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El Zagna
02-23-2010, 10:51 PM
It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism, but as life fine tuned itself through evolution, those mutations would be less and less likely to be beneficial.

So two questions: Is this logic valid, and is this happening?

DCnDC
02-23-2010, 11:05 PM
The environment and conditions on Earth are and always have been constantly changing, and life has been changing with it. Countless mutations ultimately prove to be non-beneficial, while those that are get passed on and further refined as conditions change; these mutations happen on a daily, hourly, momentary basis on every level big and small. Evolution is an ongoing, never-ending process; there is never a "final product," despite man's egotism. Specific mutations that are beneficial don't become less beneficial over time or they would not survive.

Tapioca Dextrin
02-23-2010, 11:20 PM
Life first started out about 3.5 billion years ago. It spent 3 billion years as single celled bacteria. Even today, the vast majority of life is single celled bacteria. Us multi celled weirdos are just irrelevant freaks.

HMS Irruncible
02-23-2010, 11:34 PM
It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism, but as life fine tuned itself through evolution, those mutations would be less and less likely to be beneficial.

So two questions: Is this logic valid, and is this happening?
Organisms are still (in recent geological history) evolving in very useful directions... just because they're not re-inventing the gill or the spinal cord or the opposable thumb doesn't mean that evolution isn't occurring in very interesting ways... maybe resistance to a bacteria, or ability to exploit some new plant food, things like that.

Also, regarding the pace of evolution, it has been theorized that it actually speeds up or slows down for particular species in particular environments. It has nothing to do with life 'fine tuning' itself but on the selective pressure on the organisms in that niche in time.

John Mace
02-23-2010, 11:47 PM
It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism, but as life fine tuned itself through evolution, those mutations would be less and less likely to be beneficial.

Why do you think this?

Smeghead
02-24-2010, 09:26 AM
I think what you're talking about is best represented in the scientific community by the concept of fitness landscapes. Think of a two dimensional plane representing all possible genotypes (in reality, this would have to be many, many dimensions, but for simplicity's sake, stick with two). Now, let's say that for each point on the plane, we raise that point to a height representing the relative fitness of that genotype - if it's really, really good, it'll be a high peak, while if it's only mediocre, it'll be a small mound. We're left with a landscape of peaks and valleys.

Now, one important thing about evolution is that it's only able to move in small steps, which means it's going to be difficult (though not always completely impossible) to cross big valleys. Once you're on the slope of a peak, you tend to stay there, and move only to the top of that particular peak, even if there's a much higher peak not very far away. In technical terms, you tend toward the local maximum.

So given a static fitness landscape, yes, organisms would evolve to their local peak and stay there. In other words, they'd adapt what they have as best they can and then stop evolving. However, this isn't the case. Fitness landscapes are never static. They're constantly changing and evolving themselves, as the environment changes. We see this a lot with human activity. Organisms that haven't needed to change in millions of years suddenly find themselves horribly maladapted when humans show up. Look at the dodo or any of thousands of other examples of island species for an illustration.

panache45
02-24-2010, 09:53 AM
It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism . . .

Why do you think this? It's more reasonable to assume that, since mutations are random, they can just as easily be harmful as beneficial.

Gagundathar
02-24-2010, 10:00 AM
This may be supposition, but now that there is a species that can somewhat direct its own development, might we consider that directed evolution will augment the usual genomic flow?

Chessic Sense
02-24-2010, 10:33 AM
Even humans have evolved traits fairly recently. Eurpoeans just became able to produce lactase, so that we can digest the lactose in cow milk. This happened when cows were introduced to Europe. The entire continent evolved in (I'm quoting from memory, so I'm a little off) 3 or 4 generations. It's easy to see how that "landscape" changed when cows showed up.

Really Not All That Bright
02-24-2010, 10:34 AM
Why do you think this? It's more reasonable to assume that, since mutations are random, they can just as easily be harmful as beneficial.
Indeed, now and then, 99% of mutations would disappear within a generation or two in complex organisms.

Quercus
02-24-2010, 10:40 AM
The logic is more-or-less valid, in fact it's the germ of the idea of 'punctuated equilibrium', which is part of modern biology (the arguments are really about how much punctuated equilibrium happens, compared to slow-and-steady changes), but there are a couple points to add:

First, you want to think about the starting point being not just the first instance of life, but any time the environment changes drastically. This means that even an organism that has evolved for a long time to be highly fit in the old environment is now far away from the optimum. An analogy might be taking a modern production car, which is pretty well suited for being a civilian passenger car, and putting it into a racing situation, where it's not that well suited anymore. So we tend to see bursts of evolution when an organisms surroundings change drastically (an ice age, or even a competitor arriving in the area, say from a new land bridge), and then relative stability until something else changes.

And second, it's not so much that any change is likely to be positive, as the fact that the positive changes are going to be really valuable. Again, use the analogy of a production car. Most random changes to the car are going to make it worse for both regular use and racing use (since any random change is most likely going to make the car not run). But it's hard to make it a much better street car, because it's already a good street car, so improvements are going to be small. But it's easier to make it a much better racer (e.g. drop the passenger seats), since it's not a great racer right now.

jayjay
02-24-2010, 10:42 AM
This may be supposition, but now that there is a species that can somewhat direct its own development, might we consider that directed evolution will augment the usual genomic flow?

Directed human evolution is a) difficult to practically bring into effect and b) fraught with ethical conundrums. Eugenics is basically an attempt to direct human evolution, and since its heyday in the late 19th/early 20th centuries, it's gone out of favor.

md2000
02-24-2010, 11:07 AM
Now, one important thing about evolution is that it's only able to move in small steps, which means it's going to be difficult (though not always completely impossible) to cross big valleys. Once you're on the slope of a peak, you tend to stay there, and move only to the top of that particular peak, even if there's a much higher peak not very far away. In technical terms, you tend toward the local maximum.

So given a static fitness landscape, yes, organisms would evolve to their local peak and stay there. In other words, they'd adapt what they have as best they can and then stop evolving.

If you look at history of various species, generally they start out as smaller, less specialized types... The example, perhaps, is the Kangaroo. I saw a small rodent in the Sydney zoo which they think was related to the precursor of all kangaroos. Without different competition, this thing - that looks like a mouse that hops as much as it walks - diverged into various other species, including much larger kangaroos. Why? Think of the fitness landscape mentioned above as "opportunity", or food and safety. Does what the animal have fit neatly into what's available? Is there some improvement that makes a better fit - gets them more food, protects them from predators better? Bigger size, faster jumps, different coloration? Every step in that direction means you and your offspring are more likely to survive. However, if someone else is already there, you have to be better than them to eat ther lunch. Perhaps the Kangaroos were lucky that they had limited competition in isolated Australia.

Size works in some situations - but... if you are too big, what happens if there's a drought or other food shortage? The smaller ones survive, usually unless the big ones can push them aside. A bigger animal needs more range meaning more exposure. A bigger monkey, for example, may be too heavy to reach the outer branches of a tree.

Ditto for smart or fast; smart uses up a lot of calories. That brain better be good for something, like figuring out how to get away from predators, or how to steal the chimp's lunch. Extra fast means extra lean - not an advantage if food is periodically scarce. Extra fast means no wasted matter - if that brain doesn't need to be that big, it's a disadvantage to carry it around; so you're either slow and smart, or fast and dumb. Evolution is a trade-off.

Once some species reaches that "peak", they are optimized for their conditions. As pointed out, those conditions could change.

Imagine the first pair of lemurs washed up on Madagascar - an island full of food with nothing except maybe some insects eating it; instant population explosion! As the population exceeds the food, now some mutations allow you to eat something nobody else does - you and others like you enjoy a food and population boom, but only if you mate with others who can eat more easily that food. You nutcrackers are your way to being a separate species from the fruit eaters.

To steal someone else's lunch, and push them out of an ecological niche? You need an edge. For example, warm blooded animals are able to keep moving while the cold-blooded ones slow down; so a rat has an advantage over a lizard in cooler climates; once again, it comes with a price - to stay warm it has to eat; rats wolf down everything in sight, and reptiles eat very little. So reptiles can hold their own in deserts where food is scarce, or tropical climates where temperture is not an issue.

Similarly with humans - there are obvious adaptions - skin color for sun exposure, for example. There are more subtle ones - there are some Andean Indians well-adapted for high altitudes. Eskimos have the body shape and fat deposits well-adapted for cold weather. Europeans males have the rampant facial hair needed to protect hunters from bad climates.

However, humans game the system. We make our food with tricks like irrigation or imports, so are less susceptible to food pressures.We have things like sunscreen and clothing, vaccines and antibiotics, and vitamin supplements to offset environmental or body shortcomings. We use eyeglasses, teeth braces, and plastic surgery to enhance our abilities and reproduction appeal despite the fact we will pass those bad genes on to our children. The most successful sue birth control more often, thus ensuring that in fact the least capable are more likely to reproduce. Perhaps in the case of the most successful humans, evolution is running backward.

So the short answer is - evolution happens when there is pressure to change. When an animal is well-adapted to a stable environment, they have probably reached their peak.

Mangetout
02-24-2010, 11:12 AM
It seems to me that in the early days of life forms, any given genetic mutation had a relatively high chance of being beneficial to the organism, but as life fine tuned itself through evolution, those mutations would be less and less likely to be beneficial.

Do you mean that ecological niches might be generally underexploited and even where occupied, maybe not by a vast diversity of competition? That could be correct, but in the big picture, it probably only lasted a comparatively short time.

F.Pu-du-he-pa-as
02-24-2010, 11:25 AM
I'd like to take the quantitative approach here-- how can we measure the rate of evolution, and what does it look like?

In principle, this is extremely simple. If you have an evolutionary tree based on DNA sequences, it will give you a measure of how many nucleotide changes took place along a given branch. That gives you some sort of distance measure. Then, if you know how much time each branch represented, you can use the formula distance = rate x time to find the rate of evolution.

But this is problematic for a number of reasons.

First, the distances will be an underestimate for the actual amount of evolution that took place. If one nucleotide changes to another nucleotide, then back to the original, then you would really have two changes, but you would measure zero changes.

Second, determining the amount of time a branch represents is tricky. One of the sources of calibration points is the fossil record, which of course suffers from preservation bias.

Third, there is the assumption that the rate of evolution is the same across all stretches of DNA. This is clearly wrong. Genes coding for the crucial functional sites of enzymes change more slowly than the not as crucial parts of the enzyme, or junk DNA.

So, to give a quantitative answer to your question, we would have to factor in a certain amount of error in the distances and have a complete and precisely dated fossil record of early life on Earth, which we don't have and never will. We would also have to figure out some way to answer your question that accounts for the differing rate of evolution within the genome of a single organism.

Still, quantitative assessment of rates of evolutionary change is a very hot topic in phylogenetics right now, and a lot of interesting work is going into it both on the biology and the CS/math sides of things.

El Zagna
02-24-2010, 01:42 PM
Good job as usual, Dopers. You fine tuned my question and covered it and more. Thanks.

septimus
02-24-2010, 01:52 PM
Measuring "evolution" isn't done by just counting nucleotide changes -- the vast majority of these are irrelevant, neither harmful nor helpful. Instead one looks for long stretches of DNA that are nearly constant throughout a population (of humans for example), but different from a separate related group (e.g. a different human or primate population): that's the signature of a useful mutation.

I read recently that when this is done, the evolutionary distance between two human groups (e.g., Australian and Amerindian), that is the changes in 60,000 years or so, is much greater than would be expected if evolution had continued at the same rate as it did over the millions of years separating humans from a common ape ancestor. I think a big part of the reason for this is that evolution proceeds much more readily with a large population. (When there's a survival bottleneck of a few hundred individuals you're "stuck with what you've got;" when there's many thousands, you've got "room to experiment.")

The Other Waldo Pepper
02-24-2010, 02:11 PM
Directed human evolution is a) difficult to practically bring into effect and b) fraught with ethical conundrums. Eugenics is basically an attempt to direct human evolution, and since its heyday in the late 19th/early 20th centuries, it's gone out of favor.

Forget eugenics. How far do you think we are from directed human evolution by way of practical and effective genetic manipulation rather than selective breeding?

Mosier
02-24-2010, 03:12 PM
Life first started out about 3.5 billion years ago. It spent 3 billion years as single celled bacteria. Even today, the vast majority of life is single celled bacteria. Us multi celled weirdos are just irrelevant freaks.

How are you measuring? Nearly all of the biomass on the planet comes from multi-celled organisms.

Really Not All That Bright
02-24-2010, 03:32 PM
Probably by absolute number of organisms. In terms of biomass plants alone would vastly outweigh all single-celled organisms.

md2000
02-24-2010, 04:49 PM
Forget eugenics. How far do you think we are from directed human evolution by way of practical and effective genetic manipulation rather than selective breeding?

If you can find the story "Brenda" by Larry Niven - it's an interesting take on genetic manipulation. Doubled up superman genes for clotting leads to strokes and heart attacks by age 50; doubled up nightvision genes leads to day blindmness; stronger muscles lead to limb dislocations and broken bones; quick-reacting skin pigment leads to vitamin D deficiency; etc. Fiddling with one genetic feature is like bolting a V8 into a Volkwagen Beetle. If the engine's too strong, the transmission will strip, and the clutch will burn; meanwhile, the engine mounts could break and the whole balance is off... You can't change just one thing.

I think you'll see a lot more experimentation on animals before anyone's ready to try humans. We still don't know that much about manipulating anything more than single-protein producing genes, and we can't easily and reliably even clone primates. Evolution is essentially the process of throwing out a huge mess of mutations and seeing which ones (a) are better than what we used to have and (b) breed true. Who wants to volunteer their kids to see whether they come out as athletic geniuses or vegetables?

t-bonham@scc.net
02-24-2010, 05:06 PM
This may be supposition, but now that there is a species that can somewhat direct its own development, might we consider that directed evolution will augment the usual genomic flow?Most of recent human effort on this seems to be directed counter-evolutionary.

For example, diabetes seems to be increasing in humans. But previous to the discovery of insulin and its manufacture & prescription, most diabetics died off before they were old enough to reproduce, thus ending their family line. But now diabetics can live a near-normal lifespan, and produce children, who will carry on the genetic tendency for diabetes. The same could be said for many current medical treatments for diseases.

There used to be efforts made to sterilize patients in mental hospitals, which would tend to reduce the frequency of genetic-related mental disorders in future generations. But now, such surgery is considered ethically suspect, and not generally done any more.

Quercus
02-24-2010, 05:18 PM
Most of recent human effort on this seems to be directed counter-evolutionary. No. What's happening is that human technology is changing the environment such that genes that used to be very harmful (diabetes) are now not as harmful. Evolution is still going on, it's just that what is 'fit' has changed.

It's exactly the same as what happened millions of years ago when humans started using fire. The environment that human genes evolved in changed, such that huge jaw muscles weren't as important for survival (and probably some others were more important, for instance a gene that finds roasted meat appetizing).
Or, when humans in Europe domesticated milk-giving cattle, it became advantageous to have a gene that kept lactose production active for one's entire life. Evolution happens, it's just that the 'goal' changes because the environment is different.

The Other Waldo Pepper
02-24-2010, 05:42 PM
I think you'll see a lot more experimentation on animals before anyone's ready to try humans.

So? It was only sixty-odd years from experiments with crude gliders at Kitty Hawk to landing men on the moon, and much of that was spent developing better aircraft carriers while finding new ways for pilots to break the sound barrier. I'm fine with figuring on a lot of animal trials before we get to mankind; I'm not saying it'll be incredibly soon, I'm just asking how far off we think it is.

penultima thule
02-24-2010, 06:26 PM
Perhaps the Kangaroos were lucky that they had limited competition in isolated Australia.
There is reasonable evidence since the 1980s that it was incorrect that marsupials thrived in Australia only because there were no placentals.

Fossil records suggest that both placentals and marcupials made it to the Australian continent as or just after the split up of Gondwana. Then as time tested their adaptations in the environment, marsupials proved superior and kicked the placentals off the continent, the only place where this occurred. Having a lower metabolic rate is thought one of the key advantages. There are a few, small native placentals in Australia, though these arrived later, island hopping down the Indonesian archipelego.


evolution happens when there is pressure to change.
Which, if you accept the notion that the global environment is under stress, would infer that the rate of evolution is increasing, not decreasing.

Blake
02-24-2010, 10:18 PM
There are a few, small native placentals in Australia, though these arrived later, island hopping down the Indonesian archipelego.

Just over half of all native mammal species in Australia are placentals. Not really a few.

The placentals occupied the niches that marsupials couldn't readily occupy for various reasons: the air, the niche of small omnivores and the niche of aquatic predator. In all the niches that the marsupials could readily occupy they excluded the placentals for 50 million of years.

It was only with human gross disturbance of the ecosystem and the simultaneous introduction of mammals from every other landmass on the planet that a dozen or so placentals established in other niches. IOW under ideal conditions only about 12 placental mammals are competitive with marsupials in their home range.

And around 15% of the terrestrial mammal species in South America are also marsupials.

These points pretty much refute the commonly held belief that marsupials are somehow unable to compete with placentals.

F.Pu-du-he-pa-as
02-24-2010, 10:30 PM
Measuring "evolution" isn't done by just counting nucleotide changes -- the vast majority of these are irrelevant, neither harmful nor helpful. Instead one looks for long stretches of DNA that are nearly constant throughout a population (of humans for example), but different from a separate related group (e.g. a different human or primate population): that's the signature of a useful mutation.

Of course genotype and phenotype are both legitimate ways of thinking about evolution. But I was discussing how to quantify rates of evolutionary change, and it is often much easier to measure genotype than phenotype for these purposes, and especially to set up experiments to test theories about the rate of evolutionary change.

You bring up a good point, which is that you need to pick sequences of DNA which evolve at an appropriate rate in order to produce a phylogenetic tree. If you pick sequences that evolve too slowly or too quickly for the time frame you're interested in, you won't obtain an accurate tree. I started my post assuming that you had a reasonably accurate tree, so I left out this step.

septimus
02-25-2010, 12:14 AM
Measuring "evolution" isn't done by just counting nucleotide changes ... Instead one looks for long stretches of DNA that are nearly constant throughout a population (of humans for example), but different from a separate related group (e.g. a different human or primate population): that's the signature of a useful mutation.

I'm not sure my point was clear.

The rate at which DNA changes and the rate of "evolution" have only a very loose relationship! DNA changes quickly: an individual has many 100's of actual mutations, I think, compared with parents, not even counting changes due to chromosome crossover. The vast majority of these changes have no effect. "Evolution" rate measures the speed at which a population selects and amplifies the rare useful change. (The quote outlines a method where probably-useful genetic changes can be identified automatically.) That evolution rate is related to population size was my own conjecture: I hope someone will confirm or refute that. The article I alluded to mentioned geographic dispersion, which may be more important.

Unfortunately I've lost the URL which explained this in more detail and made the claim that human evolution was much faster during the 60,000 years since dispersion, compared with during the millions of years separating man and chimp. (This can become a racist claim, unfortunately, since the measurement is made between ethnic groups.) What I was really Googling for at the time was specific examples of recent beneficial mutations in humans. Other than skin pigment and sickle-cell, lactose tolerance is the only well-known such mutation, but I've stumbled on more interesting claims I'm no longer able to find with Google. :dubious:

Blake
02-25-2010, 03:12 AM
"Evolution" rate measures the speed at which a population selects and amplifies the rare useful change.

Assuming you are using the words with their standard definition, that would be the adpatation rate, not the evolution rate.

In its broadest sense evolution is a change in allele frequencies (or more strictly a heritable change in the frequency of allele expression). So for most practical purposes the rate of evolution is the rate of change of DNA.

The rate at which a population selects for favourable change is the rate of adaptation, not evolution. To illustrate this, consider two identical populations of organisms.

Population A is subjected to only minor environmental stressors. As a result the genotype change is primarily due to genetic drift, with effectively no selection. Nonetheless after 100 generations the entire population has changed from small, green and scaly to large blue and furry.


Population B is subjected to a multitude of environmental stressors on a rotational, generational basis, however none of the stressors is novel. So a trait that confers a 90% survival advantage in one generation is worthless or even detrimental in the second and third generations, only to confer the same advantge on the fourth generation. The same applies to genes favourable to second and third generations. As a result the population remains genetically unchanged, because no single favourable mutation confers an overwhelming, multi-generational benefit. Yet the population selects for novel mutations with 90% effectiveness every single generation.


According to your defintion population A has experienced a near-zero rate of evolution because there has been nearly zero selection for favourable change, despite the fact that it has experienced radical genotypic and phenotypic changes. And conversely according to your definition population B has experienced rapid evolution because the population selects and amplifies useful traits with 90% efficacy each generation, despite the fact that after 100 generations the population remains phenotypically and genotypically unchanged.


That evolution rate is related to population size was my own conjecture: I hope someone will confirm or refute that.

All things being equal a larger population will evolve faster simply because it has a larger potential gene pool to draw from. However in the real world all things are never equal, and larger populations don't actually exhibit faster mutation rates. Rather what we find is that larger populations exhibit more radically changes more rapidly through the culling of clusters.

What I was really Googling for at the time was specific examples of recent beneficial mutations in humans. Other than skin pigment and sickle-cell, lactose tolerance is the only well-known such mutation...

I think far more people are aware of the genetic disease resistance of Eurasians compared with, for example, Americans and Australians, than are aware of lactose tolerance. So that's a far better example of a beneficial, recent mutation.

F.Pu-du-he-pa-as
02-25-2010, 11:30 AM
I'm not sure my point was clear.

The rate at which DNA changes and the rate of "evolution" have only a very loose relationship! DNA changes quickly: an individual has many 100's of actual mutations, I think, compared with parents, not even counting changes due to chromosome crossover. The vast majority of these changes have no effect. "Evolution" rate measures the speed at which a population selects and amplifies the rare useful change.

I am saying that "evolution" can be used to describe change in the genome. In support of my assertion, I quote the manual for r8s, a program which does exactly what I have been describing (http://loco.biosci.arizona.edu/r8s/)

This is a program for estimating absolute rates (r8s) of molecular evolution and divergence times on a phylogenetic tree.


That should be clear enough, but I can find more papers on this topic if you would like.

md2000
02-25-2010, 12:21 PM
I would suggest in a difficult environment, being a marsupial is probably a survival advantage. Considering the burden reproduction puts on the female, being able to jettison Junior at the sign of difficulty - or, bluntly, letting him starve to death without killing the mother too - is probably a survival advantage.

I was just saying that Australia's peculiar dominant animal type - a large hopping beastie - came from a prototype or precursor that happened to favour that means of locomotion. Whether that would win against a scurying type is an interesting debate; but lacking significant opposition or entrenched competition - the hoppers filled many niches in Australia, the lemurs in Madagascar, the finches in the Galapagos...

Similarly, the ruminants seem to have dominated over the horse-type animals in other continents, since they can digest the cellulose to a gooey mess while the latter just pass it out as fluff for the road-apples. It's all about what food is available and how well you use it, or how easily you escape being food.

md2000
02-25-2010, 12:31 PM
How far are we from real experimentation with genes?

I ssupect we are a long way from "designer babies" just because of general revulsion. I expect some decade a regime like the North Korea, where public opinion is irrelevant, may do some serious experimentation.

The problem until then is "who"? Nobody will volunteer their kids; I suspect real genetic manipulation will sneak in the back door over about 100 years; first we will have viruses inserting genes into people to cure problems. Then we use the same trick to make the cure permanent. Then our perception of the need and value of changes will shift, much as it has with plastic surgery; we've gone from something that was for movie stars who needed the looks and vain but super-rich old ladies, to something girls (and guys) ask for on their 16th birthday.

Orthodonty and braces (a form of beauty enhancement) is automatic nowadays. How long before the gene for HPV resistance or straight teeth and cavity resistance or diabetes resistance or vision correction is inserted? From there, it's only a generation to genes for blonde or height or breast size...

We have a mild tolerance for sex selection nowadays depending on motive. Wanting only boys - bad... wanting a complete set - good... So we're on our way.

Blake
02-25-2010, 06:10 PM
I would suggest in a difficult environment, being a marsupial is probably a survival advantage. Considering the burden reproduction puts on the female, being able to jettison Junior at the sign of difficulty - or, bluntly, letting him starve to death without killing the mother too - is probably a survival advantage.

Except that placentals actually have the advantage in these situations. If you compare a sheep and a kangaroo, or a rabbit and a bettong, which are marsupials and placental sin almost identical niches, you find that the placentals can actually resorb the foetus, thus recaprturing lost nutrients, whereas the marsupials simply lose all that material.


The idea that placentals will somehow die if they abort has no basis in fact.

I was just saying that Australia's peculiar dominant animal type - a large hopping beastie - came from a prototype or precursor that happened to favour that means of locomotion.

Except that it didn't. The macropods are descended from generic arboreal quadrupeds. They adopted the slatatorial locomotion when they were already substantial sized beasties, in the range of 20 kgs or so. The living rat kangaroos and hare wallabies are diminutive representatives, not ancestral.


Whether that would win against a scurying type is an interesting debate

Not really, because it has been well and trult resolved.

but lacking significant opposition or entrenched competition...

The vast majority of mammals in Australia are not and never have been macropods. So in what way is there any lack of competition?

- the hoppers filled many niches in Australia...

Let's see. They filled the niche of medium sized browsers and...... well that's it. The tree kangaroos have adopted arboreal sleeping arranngements, but they remain medium sized browsers. Even those few macropods that adopted an omnivorous lifestyle were forced to abandon saltatory movement.

So the hoppers filled exactly one niche.

In what way did hopping macropods fill many niches? The ungulates filled many more niches than the macropods ever have, existing as browsers, omnivores, aquatic herbivores and carnivores, in medium, large and fricken' huge sizes.

In fact can you give us an example of any other mammal group at all that is more conservative than the macropods (assuming comparable or higher levels of richness)?

Similarly, the ruminants seem to have dominated over the horse-type animals in other continents, since they can digest the cellulose to a gooey mess while the latter just pass it out as fluff for the road-apples.

No.

First off both modes of digestion have their advantages and disadvantages and perissodactyls will outcompete ruminants in many situations. Which is why the two groups coexist worldwide. The perissodactyls still comprise ~20% of the world's medium and large gazing animals and around 30% of the large animals. So it's not like they are uncompetitive.

The true disadvantage of the perissodactyls seems to have been there susceptibility to human hunting, probably because they are better suited to life as huge animals. Prior to human influence they comprised about 25% of the large grazing fauna of the world by numbers, and about 70% of the "huge" animals".


It's all about what food is available and how well you use it, or how easily you escape being food.

No, it's so much more complex than this that the statement you made is almost completely untrue.

md2000
02-25-2010, 08:26 PM
Except that placentals actually have the advantage in these situations. If you compare a sheep and a kangaroo, or a rabbit and a bettong, which are marsupials and placental sin almost identical niches, you find that the placentals can actually resorb the foetus, thus recaprturing lost nutrients, whereas the marsupials simply lose all that material.
OK, you know more about goats than I do. The placental mammal I'm most familiar with does not usually resorb the fetus, except for the apocryphal disappearing twin. Given sufficient stress, they may miscarry, but if we're talking environmental problems in evolutionary history - exposure, starvation, chased by predators, etc. - at a certain point the female is more sitting duck for serious problems. Even before the evolution of doctors who didn't believe in handwashing, death from childbirth was a common problem. As the prevalence of back-alley abortion deaths pre Roe-v-Wade demonstrated, getting rid of a fetus is by no means a simple task.

Marsupialism seems to solve 2 problems - allowing the female to jettison the child when necessary, but allow her to carry it far longer than would be feasible for a placental mammal. Since placental mammals seem to have won in most other places, odds are this is not a great advantage, or something else about placentals carries a bigger advantage.

Except that it didn't. The macropods are descended from generic arboreal quadrupeds. They adopted the slatatorial locomotion when they were already substantial sized beasties, in the range of 20 kgs or so. The living rat kangaroos and hare wallabies are diminutive representatives, not ancestral.

I'm not sure I understand. The little I've been able to find suggests that all kangaroos evolved over the last 25M years or so from potoroines; a little critter similar to a potoroo (hence the type name?) These little 2-foot hoppers (macropods) evolved somehow from a small, 4-legged hopping and scurrying precursor. Somewhere in their development, 2-legged hopping became more efficient. I've found some description that suggests hopping is more efficient because the tendons act like elastics - an interesting development.

So tiny hoppers expand to fill a large number of niches - how is this different from what I said?

They were successful. The question is why? The likely answer, from what I've been able to find, is that the climate of Australia changed drastically - creating new ecological niches where, presumably they could outperform anything else competeing for that niche. The island geography prevented alternatives from migrating in and taking over the niche before they better filled it. They vary in size and range, from the desert to the forest, ground and tree. That's variety.

Was everything in Australia a kangaroo? No. For example, there is fossil evidence of one killer kangaroo, but generally they are herbivores. Similarly in other discussions, successful as ungulates may be, they are still generally herbivores. et cetera...
(I'll bite. Which is a carnivorous hoofed mammal?)

No, it's so much more complex than this that the statement you made is almost completely untrue.
Well, yes, you can write a book on the many details of the Theory of Evolution - come to think of it, that's how the theory got started - but essentially it's about differential breeding; finding the food to get there is the first and major step. Note how most animals are primarily defined by the food they eat and or the defensive strategies they employ to not be food in their environment.

Population A is subjected to only minor environmental stressors. As a result the genotype change is primarily due to genetic drift, with effectively no selection. Nonetheless after 100 generations the entire population has changed from small, green and scaly to large blue and furry.


Population B is subjected to a multitude of environmental stressors on a rotational, generational basis, however none of the stressors is novel. So a trait that confers a 90% survival advantage in one generation is worthless or even detrimental in the second and third generations, only to confer the same advantge on the fourth generation. The same applies to genes favourable to second and third generations. As a result the population remains genetically unchanged, because no single favourable mutation confers an overwhelming, multi-generational benefit. Yet the population selects for novel mutations with 90% effectiveness every single generation.

But the falacy is - if we suggest that random mutations occur at a regular rate, provided that they do not affect ability to meet ecological demands, then both A and B will have the same change rate. If it doesn't matter what fur or scales or colour and B's pressures do not affect coloration or cover, both sets of genes are equally likely to wander in that regard. Only the B genes constrained by ecological pressures do not wander. I assume the variations tested for, that change regularly over time, are the unexpressed or "garbage" DNA sequences between productive genes.

Blake
02-26-2010, 03:45 AM
The placental mammal I'm most familiar with does not usually resorb the fetus, except for the apocryphal disappearing twin. Given sufficient stress, they may miscarry, but if we're talking environmental problems in evolutionary history - exposure, starvation, chased by predators, etc. - at a certain point the female is more sitting duck for serious problems. Even before the evolution of doctors who didn't believe in handwashing, death from childbirth was a common problem. As the prevalence of back-alley abortion deaths pre Roe-v-Wade demonstrated, getting rid of a fetus is by no means a simple task.

So you're trying to deduce the advantages of the macropod body plan by comparing it to humans. That is so invalid as to be ridiculous. Stick to comparing them to browsers of similar sizes and ecosystems.

Marsupialism seems to solve 2 problems - allowing the female to jettison the child when necessary, but allow her to carry it far longer than would be feasible for a placental mammal.

The trouble with this hypothesis is that marsupials never faced those problems. Marsupials didn't evolve from placentals, they evolved form monotreme like ancestors. So obviously these can not be the problems that Marsupialism evolved to solve.Traits can not evolve to pre-empt problems that never existed.

Since placental mammals seem to have won in most other places, odds are this is not a great advantage, or something else about placentals carries a bigger advantage.

No, as we have already explained in some detail, there is absolutely no evidence at all that placentals have any advantage at all, and considerable evidence that they lack any advantage.

The most widely held belief is that the placentals came to dominate simply due to random chance. If two groups of organisms compete then necessarily one must become extinct. There is a 50/50 chance of either group succeeding. At the time that the mammals were ascending there were three main land masses: Euramerica and Africa, Australasia and South America. The placentals dominated Euramerica, the marsupials dominated Australasia and South America was dominated by marsupial carnivores and placental herbivores. This is precisely the outcome we would expect if there was neithe rgrpup had any advantage. It is precisely the opposite to what we would predict if placentals had any advantage.

The little I've been able to find suggests that all kangaroos evolved over the last 25M years or so from potoroines; a little critter similar to a potoroo (hence the type name?)

As I've already said, these were not little critters as far as we can tell. they were medium sized critters in the 20 kg size range.

These little 2-foot hoppers (macropods) evolved somehow from a small, 4-legged hopping and scurrying precursor.

Not scurrying, no. The ancestor would have been a bounder, somewhat like a modern rabbit.

So tiny hoppers expand to fill a large number of niches - how is this different from what I said?

It's no different at all. That;s the problem. You are repeating the same erroneous statments over and over.

The organisms in question weren't tiny, and they never evolved to fill a large number of niches.

I ask you again: what niches have they evolved to fill? Complete this sentence: "Macropods started out as medium sized browsers and they have since radiated into alternative niches such as......". And once agian I ask: what other group of mammals of comparble diversity has evolved to occupy fewer niches than the macropods?


They were successful. The question is why? The likely answer, from what I've been able to find, is that the climate of Australia changed drastically - creating new ecological niches where, presumably they could outperform anything else competeing for that niche. The island geography prevented alternatives from migrating in and taking over the niche before they better filled it.

That's a tautology. All you are saying is that macropods adaptation allowed them to outperform competitors in the niche they occupied because they were able to adapt to outperform competitors in the niche they occupied.

It's true enough. It's equally true of ungulates and porcupines and duck-billed platypuses. It is so universally true that it is meaningless


They vary in size and range, from the desert to the forest, ground and tree. That's variety.


Compared to what exactly?Once again I ask, what comparble group of mammals shows less variety? Rather than being remarkably diverse they appear to me to be remarkably conservative.

Similarly in other discussions, successful as ungulates may be, they are still generally herbivores. et cetera...

Well, no. The pigs for example are true omnivores and quite predatory.

(I'll bite. Which is a carnivorous hoofed mammal?)

You're rather spoiled for choice, there were dozens if not hundreds of species. The most spectacular examples were the Mesonychid group.

Well, yes, you can write a book on the many details of the Theory of Evolution - come to think of it, that's how the theory got started - but essentially it's about differential breeding; finding the food to get there is the first and major step.

No, it isn't.

Note how most animals are primarily defined by the food they eat and or the defensive strategies they employ to not be food in their environment.

No, they aren't. Name me a single classification system that primarily defines animals by the food they eat. Just one.


But the falacy is - if we suggest that random mutations occur at a regular rate, provided that they do not affect ability to meet ecological demands, then both A and B will have the same change rate.

Of course they won't.

If it doesn't matter what fur or scales or colour and B's pressures do not affect coloration or cover, both sets of genes are equally likely to wander in that regard.

No, they are not.

[quote]Only the B genes constrained by ecological pressures do not wander./quote]

Right, now you're getting it. And since we know that far more genes are constrained in this manner for population B, are they going to have more or less genetic drift?

Look, md2000, at this juncture I really don't have much faith that you have a clue what you are talking about. You keep repeating the same erroneous statements that have already been pointed out as erroneous. You are attempting ridiculous comparison, you make several factually incorrect statements in every post and you refuse to answer basic question.

If you don't start answering questions and proving some evidence for your more controversial claims I;m going to wash my hands of you and leave others to decide the worth of your claims.

Arguing with someone who brings nothing to the table but factually incorrect assertions is fruitless.