Natural Selection, why not even better?

[Gerinski]

I'l start by saying that I'm not a creationist or something like that, I fully support evolutionism and the scientific approach.

However, while natural selection can powerfully explain the traits of all current species, it can be hard to understand why if this is the mechanism driving evolution, it has not produced even better organisms.

I mean, surely we would be better off if we had eyes also in our back, multiple eyes have evolved in other organisms so why not in us or in many other animals? Or, we would be better off if we could dive in the water for a long time as cetaceans do. Or if we had echolocation. Or if we could see well in the dark as other animals do. Or if we had the smell of dogs. Or if we could survive under a wider range of ambient temperatures as many other animals do. And so on (I will not go to the extreme of "and if we could fly").

I am putting these examples from a human perspective to be more clear but the argument is general, given that many advantageous traits have evolved in many animals, it is somehow strange that these have not evolved in many other species, when they would surely mean also a benefit to them.

It seems that organisms have evolved just a few advantageous traits, and that "few" is a different set for each species. It seems strange that no organism has harnessed most (or a lot) of the best mutations but each species has only a few of them.

Humans have developed intelligence and the ability to manipulate things with our hands but have not developed many of the advantageous traits found in other species. If mutations are really random, it is hard to understand why a mutation for, say, being able to see a wider spectrum of electromagnetic radiation than our visible light spectrum has never happened to the human ancestors as it did in other species. Because if it ever did, it should have provided a survival advantage to its possessor and should have therefore stayed in time.

 

[Borg]

Evolution relates to mutations that improve a species' survival in their environment. For example, humans don't live in the water so there is no survival advantage to having gills.

when members of a population die they are replaced by the progeny of parents better adapted to survive and reproduce in the environment in which natural selection takes place.

In fact, species can often lose capabilities that they no longer need based on their environment such as fish that live in caves. Over time, several species of fish that originally had eyes, no longer have them. In a completely dark environment, eyes provide no advantage so fish without them have the same survival chances as a fish that does.

 

[DiracPool]

If mutations are really random, it is hard to understand why a mutation for, say, being able to see a wider spectrum of electromagnetic radiation than our visible light spectrum has never happened to the human ancestors as it did in other species. Because if it ever did, it should have provided a survival advantage to its possessor and should have therefore stayed in time.

As Borg pointed out, animals typically only retain traits that are essential to their survival and reproduction. It's a basically a trade-off between the usefulness of the trait and the energy cost of retaining the trait. If the trait isn't absolutely necessary, the energy investment typically isn't worth retaining or developing the trait.

As far as your example above, there really isn't a compelling survival advantage to expanding the capacities of the eye-brain network in humans to process ultraviolet light signals. The neural resources are better spent elsewhere. However, for a bee, there is a clear survival advantage, so they have developed the capacity to see into the UV range:

http://whyfiles.org/2012/bee-vision/

 

[Gerinski]

^ mmm, I can't see why being able to see UV or infrared should not provide any advantage compared to individuals able to see only visible light. Whatever the amount of "advantage", it can never be negative, seeing more is better than seeing less. And I don't see why this should mean a sensibly higher energy expenditure, many animals detect a wider electromagnetic spectrum than humans and I don't think they have that much more energy expenditure, I believe that humans have energy enough to cope with that without any significant loss in any other areas. Sorry but I don't buy that argument.

 

[Evo]

What makes you think that humans would ever acquire this ability in the first place? It's not like one day the human body goes "oooh, let's add *this*".

 

[Gerinski]

What makes you think that humans would ever acquire this ability in the first place? It's not like one day the human body goes "oooh, let's add *this*".

Of course not. But there are so many useful features found in "lower" animals which have not gotten their place in humans or other "higher" animals. It is hard to understand why mutations which helped other species to see better, to hear better, to smell better, to breath better, to echolocate themselves, to survive in a wider range of temperatures etc, none of them ever happened to the ancestors of humans. I guess the academical answer is just "well, it certainly could have happened but it just didn't", but the fact that they did for many other species still leaves an open question as to why similar mutations did not happen in many other organisms such as the human ancestors.

 

[Borg]

Of course not. But there are so many useful features found in "lower" animals which have not gotten their place in humans or other "higher" animals. It is hard to understand why mutations which helped other species to see better, to hear better, to smell better, to breath better, to echolocate themselves, to survive in a wider range of temperatures etc, none of them ever happened to the ancestors of humans. I guess the academical answer is just "well, it certainly could have happened but it just didn't", but the fact that they did for many other species still leaves an open question as to why similar mutations did not happen in many other organisms such as the human ancestors.

The problem with your logic is that the useful features that you give as examples aren't simple mutations. They are the combined result of mutations that occurred over a very long time due to environmental conditions for that species.

Think about this. How do you think that eyes formed? Did they just suddenly appear on some creature one day? Why don't plants have eyes?

 

[Ryan_m_b]

As others have pointed out traits to not simply mutate fully formed (at least of the sort you are describing). Whilst the end product might confer a survival/reproductive advantage the steps required to get there clearly do not. In other organisms though these individual steps did. The difference is one of environment (at various times in their evolutionary history their environment favoured those mutations) and lineage (their evolutionary history itself made it possible for these mutations to occur).

 

[.Scott]

^ mmm, I can't see why being able to see UV or infrared should not provide any advantage compared to individuals able to see only visible light. Whatever the amount of "advantage", it can never be negative, seeing more is better than seeing less. And I don't see why this should mean a sensibly higher energy expenditure, many animals detect a wider electromagnetic spectrum than humans and I don't think they have that much more energy expenditure, I believe that humans have energy enough to cope with that without any significant loss in any other areas. Sorry but I don't buy that argument.

If the information we need is in the visible spectrum, adapting our optics to include infrared or uv will comprises what we need for something we don't need.

Just as an example of what we don't need, take a look at the bands collected by the Thematic Mapper (a general purpose imaging satellite):
http://igett.delmar.edu/Resources/Remote Sensing Technology Training/Landsat_bands-sm.pdf
It has 8 bands, including visible and infrared - but no ultraviolet.
There are modern advantages to look in the infrared. But if you review Landsat imagery, I think you can see that there would be nothing of critical use to our ancestors.

 

[Ygggdrasil]

We have the capability to make cars that drive extremely fast (>250 mph/400 kmph), so why can't the typical car you see around town reach these speeds?

The answer to this question and the question you ask is the same: cost. Many traits come with a cost, for example, in terms of the resources required to form and develop new organs or structures. If these traits confer only marginal fitness benefits to a particular organism, organisms lacking that trait would likely be able to outcompete those with that trait just because they use their resources more efficiently.

In another thread, I discuss and link to a news piece from Science that talks about the evolutionary trade-offs associated with increased intelligence that explains why some animals have not evolved increased intelligence.

 

[Drakkith]

^ mmm, I can't see why being able to see UV or infrared should not provide any advantage compared to individuals able to see only visible light. Whatever the amount of "advantage", it can never be negative, seeing more is better than seeing less.

Not if the cost of seeing these wavelengths results in a situation that lowers the fitness of an individual. For example, the human eye can see UV in a limited manner, but only if you remove the lens. However, removing the lens would lead to a loss of fine focus and would be very detrimental to our fitness, so it will not happen.

And I don't see why this should mean a sensibly higher energy expenditure, many animals detect a wider electromagnetic spectrum than humans and I don't think they have that much more energy expenditure, I believe that humans have energy enough to cope with that without any significant loss in any other areas. Sorry but I don't buy that argument.

It's not always about energy expenditure. Changing the structure of the eye could mean it is more susceptible to injury, which would lower our fitness. You also have to consider that even if there is a way to alter our eyes without making them more fragile, there may be no way to get from our current eye to the new one without going through steps in between that reduce our fitness. A good example is the recurrent laryngeal nerve (RLN), the nerve that controls your larynx. You would expect that the nerve would split off from the main vagus nerve around the level of the larynx and run directly to it. But this isn't the case. The two branches of the RLN run all the way down into your chest before splitting off and running back up to your larynx. The left RLN in particular takes a very indirect approach, looping under your aortic arch before running all the way up to your larynx.

You would think that evolution would change this nerve, as it is susceptible to damage from blows to the chest and moving it would shorten and protect it. But evolution can't. To do so would require several major changes during the development of the embryo, and each of these changes would affect the fitness of the person. There's simply no intermediate stage that works, so evolution leaves it as it is.

 

[DiracPool]

You would think that evolution would change this nerve, as it is susceptible to damage from blows to the chest and moving it would shorten and protect it. But evolution can't. To do so would require several major changes during the development of the embryo, and each of these changes would affect the fitness of the person. There's simply no intermediate stage that works, so evolution leaves it as it is.

This reminded me of an old paper I read by Terrence Sejnowski where he draws an anology between brain evolution and the upgrading of an old power plant that couldn't afford to be taken "offline."

It took me a few minutes here to find the reference, and it was actually in a review article by Sejnowski on John Allman's book, Evolving brains. I actually read that book too and own it, but remember the passage from Sejnowski's review. In any case, here it is:

Evolution tends to build on the past. Allman illustrates this by re-counting a visit to the boiler room of an old power plant, where he noticed an intricate array of small pneumatic tubes next to a bank of vacuum tubes, alongside several generations of computer control systems. Because the plant was needed for continuous power output, it could not be shut down and retrofitted with each new technology, so the old control system remained in place and the new ones were integrated into the existing system. So too with evolving brains: Nature cannot afford to throw out an old brain system, but makes do by tinkering with the existing developmental plans and occasionally adding a new layer of control.

https://papers.cnl.salk.edu/PDFs/A High Point for Evolution 1999-2940.pdf

The article and Allman's book also go into some discussion on the evolution of the visual system and the tradeoffs in development vs energy concerns discussed in this thread in case the OP is interested.

 

[Borg]

Couldn't resist. :tongue2:

 

[Pythagorean]

Evolution is more "lazy" than you presume. It need only do the minimum required to sustain a genetic lineage. It needn't go further.

The selection process uses death to weed out the unfit, not hearing amd seeing tests. If you can survive (and reproduce) without hearing or seeing, you pass.

 

[Darryl]

I think it depends on your definition of 'better', as far as humans go, most things seem to be worse that other animals, or our past selves.

Evolution does not have a direction, I also think it should be "nature selection" not 'natural', artificial or 'unnatural' don't really apply (again IMO).

Apart from a big brain, we have lost most anything else that is considered 'advancement', we are weak, slow, don't see or smell well, we can't run fast or that far.

As for our intelligence, it gets us by and is sufficient to give us the required edge to survive as a species, but does that mean MORE intelligence would give us more advantage? Is there any real selective pressure based on a persons level of intelligence now?

In the past being a skilled hunter meant that you were also a able provider for a family, but once you have the level of intelligence to skilfully hunt and provide does more intelligence help you much?

Height is one thing that seems influenced by evolution, people see (generally) tall people to be more successful and potentially more able to be a good provider, so tall people get all the nice looking tall girls, (not me, I am short), the tall gene is then more favoured that shorter, so over time we evolve to be taller, (but not too tall).

 

[Gerinski]

^ I think not even that is relevant anymore for humans, now (in developed countries at least) everybody has basically the same chance of survival and reproduction, tall or short, clever or dumb, physically strong or weak. Of the relatively few people who die without any offspring, I don't think for many of them the cause was a weak genetic profile (except for those few who die very young due to some severe health issue). And it's not the case anymore either that the 'fittest' have more offspring than the average person.
In the developed world natural selection is not present anymore. Will this have any long-term effects in the quality of the human gene pool? who knows...

 

[Gerinski]

BTW, what is the proposed explanation for the persistence of genetic disorders which affect the chances of reaching adulthood or of reproduction in the gene pool? Such as cystic fibrosis, which causes infertility (in 97% of the males and 20% of the females according to Wiki). A simplistic Darwinian interpretation would suggest that such a disorder should have long been eliminated from the human gene pool but the fact is that it is still relatively common in Caucasians. This is another form of the question "why not even better?". Why not having eliminated such genetic disorders altogether?

 

[Chronos]

Evolution is a crap shoot. For every successful mutation, there are innumerable failed mutations. Furthermore, there are a number of genetic disorders that are just a matter of bad luck. For example, it has been shown that acquiring one copy of the gene responsible for sickle cell anemia confers protection from malaria. Acquiring this gene from both parents results in sickle cell anemia.

 

[Ryan_m_b]

^ I think not even that is relevant anymore for humans, now (in developed countries at least) everybody has basically the same chance of survival and reproduction, tall or short, clever or dumb, physically strong or weak. Of the relatively few people who die without any offspring, I don't think for many of them the cause was a weak genetic profile (except for those few who die very young due to some severe health issue). And it's not the case anymore either that the 'fittest' have more offspring than the average person.
In the developed world natural selection is not present anymore. Will this have any long-term effects in the quality of the human gene pool? who knows...

There is no such thing as a "weak genetic profile". Whether or not a gene is advantageous, neutral or deleterious is almost always entirely contextual. A gene granting resistance to a certain disease is of no benefit in a society with adequate hygiene and healthcare. In fact it might be a disadvantage if there are side effects to resistance (e.g. sickle cell trait and malaria).

You seem to be leaning towards a way of thinking that "fitness" in an evolutionary sense is equivalent to everyday human usage implying that with strong selective pressure we would all be strong, smart, healthy individuals. Aside from the fact that being strong, smart etc isn't always the best survival strategy when it comes to health you need to consider the evolutionary arms race between predator/prey and host/disease. Essentially whilst natural selection might lead to a species developing resistance towards a certain disease it can also lead to the disease adapting to work around said resistance. Most of the time this means that no species comes out dominant but rather there is a constant too-and-fro, this is called the red queen effect.

If you apply this to the way of thinking that using technology to keep people alive makes us weaker you realize that this isn't the case. Without technology we would have a situation in which generations adapt to fight disease but disease keeps up ensuring a constant threat.

BTW, what is the proposed explanation for the persistence of genetic disorders which affect the chances of reaching adulthood or of reproduction in the gene pool? Such as cystic fibrosis, which causes infertility (in 97% of the males and 20% of the females according to Wiki). A simplistic Darwinian interpretation would suggest that such a disorder should have long been eliminated from the human gene pool but the fact is that it is still relatively common in Caucasians. This is another form of the question "why not even better?". Why not having eliminated such genetic disorders altogether?

Cystic fibrosis is caused by a recessive mutation. You can carry a copy of a damaged CFTR gene and be absolutely normal if your second copy works fine. You can only have cystic fibrosis if both your parents are carriers and through chance (a 1/4 chance) passed on two faulty copies to you. Thus whilst a person with CF is unlikely to reproduce due to infertility and reduced life expectancy genes leading to CF persist in the population due to carriers.

 

[Jupiter60]

Why haven't any animals evolved wheels? Surely that would be an improvement for speed.

 

[PeroK]

Why haven't any animals evolved wheels? Surely that would be an improvement for speed.

http://en.wikipedia.org/wiki/Rotati...ystems#Known_instances_of_rotation_in_biology

 

[Drakkith]

Why haven't any animals evolved wheels? Surely that would be an improvement for speed.

There's only one case in which they have, but that's restricted to certain types of flagellum used by bacteria.

In large, multicellular amimals there is no evolutionary path from legs to wheels that isn't fatal or nearly fatal to an animal. The intermediate steps between legs/arms and wheels are effectively useless for anything else. In addition, a wheel and axle is a very specific configuration that is much more complicated than other types of joints. A free spinning biological joint is practically impossible. The species would need to evolve totally unique ways of supplying oxygen and nutrients to the tissues of the wheel without using blood vessels, and it would need to evolve this mechanism before evolving the wheel.

Put simply, the adaptions necessary for wheeled locomotion are extremely unlikely to evolve via natural selection.

More here: http://web.archive.org/web/20070221.../Dawkins/Work/Articles/1996-11-24wheels.shtml

From the link above:

The fact that only very small creatures have evolved the wheel suggests what may be the most plausible reason why larger creatures have not. It’s a rather mundane, practical reason, but it is nonetheless important. A large creature would need large wheels which, unlike manmade wheels, would have to grow in situ rather than being separately fashioned out of dead materials and then mounted. For a large, living organ, growth in situ demands blood or something equivalent. The problem of supplying a freely rotating organ with blood vessels (not to mention nerves) that don’t tie themselves in knots is too vivid to need spelling out!

Human engineers might suggest running concentric ducts to carry blood through the middle of the axle into the middle of the wheel. But what would the evolutionary intermediates have looked like? Evolutionary improvement is like climbing a mountain (“Mount Improbable”). You can’t jump from the bottom of a cliff to the top in a single leap. Sudden, precipitous change is an option for engineers, but in wild nature the summit of Mount Improbable can be reached only if a gradual ramp upwards from a given starting point can be found. The wheel may be one of those cases where the engineering solution can be seen in plain view, yet be unattainable in evolution because its lies the other side of a deep valley, cutting unbridgeably across the massif of Mount Improbable.

Even more info here: http://en.wikipedia.org/wiki/Rotating_locomotion_in_living_systems

 

[BasicComplex]

Natural selection happens, it's just not a blind or random process as was assumed. I think creationists had that right though

 

[aroc91]

It was never assumed that selection was blind or random. Mutation, on the other hand...

 

[fedaykin]

Wheels would be enormously different from current structure in place. How would you grow and maintain an axle? How would you prevent infection there? Any steps on the way to making them would probably be non-functional, and wheels would probably be less agile than limbs for most terrain.

 

[Torbjorn_L]

However, while natural selection can powerfully explain the traits of all current species, it can be hard to understand why if this is the mechanism driving evolution, it has not produced even better organisms.

I can understand the question, but it is a non sequitur. The resident article on evolution ought to answer this.

However, let me try to respond to this and other confusions in the thread:

- Evolution is a process on populations over generational time scales.

What drives the process is differential reproduction, that descendants that 1) survives and 2 ) do better in bed are promoted. The requirement is to "do well enough", not to maximize traits.

A test for that is that since life started out as simple cellular populations it should at best diffuse somewhat from that and we would expect most organisms are simple cellular populations. That is indeed the case, most of the biomass are prokaryotes.

- Evolution has several mechanisms that drives it. Among them are

1. Variation, that is _indifferent_ to selection, not "random". In most cases but not all variation is random mutational, but there are both non-mutational (say, polyploidy) and non-random variation (say, radioactive radiation responses increasing with dose).

2. Selection, that is acting on large enough negative and positive fitness. In large enough populations (say, humans) small fitness differences can be picked up and driven towards fixation (say, being the only allele) without random "noise" interfering.

In such cases a local fitness maximum can be found. There are ways that populations can climb out of such traps or scale wide differences between maxima, but there isn't any guarantee that a global minimum will be found. It is quite easy to see that it doesn't happen, say the genetic code which is robust against variation but not the best in that regard. It is "a frozen accident", the process was frozen before it found the global maximum (if it could).

3. Near neutral drift, where random "noise" interferes with selection by the fitness differences being too small so "good enough" applies. Usually happens in small populations.

- And of course since we look at populations, these mechanisms are dynamical.

If selection is forceful, much variation will be extinguished. Future generations will then have problem to respond adaptively and have very hard to "sense" the available fitness surface.

If selection is weak, the population will amass variation (alleles). It is time dependent, which is why we see most variation in old populations even where selection has been forecful. The Same/Finn populations have most variation, is oldest, in Europe despite being a small remnant population of descendants of the original hunter-gatherers that were driven, selected, to the edge of the landmass.

TL;DR:

- We see what we expect of the process and its mechanisms, differential reproduction of "good enough" in populations over geological time, and hence little complexity seen over the entire biosphere.

- To expect that evolution is only its mechanism of selection is a strawman. It's akin to say that falling is only gravity, not the presence of masses or black holes that make you go "ouch" at times.

And to expect that evolution will seek out a global maximum for all, or even a few, populations is a common misunderstanding. It's akin to say that falling will always lead to approaching the universal speed limit. oo)

 

[Torbjorn_L]

In large, multicellular amimals there is no evolutionary path from legs to wheels that isn't fatal or nearly fatal to an animal. The intermediate steps between legs/arms and wheels are effectively useless for anything else.

Also, these mechanisms aren't usually robust against wear and tear.

If you are a young plant hopper, leaping one metre in a single bound, you need to push off with both hind legs in perfect unison or you might end up in a spin. Researchers have discovered that this synchrony is made possible by toothed gears that connect the two legs when the insects jump.
.

When the insect jumps, the cog teeth join so that the two legs lock together, ensuring that they thrust at exactly the same time (see video above and image at left). “The gears add an extra level of synchronization beyond that which can be achieved by the nervous system,” says Burrows.

Curiously, the gears are found only in the plant hopper's nymph stage. When it sheds its exoskeleton for the last time to reach full adulthood, the gears disappear. In adults, the legs are instead synchronized by a simple, frictional-contact method.

Burrows and Sutton are not yet sure why this is so, but it might be because of ease of repair. “If a gear breaks it can’t be replaced in adults,” says Burrows. “But in nymphs, a repair can be made at the next of several moults.” Another reason might be that the larger and more rigid adult bodies make the frictional method work better than the gear mechanism, he adds.

[ http://www.nature.com/news/insect-leg-cogs-a-first-in-animal-kingdom-1.13723 ; my bold]

I note though that animals and plants have evolved large scale wheels, the whole organism (tumble weeds, caterpillars, armadillos, ...). Such structures have survivable intermediate steps (even a partial roll structure protects and moves) and can be repaired (skin replacement) or toughened (scales).

Spiders have evolved something better than wheels, wheels are for training:



Cartwheeling makes them twice as fast than wheeling would, which is seen in other spiders in similar situations. (The predators that made them evolve this flight response are fast, winged egg laying parasites, wasps or flies IIRC.) It is barely sustainable though, unlike humans doing it a handful [sic!] of times during a day will kill them.

 

[Masceritoy]

There is a thought I want to put out there. At some point in our history, we developed, for lack of a better word, humanity. The desire to not kill or let our fellow humans die, no matter their characteristics, or weak or strong mutations. This was a prelude to a "societal norm," where people who were different, good or bad, were ostracized and unable to pass on that trait, which has led to a general stagnation of our species, where the only things really changing are height and skin color. I just want to see if this could be considered an accurate theory.

 

[Evo]

There is a thought I want to put out there. At some point in our history, we developed, for lack of a better word, humanity. The desire to not kill or let our fellow humans die, no matter their characteristics, or weak or strong mutations. This was a prelude to a "societal norm," where people who were different, good or bad, were ostracized and unable to pass on that trait, which has led to a general stagnation of our species, where the only things really changing are height and skin color. I just want to see if this could be considered an accurate theory.

Hi Masceritoy. No, people are still good or bad and many, many things change besides height and skin color.

Also, please be careful of using the word "theory" on a science forum. :) What you have there is a guess. We also do not allow actual personal theories on this forum, it's in the rules.

 

[Drakkith]

I note though that animals and plants have evolved large scale wheels, the whole organism (tumble weeds, caterpillars, armadillos, ...). Such structures have survivable intermediate steps (even a partial roll structure protects and moves) and can be repaired (skin replacement) or toughened (scales).

Wheels are defined as a circular component that rotates around an axial bearing. Rolling or cartwheeling organisms are not wheels. The challenges of evolving an axle and wheel are absolutely nothing like evolving the ability for an organism to roll.

An interesting link, though. I've never seen an example of gears in an organism before. Thanks for the info.

 

[Torbjorn_L]

At some point in our history, we developed, for lack of a better word, humanity. The desire to not kill or let our fellow humans die, no matter their characteristics, or weak or strong mutations.

Such a humanity evolved very early. The earliest example may be eusocial ants, whose lineages split over 100 million years ago. [ http://en.wikipedia.org/wiki/Ant ]

"Eusociality is the highest level of social organization. It is characterised by:

Overlap of adult generations
Reproductive division of labour
Cooperative care of young"

[ http://en.wikipedia.org/wiki/Social_animal ]

Whether eusocial organisms weed out differences in youngs are specfics - we do it as well - they certainly may euthanize sick individuals - we do it as well (Belgium IIRC, sufferers may ask and get it).

This was a prelude to a "societal norm," where people who were different, good or bad, were ostracized and unable to pass on that trait, which has led to a general stagnation of our species, where the only things really changing are height and skin color.

The link above goes into evolution of sociality, which is an old phenomena indeed.

As for humans, as you could surmise from my earlier comments, evolution works better than ever. Since we are so numerous (efficient evolution) and since it happened recently (change in environment), we should be among the fastest evolving animals these days. And, arguably, that is what science has found:

"Still, a very small fraction of the mutations in any given population will be advantageous. And the longer a population has existed, the more likely it will be close to its adaptive optimum -- the point at which positively selected mutations don't happen because there is no possible improvement. This is the most likely explanation for why very large species in nature don't always evolve rapidly.

Instead, it is when a new environment is imposed that natural populations respond. And when the environment changes, larger populations have an intrinsic advantage, as Fisher showed, because they have a faster potential response by new mutations.

From that standpoint, the ecological changes documented in human history and the archaeological record create an exceptional situation. Humans faced new selective pressures during the last 40,000 years, related to disease, agricultural diets, sedentism, city life, greater lifespan, and many other ecological changes. This created a need for selection.

Larger population sizes allowed the rapid response to selection -- more new adaptive mutations. Together, the the two patterns of historical change have placed humans far from an equilibrium. In that case, we expect that the pace of genetic change due to positive selection should recently have been radically higher than at other times in human evolution. ...

So to test the null hypothesis, we should look for evidence of these fixed selected substitutions. ...

This large number of completed sweeps should have vastly reduced human genetic variation, because polymorphisms tend to hitchhike along with nearby selected alleles. Hitchhiking up to fixation tends to eliminate variation. When we look at the effect of hitchhiking under this constant selection hypothesis, the genome-wide average diversity should be less than a tenth of what we actually observe. So that also disproves the null hypothesis. ...

Our evolution has recently accelerated by around 100-fold. And that's exactly what we would expect from the enormous growth of our population."

[ http://johnhawks.net/weblog/topics/evolution/selection/acceleration/accel_story_2007.html ; my bold]

That our species has "stagnated" is a poor guess, and it seems contradicted by observation.

 

[Torbjorn_L]

Wheels are defined as a circular component that rotates around an axial bearing. Rolling or cartwheeling organisms are not wheels.

Ah, thanks! We have a language difficulty, english isn't my first language, and that was a nuance lost on me.

On the other hand, there is an animal that has evolved an axial bearing, but it is a screw:

"Here’s the only figure in the article. It shows reconstructions of the ‘coxa’ (the first leg joint, where the leg joins the thorax) in green, and the trochanter, the first leg segment, in yellow. The coxa is the thread, the trochanter the screw. There’s also a nice scanning electron microscope image of the trochanter. The system works pretty much like your hip does – except the knobbly bit at the end of your hip has turned into a screw shape. (All the nerves and stuff run down the centre of the trochanter.) Most insect joints work like hinges."

So why doesn’t the leg unscrew itself? Most of the weevil’s leg motion will not involve a 360 degree rotation, or the hapless insect would get stuck after a couple of paces. Instead it will simply swing back and forth. Doesn’t the leg get blocked when it’s screwed into the max? Presumably so, in which case careful observation should show weevils back-pedalling to unwind their legs. And conversely – why doesn’t the leg come unscrewed? The muscles appear to hold it in place. Phew!"

"Now why does the weevil have this odd arrangement? The obvious advantage is that you can rotate the leg right round. But in that case, why not evolve the axle/wheel combination? The authors speculate that the screw might be better:

We suggest that an advantage of this construction is that the leg comes to a stable resting position, preventing passive straining of leg muscles, which would not be accomplished by an axle construction.

Above all, they think that it might be the weevil’s unique feeding posture, where it shoves its rostrum (its ‘snout’) into its food, that holds the key. Substantial forces will be generated on the weevil’s legs as it tries to grip the substrate; having a screw would effectively block the rotating joint, stopping the weevil from ending up with its head smashed in the food."

[ http://whyevolutionistrue.wordpress.com/2011/06/30/the-weevil-with-screws-in-its-legs/ ]

So it is has a partial wheel action, if I understand the objection correctly, and it could have evolved a another type of axis if the evolutionary constraints had been slightly different.

[On another note, it is odd if this 2011 paper hasn't gotten more around more re this topic.]

The general reason why wheels aren't evolved is likely that they are impractical and inefficient for most animal or plant uses (uneven "road", as seen here tiring for muscles).

Gears, screws, ... what will they come up with next? Windows and lenses!?

Slit lamp image of the cornea, iris and lens (showing mild cataract) [ http://en.wikipedia.org/wiki/Cornea ]

 

[Drakkith]

Ah, thanks! We have a language difficulty, english isn't my first language, and that was a nuance lost on me.

No problem!

 

[Rocky9242]

>>Apart from a big brain, we have lost most anything else that is considered 'advancement', we are weak, slow, don't see or smell well, we can't run fast or that far.

Apparently, we can run "that far." See this Wikipedia article on "persistence hunting:"
http://en.wikipedia.org/wiki/Persistence_hunting

 

[Rocky9242]

Why haven't any animals evolved wheels? Surely that would be an improvement for speed.

A lot of animals have evolved rotary bearings. It would not be much of a stretch from there to place a wheel on the end of that axel.

Some animals make themselves into "wheels" (or balls or something) that can roll downhill.

This Wikipedia article has a good discussion of these structures and issues related to their evolution:
http://en.wikipedia.org/wiki/Rotating_locomotion_in_living_systems