survival and fierce competition. If you imagine the period when they started making the first tools and the first weapons I suppose a real race started between humans to create better weapons, otherwise the risk of being litterally wiped out it was very likely to happen. I can imagine how many wars and how many deaths, the second world war it's just the tip of the iceberg.

[questionposter]

By competition I mean both, fight for survival and fierce competition. If you imagine the period when they started making the first tools and the first weapons I suppose a real race started between humans to create better weapons, otherwise the risk of being litterally wiped out it was very likely to happen. I can imagine how many wars and how many deaths, the second world war it's just the tip of the iceberg.

 

Animals don't live to compete, they live to survive, that's why we can be friends with other animals like birds and dogs since in that case , both are surviving, even when there might be a limited amount of resources, humans and other animals can stick together.

[Greg Boyles]

Experts in evolution please answer me. I know that every change in evolution happened because of genetic mutations, therefore by accident. Chimps could develop bigger brains only thanks to a genetic mutation in the size of the skull. But I don't understand why this happened only to primates. Why this mutation didn't happen to a zebra or a lion?

 

Similar reason that zebras devloped hooves but primates didn't I guess. Different genetic make ups to start with so different horses for courses. As to precise reasons, genetic evolution is a complicated business and our understanding of it is as yet rather rudimentary.

[Arete]

Different genetic make ups to start with so different horses for courses.

 

Ungulates and Primates share a common Eutherian ancestor and thus originate from a common gene pool. The reason they phenotypically differ is not distinct ancestry, but divergent selection and ecological specialization.

 

As to precise reasons, genetic evolution is a complicated business and our understanding of it is as yet rather rudimentary.

 

No one understands "genetic evolution" as the term is nonsensical. Genetic mutation is a component of evolutionary theory and our understanding of it is quite detailed.

 

i.e. - mutational models: http://mbe.oxfordjou.../25/3/568.short http://mbe.oxfordjou.../3/537.abstract http://mbe.oxfordjou...0/2275.abstract

- mutation rate estimation: http://mbe.oxfordjou...27/6/1289.short http://mbe.oxfordjou...4/12/2669.short http://sysbio.oxford.../59/2/119.short

- phylogenetic reconstruction: http://bioinformatic.../25/7/971.short http://sysbio.oxford.../56/3/504.short http://sysbio.oxford.../56/3/400.short

- functional genomics: http://www.sciencema.../5863/620.short http://www.sciencedi...5920/1481.short

 

e.t.c.

 

What is difficult is inferring a precise selective cause for a diversification event. This is an intrinsic difficulty irrespective of the theoretical and empirical knowledge of genetic mutation and the action of selection, as environments are highly complex and dynamic. Even when selective divergence is directly observable (e.g. http://www.sciencema.../5915/737.short http://www.plosgenet...al.pgen.1000929 http://rstb.royalsoc...1506/3071.short) inferring exact causes is excruciatingly difficult as the precise knowledge of environmental conditions, ecological interactions and systemic dynamism are difficult to determine even in systems we can directly observe. Many, many selective pressures act on organisms simultaneously so being able to directly infer a cause and effect event is a formidable challenge in experimental design, information gathering and statistics.

 

Inferring such events in a prehistoric context is functionally impossible and its not through any lack of theoretical understanding. We have extremely well supported theories regarding what sort of events cause selective divergence, how genetic mutation in functional genes manifests and how those two interact, so we can infer broad patterns across extinct taxa and propose broad hypotheses explaining trait selection and divergence. The reason we will be unlikely to determine the exact selective cause of a trait like "intelligence" or "hooves" is that the mind boggling complexity of environmental variables and the similarly complex nature of temporal fluctuations means that the number of plausible explanations is huge, coupled with the practical impossibility of inferring these parameters in long past systems with any degree of accuracy. Not our supposed "rudimentary knowledge."

Edited August 15, 2011 by Arete

[questionposter]

What is difficult is inferring a precise selective cause for a diversification event.

 

The precise selective cause is the environment. If something cannot survive the environment on Earth, it will die. What's complex about evolution is how mutations build off of each other like to give dinosaurs feathers or to give giraffes long necks. What happens is you have a gene that gets altered which allows a species to survive better, and then after that original mutation becomes common in the species, as in nearly all the members have it, there's a good chance that the alteration of that gene will then alter again. Since its so successful and so many members have the alteration, there's a greater chance for that alteration to alter yet again. So giraffes had a gene that only made their necks like 2 inches longer, and then when that become common, the probability of that gene altering again increases, so then now it gets altered to make their necks 4 inches long, and since it helps the giraffes still, then that same process just happens with that alteration and this process happens again and again until that gene can't be altered without some back-draw which is why that gene stays the way it is. So right now, Giraffes necks might be as long as they can get before being a burden, which is why they don't alter anymore, because an alteration in that gene again would only do harm, so the organism containing it wouldn't survive to pass it on to the species in order for the alteration process to happen to it again. Or like with humans, there's not a whole lot that could get bigger or smaller and still be as helpful. We could have a bigger heart, but then there could be less room for other organs. We could make more room for other organs by having a smaller heart, but then it would be easier to have a heart attack. Eventually a gene alters in the same way to a point where it doesn't change a species any more due to the fact that its unsuccessful and any members carrying a surplus of alteration die before they can pass it on, so then evolution selects some other gene to alter.

 

Although there's a sort of loophole that can happen to this.

 

This surplus of alteration isn't always completely dead, it could also be a recessive gene. We can still carry it, but it wouldn't make our offspring successful in the right circumstances so then that DNA wouldn't get passed on to a future generation, unless it was also recessive in the offspring and then recessive in the offspring's offspring and etc. But the spruplus thing I'm talking about is true for dominant genes. If a Giraffe's neck is "too" long, then it might not be able to carry it's neck well enough or it might burn calories too fast or it's center of balance would be too off or etc.

Edited August 17, 2011 by questionposter

[Greg Boyles]

Ungulates and Primates share a common Eutherian ancestor and thus originate from a common gene pool. The reason they phenotypically differ is not distinct ancestry, but divergent selection and ecological specialization.

 

 

That exactly what I meant, but just chose to express it more creatively.

 

 

No one understands "genetic evolution" as the term is nonsensical. Genetic mutation is a component of evolutionary theory and our understanding of it is quite detailed.

 

Don't doubt that for a second and it is truly amazing what we have managed to unravel from the mystrery of our denes.

 

But what we do currently know is still the tip of the iceberg compared to what we are yet to figure out, hence our understanding is as yet relatively rudimentary.

Edited August 17, 2011 by Greg Boyles

[Arete]

The precise selective cause is the environment.

 

As an arm-waving generalized hypothesis, yes. If you want to take selection through environmental variation further, empirical demonstration of how a particular environmental parameter causes a selection for a particular phenotypic trait is necessary and particularly pertinent when suggesting sympatric, ecologically driven speciation. For example - Darwin's finches, parallel evolution of limetic and benthic stickleback, the cichlid radiation, Caribbean anoles etc. see http://www.sciencedi...169534709001268 for a nice summary piece. Otherwise, stochastic genetic drift in allopatry can explain phenotypic diversity in the absence of selection.

 

What's complex about evolution is how mutations build off of each other like to give dinosaurs feathers or to give giraffes long necks. What happens is you have a gene that gets altered which allows a species to survive better, and then after that original mutation becomes common in the species, as in nearly all the members have it, there's a good chance that the alteration of that gene will then alter again.

 

Hmmm not quite. Most phenotypic traits, such as neck length in giraffes will display population-level phenotypic variablity and a degree of heritability, generally with a normal distribution around the mean. Under a selection model, reproductive success across the distribution is not even, leading to a shift in the mean in the next generation. Over progressive generations the mean neck length shifts in response to selection. There is no need for a trait to become fixed or even common in a population for it to be selected for - see selective sweep http://www.nature.co...ive-sweep-24827 and population bottleneck http://en.wikipedia....tion_bottleneck . Conceptually, it's an elegant theory, brilliant in its simplicity.

Of course the deveopment of novel phenotypes - like feathers is more complex in that multiple mean shifts, interaction between selective parameters and phenotypic characters leading from one state to another, but essentially the same principles apply on a broader scale.

 

As per my previous post - empirically supporting hypotheses suggesting the precise stimuli for uneven reproductive success based on phenotype/environment interaction is difficult. Did a longer neck allow giraffes to exploit a new ecological niche by reaching higher branches? Why don't we see a bimodal distribution of neck lengths? Competition with conspecific species? Was it predator detection? Something we haven't thought of? A combination of multiple environmental factors? Mate choice driven sexual selection? Developing conceptual or experimental frameworks to test independent hypotheses - even with system like giraffe necks we can directly manipulate and observe is very difficult, so trying to do it with now non existent environments and non-emergent traits is functionally impossible - leading to hypotheses being speculative and virtually untestable.

 

So right now, Giraffes necks might be as long as they can get before being a burden, which is why they don't alter anymore, because an alteration in that gene again would only do harm, so the organism containing it wouldn't survive to pass it on to the species in order for the alteration process to happen to it again. Or like with humans, there's not a whole lot that could get bigger or smaller and still be as helpful. We could have a bigger heart, but then there could be less room for other organs. We could make more room for other organs by having a smaller heart, but then it would be easier to have a heart attack.

 

Stabilizing selection leads to phenotypic stasis. Another elegant, simple theory I discussed in the first point of my first post in this thread.Giraffe neck length or human heart size are exceptionally poor examples in extremely recently evolved species (H sapiens have been around for ~13 000 generations being generous. Coalescent theory suggests allelic population stability occurs in 4xNe generations. There's no way humans are even close to genetic equilibrium darwin.eeb.uconn.edu/eeb348/lecturenotes/coalescent.pdf).

A good example of a trait under stabilizing selection might be something like the functional skull morphology of the crocodilians. It is phenotypically similar across the group and has remained relatively stable since the late Triassic. This suggests that significant modification of this trait has been selectively deleterious under environmental fluctuations for 40 odd million years and thus under strong stabilizing selection. Human hearts and giraffe necks have not been consistent in size or length nearly long enough to infer stabilizing selection.

 

Eventually a gene alters in the same way to a point where it doesn't change a species any more due to the fact that its unsuccessful and any members carrying a surplus of alteration die before they can pass it on, so then evolution selects some other gene to alter.

 

Although there's a sort of loophole that can happen to this.

 

This surplus of alteration isn't always completely dead, it could also be a recessive gene. We can still carry it, but it wouldn't make our offspring successful in the right circumstances so then that DNA wouldn't get passed on to a future generation, unless it was also recessive in the offspring and then recessive in the offspring's offspring and etc. But the spruplus thing I'm talking about is true for dominant genes. If a Giraffe's neck is "too" long, then it might not be able to carry it's neck well enough or it might burn calories too fast or it's center of balance would be too off or etc.

 

Evolution is continuous process, as random mutation is continuous and the environment is continually in flux. Even the most strongly selectively constrained trait exhibits population level variation, which allows for the action of selective environmental parameters. Species don't simply reach equilibrium and stop evolving. In addition, genes under environmental selection display non-Mendelian models of inheritance and most phenotypic traits are the result of multiple gene interactions.

 

That exactly what I meant, but just chose to express it more creatively.

 

Hrmmm. What you said was :

 

Different genetic make ups to start with so different horses for courses.

 

Having "different genetic make ups to start with" is the opposite of sharing a common ancestor. Your statement was pretty concise - now sure how creativity involves interpreting a statement to mean the opposite of what is written.

 

But what we do currently know is still the tip of the iceberg compared to what we are yet to figure out, hence our understanding is as yet relatively rudimentary.

 

You directly stated that understanding the precise selective driver for the trait of human skull morphology was unknown because our understanding of "genetic evolution" was rudimentary. The reason we can't infer the precise selective driver for ancestral traits is due to our inability to measure the variables and control for each of them, not a lack of knowledge. If those damn physicists could hurry up and make a time machine and a generation speeder upperer to use on our ancestors, we can experimentally test our theories using existing knowledge. In the meantime we rely on model species and systems, extrapolation and inference. Similarly, tectonic theory relies on the extrapolation of observation the suggest the causation of past events, as do many other sciences.

 

Of course, there is more to know about evolutionary theory and empirical proof thereof, otherwise I'd be out of a job but the difficulty in answering the question at hand is due to the above-mentioned practicalities and not a lack of knowledge.

Edited August 17, 2011 by Arete

[Greg Boyles]

Hrmmm. What you said was :

 

 

 

Having "different genetic make ups to start with" is the opposite of sharing a common ancestor. Your statement was pretty concise - now sure how creativity involves interpreting a statement to mean the opposite of what is written.

 

 

Sorry I understand what you mean. If horses and primates shared a common ancestor then why did one start heading down the path of hooves and the other down the path of large skulls and brains.

 

Well I would say that initially, when both were very close to the ancestral line there could have been potential for both lines to swap evolutionary paths....theoretically.

 

But once they have travelled a certain distance down each pathway, the accumulated genetic changes locks them into that path I would guess. Or at least locks them out of the other path. I.E. you are very unlikely to get hooved animals with primate like brains ans skulls due to hooved animals being specialised for a particular ecological niche. Development of characteristics designed for another ecological niche would decrease their fitness for their quadraped herbivore niche most likely.

 

That sound reasonable?

 

The reason we can't infer the precise selective driver for ancestral traits is due to our inability to measure the variables and control for each of them, not a lack of knowledge.

 

Same thing I would have thought. Anyway genetics is not a simple linear system but rather a very complex system with many cross intereaction between genes and groups of genes. Understanding single bits of the whole by controlling variables etc, does not necessarily give you an understanding of the whole.

 

There is a whole new field of epigenetics now which seems to involve what was previously regarded as junk DNA to a great degree.

 

I also pondered that, if the genes represent the computer data, then perhaps large parts of the fomerly junk DNA represent the operating system. That research has probably come a long way since I was at uni however.

Edited August 17, 2011 by Greg Boyles

[questionposter]

Species don't simply reach equilibrium and stop evolving. In addition, genes under environmental selection display non-Mendelian models of inheritance and most phenotypic traits are the result of multiple gene interactions.

 

I'm not saying a species ever stops evolving (unless it doesn't have DNA), I'm saying that a particular gene will not alter the whole of a species when any large alteration to it will only cause harm to the organism carrying it therefore it won't be passed on. I never said humans were "Done" evolving, but some alterations obviously won't make humans any more successful, such as alterations to the gene responsible for controlling the processes cell cycle, since alterations to those are either cancer or a shortened life span (that one disease where you age faster, I can't remember what it's called), which is why these alterations usually aren't caused by evolution since it would have been eliminated a while ago by the process of natural selection.

 

Yeah, giraffe's necks can be a little shorter or longer than average, that's fine, it's obviously a subtle enough alteration where it doesn't effect the well-being of Giraffe's enough. But something like a decrease of over two feet or increase of over two feet (in proportion to the giraffe) might cause some problems, and so a giraffe carrying that extreme alteration might not survive to pass on those more extremely altered genes.

Edited August 18, 2011 by questionposter

[Arete]

I'm not saying a species ever stops evolving (unless it doesn't have DNA), I'm saying that a particular gene will not alter the whole of a species when any large alteration to it will only cause harm to the organism carrying it therefore it won't be passed on. I never said humans were "Done" evolving, but some alterations obviously won't make humans any more successful, such as alterations to the gene responsible for controlling the processes cell cycle, since alterations to those are either cancer or a shortened life span (that one disease where you age faster, I can't remember what it's called), which is why these alterations usually aren't caused by evolution since it would have been eliminated a while ago by the process of natural selection.

 

Yeah, giraffe's necks can be a little shorter or longer than average, that's fine, it's obviously a subtle enough alteration where it doesn't effect the well-being of Giraffe's enough. But something like a decrease of over two feet or increase of over two feet (in proportion to the giraffe) might cause some problems, and so a giraffe carrying that extreme alteration might not survive to pass on those more extremely altered genes.

 

You seem to be describing deleterious mutations. They happen all the time, e.g. http://www.ncbi.nlm.nih.gov/omim. No one ever argued that deleterious mutations do not occur and their existence does not prevent potentially advantageous mutations - i.e. selectively driven evolution.

 

Your previous post stated :

Eventually a gene alters in the same way to a point where it doesn't change a species any more

 

If I'm interpreting correctly what you're saying is that traits will follow directional selection to a point of equilibrium, where any mutation from the current form will be deleterious. This is false:

1) Environments change. What is selectively disadvantageous today might be selectively advantageous tomorrow. Continuing your giraffe example - say a disease decimates acaias in Africa and being tall no longer offers any advantage as the food is now grass on the ground. The shortest giraffes are at a selective advantage and thus contribute more to the gene pool of the next generation than the tallest. Giraffes get progressively shorter - a giraffe 2 foot shorter than the mean is at a distinct selective advantage.

2) Predicting every possible mutation, and the phenotypic outcome of that mutation is impossible. Not only are there a myriad of possibilities due to point SNP mutation, but recombination can render entire genes non functional or switch on silent gene copies, etc. Using your "cell cycle" example - are you saying there's no possible way a mutation could make the process of mitosis more efficient?

 

It's simply not sensible to suggest that ANY mutation to a particular gene will be deleterious given the spatial and temporal variability of environmental selection and the sheer number of potential mutations. The presumption that paticular genes stop evolving as suggested in this part of your post:

 

Eventually a gene alters in the same way to a point where it doesn't change a species any more due to the fact that its unsuccessful and any members carrying a surplus of alteration die before they can pass it on, so then evolution selects some other gene to alter.

 

is simply not cohesive with evolutionary theory.

[questionposter]

is simply not cohesive with evolutionary theory.

 

Well actually, I was assuming that the environment was static, but if the environment changes dramatically, then yes, something that was a disadvantage before CAN be an advantage then. But what about heart problems? Someone with too big of a body in proportion to their heart simply dies at an early age, that's it, how can that ever help them? How does being born with your eyes too egg-shaped help you in any environment? You would still only be able to pick up optical light, and you wouldn't be able to even do it well.

Edited August 27, 2011 by questionposter