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aknerd
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aknerd
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Peasant

Evolution!

People LOVE to "debate" evolution. But that's silly, and doesn't really solve anything. If you are in a debate about whether or not evolution is a valid theory, you are either debating someone who has little to no idea what what evolution is, or ARE the person who has little to no idea what evolution is. That doesn't sound like very much fun, so let's not do that, okay?

Instead, this thread will be about topics in evolution, because it is much more entertaining to talk about specific cases and ideas than one big overarching theory. The topics will be chosen by whoever has the best topic, with all "lesser" topics being ignored and forgotten.

Now, I'll start us off with what actually made me want to start this thread: randomness. I was reading Mage's post at the bottom of this thread, and immediately thought about genetic drift.

Here is a classic example of genetic drift in a fruit fly population:

Basically, genetic drift states that random sampling has a lot to do with the evolution of small populations. Think about it: say you have a population of four individuals, two males and two females. One female homozygous allele for blue fur, the others all have a homozygous allele for red fur. Mating between blue and red fur produces a heterzygous purple fur creature. We would therefore expect the next generation to have some purple and red individuals, and the one after that to have all three colors represented. Basic Mendelian stuff.

Now, it gets interesting. Lightening strikes the blue female. She's dead, and will never reproduce. Now, all individuals in this population will be forevermore purely red. Note that this is regardless of the fitness of these genes. Blue fur might have been much more beneficial (perhaps these creatures lived in blue grass, and it provided camouflage), due entirely to random events (as opposed to evolutionary pressures) it is RED fur that becomes fixed in the population.

Going back to and contradicting Mage's comment from before, due to genetic drift, having the same selective factors won't guarantee a particular evolutionary outcome, due to simple random events.

So.... Discuss?

  • 175 Replies
aknerd
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aknerd
1,416 posts
Peasant

(Its worth noting that my example is actually more about bottlenecking than genetic drift. Genetic drift typically refers to random sampling involving breeding/genetic recombination as opposed to random events involving death. But, in essence, they have the same result. Just adding this to avoid any later confusion)

Also, some conversation topics:

How important is genetic drift really? When is it important (ie, for which populations)? How big can the effects be?

HahiHa
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HahiHa
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Regent

Genetic drift doesn't lead to a smaller gene pool like bottlenecks. Bottlenecks really just mean that only a few genes gets passed on to the next generations (due to extinction events or migration) and those genes then build all of that population. Genetic drift means that the frequency of certain alleles change and replace other alleles over time. So no, not quite the same.

The effects of genetic drift can be that two populations, if separated, can evolve differently and form two distinct species over time. It is an important factor in long-time evolution.

MageGrayWolf
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MageGrayWolf
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Farmer

Going back to and contradicting Mage's comment from before, due to genetic drift, having the same selective factors won't guarantee a particular evolutionary outcome, due to simple random events.


The particular random event would also be a factor. This is what I meant when I said "If you were able to reproduce all the same selective factors" In such a case you would also have to reproduce the death of the blue furred female in order to reproduce all the selective factors involved.
aknerd
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aknerd
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Peasant

Genetic drift doesn't lead to a smaller gene pool like bottlenecks. Bottlenecks really just mean that only a few genes gets passed on to the next generations (due to extinction events or migration) and those genes then build all of that population. Genetic drift means that the frequency of certain alleles change and replace other alleles over time. So no, not quite the same.


Yes, the end result is. If only a few genes get passed on, then clearly the frequency of the genes is changing. The end result is that the population tends to move away from heterozygosity, though the mechanism is different (which is pretty much exactly what I said). In my example, the frequency of Blue alleles changed from .25 to 0 over time. A bit more sudden than in most genetic drift cases, to be sure. Don't worry, I'm putting in a better example later in this post.

The particular random event would also be a factor. This is what I meant when I said "If you were able to reproduce all the same selective factors" In such a case you would also have to reproduce the death of the blue furred female in order to reproduce all the selective factors involved.


Lightening is a selective factor? Usually, when we talking about selection, we mean selection due to environmental pressures such as competition for food. Its not like the blue furred creatures were especially attractive to lightening or anything. But no matter, I have a far better real life example!

Here's a graph and caption from a classic genetic drift lab experiment:

http://media.wiley.com/mrw_images/els/articles/a0001698/image_n/nfgz001.gif
Figure 1. The effects of genetic drift on 107 laboratory populations of the fruit fly, Drosophila melanogaster. The populations were maintained at a size of 16 individuals, giving a total of 32 possible different alleles (two per individual). The two horizontal axes respectively give the number of generations since the populations were all started (initial allele frequency of 0.5) and the number of mutant brown eye alleles in the population. The vertical axis reports how many of the populations possess that many alleles at a given time period. Looking across all 107 populations shows that genetic drift generates variation among the populations that increases with time, eventually ending in fixation of one or the other of the two alleles. Data from Buri (1956); figure from Hartl and Clark (1997).


So, in this experiment, we basically have 107 populations of fruit flies, that are then taken through 19 generations of breeding. Each population is maintained at 16 individuals. So basically, for each generation they would let all of the populations breed and produce eggs, then for the next generation they would randomly select 16 of those eggs out of each population to grow and become the next generation. And so on. What we see happen is that while the first generation tended to be well mixed for the eye color gene (white or brown), over time the populations moved to fixation, with the entire population being either entirely white eyed or brown eyed.

The point here is under the exact same environmental condition, some individuals populations became white eyed, and the others became brown eyed. This is because there was NO environmental pressure that determined eye color.
MageGrayWolf
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MageGrayWolf
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Lightening is a selective factor?


Random doesn't really mean something doesn't have a pattern, just that the pattern is one in which we are unable to follow resulting in an uncertain outcome.

Its not like the blue furred creatures were especially attractive to lightening or anything.


The argument was that if we did the same things we would end up with the same results. There was an attempt to counter this with how we don't get this with evolution. However if all the same random events took place that influenced that species evolution (which would have to be included if we are doing all of the same things) you would end up with the same results.

I did admit that the reproduction of events at such a level is next to impossible. So let's just say for the sake of your argument I agree.
aknerd
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aknerd
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Peasant

However if all the same random events took place that influenced that species evolution (which would have to be included if we are doing all of the same things) you would end up with the same results.


How can it be a random event then, if it happens again the second time? I have had an example of this up on my about for a long time now.

When we look at things on the level of lightening striking, or the random selection in the fruit fly example, it no longer comes down to our ability to recreate an event, but rather or not there exists such a thing as true randomness. In other words, I feel like you are arguing that evolution is an example of causal determinism. I don't want to put words in your mouth, so correct me if I'm wrong on that point.

Actually, thats a far better topic of discussion than the current one:
Is evolution purely deterministic? Or, do some evolutionary events just... sort of happen for no reason?
Kasic
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How can it be a random event then, if it happens again the second time? I have had an example of this up on my about for a long time now.

When we look at things on the level of lightening striking, or the random selection in the fruit fly example, it no longer comes down to our ability to recreate an event, but rather or not there exists such a thing as true randomness. In other words, I feel like you are arguing that evolution is an example of causal determinism. I don't want to put words in your mouth, so correct me if I'm wrong on that point.


Theoretically, if one were to know all the existing conditions, and I mean all of them, a lightning strike is not random. Nor are any other form of natural disaster. As for what gets caught where, if one had a perfect understand of how something would react to specific stimuli, predicting what something would do would be possible.

I saw theoretically, because as Mage said, that's so extreme it may as well be impossible, even if technically it is possible.
aknerd
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aknerd
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Peasant

Theoretically, if one were to know all the existing conditions, and I mean all of them, a lightning strike is not random.


And what theory, exactly is that? Lets suppose that we ignore all aspects of quantum physics that tell us that there are probabilistic forces at work which could change (ie randomize) the path of least resistance between the cloud and the ground, and thus the path of the lightening strike. Let's just set all that aside for now, and say no matter what, given the exact same conditions, the lightening strike will always happen the same way.

But.... what about the blue furred animal? Basically, you would have to make the claim that it has no free will, and will always be in the spot where the lightening hits it. Because if it had free will, then even with the exact same conditions, it might decide to meander somewhere else, and thus survive. What does free will have to do with evolution?
Kasic
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And what theory, exactly is that?


I said theoretically, meaning technically possible but not really possible.

As I said, if we knew all of the factors, it would not be considered 'random.' I'm not saying it's actually possible.

Basically, you would have to make the claim that it has no free will, and will always be in the spot where the lightening hits it.


I would argue that free will doesn't really exist. At the most base level, we process stimuli and react biologically. The only reason we consider it free will is because it's so complex it may as well be that. If it were understood how someone would react when presented with any given stimuli, is that free will?

What does free will have to do with evolution?


This is the interesting point, philosophically. Did someone -choose- to perform a specific action which resulted in not dying, or were they simply responding to surrounding stimuli? Would they always react in that say way, if the exact same conditions took place? I believe that they would.
HahiHa
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HahiHa
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Yes, the end result is.

No. You did not read closely enough.

Bottlenecks are very fast events where the total diversity of alleles in a population decreases. Of course that changes the frequency of the surviving alleles, but it means the population has less different alleles, which results in lower fitness. This is a substantial difference to genetic drift where over a comparatively longer time, allele frequencies change without losing diversity. Any allele will eventually fade away due to genetic drift and be replaced by new ones.


Randomnes in evolution is really an interesting topic. Because when most people hear "evolution is not random", they think it has a goal. It hasn't, as it's not a process with a will behind. But it is not random, as, as Kasic said, it responds to certain factors in a certain way, and follows certain rules dictated by biology, chemistry etc.

Best example of the non-randomness of evolution is convergent evolution. Ex.: wings. Wing-like structures evolving from the arm of vertebrates happened independently in pterosaurs, birds and bats, so in different groups in different times. The detailed structure is different in each case (different bones are involved in each group) but the function is exactly the same.

http://www.inkart.com/images/lineart/wings_evolution.gif
MageGrayWolf
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Randomnes in evolution is really an interesting topic. Because when most people hear "evolution is not random", they think it has a goal. It hasn't, as it's not a process with a will behind. But it is not random, as, as Kasic said, it responds to certain factors in a certain way, and follows certain rules dictated by biology, chemistry etc.


I guess the simplest way to put it,
Random = Unable to follow the pattern.
Non-Random = Able to follow the pattern.

"Philosophers and scientists use âchanceâ only in the sense of unpredictability. Chance means essentially that you cannot predict the outcome of a particular event. For example, you cannot predict whether your next child will be a son or a daughter, even though you can specify the probability or likelihood. âChanceâ does not mean lack of purpose or goal in science. If it did, we could say that absolutely everything in the natural world is by chance because we donât see any purpose or goal in storms, in ocean currents, or anything else. Evolution certainly does involve randomness; it does involve unpredictable chance. For example, the origin of new genetic variation by mutation is a process that involves a great deal of chance. Genetic drift, the process I referred to earlier, is a matter of chance." -Douglas Futuyma

Best example of the non-randomness of evolution is convergent evolution.


Another good example is with bears. The polar bear evolved from the brown bear. In the forest setting the occasional white furred baby isn't likely going to do well. That is a non-random outcome due to the environment. Now a group the these bears move north, where we have snow all the time and the weather is colder. That same white bear can now hide better and out compete the brown furred bears and over time a number of other features evolved so that those bears were better suited for the cold. Again everything that we could predict happening to a species making such a transition.
This is of course a basic example of natural selection, probably the key non-random process involved in evolution.

Anyway I would like to open up a couple of points for discussion.

First, the definition of species.
Now I tend to avoid the topic since we are usually dealing with someone who doesn't have a grasp of the basics of evolution, but I think we can get into it more with this thread.
I in general tend to use the define two groups as different species when they are either unable to reproduce or can do so with great difficulty. (Example: lions and tiger or horses and donkeys) However this isn't always the case (I think polar bears and brown bears maybe an example of this), two groups may be regarded as different species due to one or both only being able to live within narrow ecological niches or due to large morphological differences.
This difficulty of pinning down the definition of species stems from the way evolution works. More on the species problem.

The second thing I would like to bring up is gradualism and punctuated equilibrium. I tend to think both to some extent are at play given a species evolutionary change are in response to the environment in which they live. Since that environment can change gradually or rapidly a species can evolve gradually over time through through small changes and can evolve rapidly with major changes over relatively few generations. I would say the punctuated equilibrium may play a larger role in diversifying species. Of course this throws out the idea of evolution taking place at a more or less steady rate.
I would like to hear what you guys think?
aknerd
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aknerd
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Peasant

This is a substantial difference to genetic drift where over a comparatively longer time, allele frequencies change without losing diversity.


Did you not see the fruit fly example of drift? Because in that case, diversity was definitely lost. If you're willing to trust wiki on this:
Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.
If you don't want to take wiki's word for it, read any other one of these articles. Basically, bottlenecking is a type of genetic drift. Or rather, genetic drift is a common result of bottlenecking. That's what I mean when I say that the end result is the same. I also said that it wasn't a very good example, and then promptly supplied a better one, so it might be time to move past semantics.

Okay, back to randomness.

I agree that the level to which you can ascribe randomness to an event depends on the extent to which to examine the event. In the case of lightening, if you just look at the pattern of strikes, you might say something like "well it looks like it seems to hit higher things more often, but on the whole each strike is rather random". But, if you where to examine the actual air molecules prior to the strike, you might notice that lightening always follows the path of least resistance. So, maybe lightening is not so random after all. But.

Evolution isn't perfect, and it doesn't react instantaneously to every environmental pressure. By that I mean it isn't necessarily true that the most "fit" creatures survive to reproduce in every instance. In my original example, the two varieties of organisms might be equally conductive and equally unaware of the forces that influence lightening strikes.

Wouldn't you then say that they are equally fit in terms of lightening strike survival? The only difference between the two organisms is the hue of their fur, something that in this hypothetical example is completely unrelated to lightening strikes.

In your example of the convergent evolution of flight, the ability to fly is something that increased the fitness of the individual. Being able to avoid lightening really doesn't increase your fitness on an evolutionary scale, because lightening strikes are so infrequent.

For that reason, I don't think we can really say that the blue organisms are less fit that the red ones. The Blue organism was just unlucky. Without reverting to causal determinism, whose to say that if the scenario were to be repeated, it wouldn't be one of the red organisms to be hit? Or none of them at all?

What do you guys think? Is there such a thing as "equal fitness", with regards to some event/pressure?
HahiHa
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HahiHa
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Regent

Is there such a thing as "equal fitness", with regards to some event/pressure?

If out of two populations both differ only in a point irrelevant to the event/pressure, they can be qualified as equally fit in relation to said event/pressure.

Of course the lightning, if atomized, comes out as non-random. But in perspective of the population, the lightning will not differ between more fit and less fit individuals. Being fit doesn't guarantee you to survive over others anyway. It just gives you a more or less bigger probability of surviving. Even small differences can influence evolution over time, it's just probabilistics. But even fit organisms can fall prey to predators or such.

Another good example is with bears. The polar bear evolved from the brown bear.

Actually this is funny, because brown bears and polar bears aren't yet two distinctive species; they can cross-reproduce without problems, this has already happened. They're sort of borderline. I think it is a good example of phenotypic plasticity, also an interesting point. The genotype doesn't dictate the phenotype with 100% accuracy. In fact, under certain circumstances, two genetically identical animals can show different phenotypes. An example are certain salamanders or some similar animals, that in one lake have been found to grow normally, and in another lake, due probably to predators, only grow up to larval stage. Neoteny, I think it's called.
MageGrayWolf
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Actually this is funny, because brown bears and polar bears aren't yet two distinctive species; they can cross-reproduce without problems, this has already happened.


This may actually play into what I was getting at with the definition of species. despite the fact they don't meet the general qualifying factor they tend to be classified as two separate species, Urus maritimus and Urus arctos. I suppose technically the polar bear might be better classified as a subspecies of brown bear.
aknerd
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I think it is a good example of phenotypic plasticity, also an interesting point.


I don't know if I'd say that... Indeed, one of the main reasons that biologists still place them into different species is due to the fact that neither one of them can survive in the other's environment for very long. If you raise a polar bear in a brown bear's habitat, it'll still have white, insulating fur, and (relatively) smaller, sharper carnivorous teeth. While they do and can interbreed, this is hardly a common event. And when you look at the morphological and dietary differences (as Mage said), there are some pretty severe distinctions between the two groups.

That's really the problem with the reproduction-isolation definition of speciation. It is WAY to simple. Sure, its nice to look at the genetics of polar bears and say "Oh look, the mtDNA of this brown bear population is more similar to polar bears than to other brown bears, pointing to interbreeding. Therefore, the polar bears are a mere subspecies!". But then, I feel like you are missing the point that polar bears are purely carnivorous arctic marine mammals, whereas brown bears are omnivorous temperate terrestrial mammals. Its kind of a big deal.

But even more than that, there is another huge problem with this definition: what about fossils? If we base speciation solely on reproduction characteristics, how can we say anything about fossils, when we have little to no DNA data and know very little/nothing about mating habits of a particular "species"?

What about bacteria, some of which can directly incorporate the DNA from similar organisms (though not necesarily from the same species) right into their own code? This throws a serious monkey wrench into the problem, as they can hybridize without reproducing. Which is really cool, but also makes things rather complicated.

This is getting long, so I'll just end with one of my favorite new quotes:
imagine that acacia trees could exchange DNA with lions and that the resulting new tree developed "limbs" that allowed them to attack grazing giraffes. This is in a sense what prokaryotes do all the time.
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