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Evolution

Posted Feb 23, '13 at 4:47am

HahiHa

HahiHa

5,808 posts

Moderator

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
 

Posted Feb 23, '13 at 4:23pm

MageGrayWolf

MageGrayWolf

9,798 posts

Knight

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?
 

Posted Feb 23, '13 at 4:52pm

aknerd

aknerd

1,431 posts

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?
 

Posted Feb 23, '13 at 6:29pm

HahiHa

HahiHa

5,808 posts

Moderator

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.
 

Posted Feb 23, '13 at 9:11pm

MageGrayWolf

MageGrayWolf

9,798 posts

Knight

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.
 

Posted Feb 24, '13 at 8:33pm

aknerd

aknerd

1,431 posts

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.
 

Posted Feb 24, '13 at 10:07pm

Kasic

Kasic

5,746 posts

That's really the problem with the reproduction-isolation definition of speciation


I personally think ecological niches are the way to go for classifying what species we can currently observe. The phylogenetic tree is fine with a mixture of dating and logical inference.

The whole species problem is a massive tangle that really isn't even -that- important. Only when there are very minute differences is classification an issue and it's still basically a non-issue.
 

Posted Feb 25, '13 at 12:39am

MageGrayWolf

MageGrayWolf

9,798 posts

Knight

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?


Horizontal gene transfer technically isn't evolution. Though it still offers an issue with the definition of species.

I personally think ecological niches are the way to go for classifying what species we can currently observe. The phylogenetic tree is fine with a mixture of dating and logical inference.


We can have two completely different species fulfilling the same ecological niche so long as they are isolated from one another.

The whole species problem is a massive tangle that really isn't even -that- important.


Why don't you think it's important? We are talking about a process that gives rise to species, so it would seem rather relevant.

Thinking about it I wonder if ultimately some form of gradient might be necessary, rather than a set definition.
 

Posted Feb 25, '13 at 1:11am

Kasic

Kasic

5,746 posts

We can have two completely different species fulfilling the same ecological niche so long as they are isolated from one another.


Exactly. Perhaps I wasn't clear. I meant using that as a qualifier when disputing whether something should be considered another species.

Such as the brown bear/polar bear example. Each lives in a different environment. Even though they can interbreed, they occupy separate areas. Logically, the same species would occupy the same niche. A different niche would thus imply a different species.
 

Posted Feb 25, '13 at 1:13am

Kasic

Kasic

5,746 posts

Why don't you think it's important? We are talking about a process that gives rise to species, so it would seem rather relevant.


Blargh, submitted too soon.

It's an issue, but not really. Whether we call a polar bear the same species as a brown bear, for example, doesn't change anything. It's only for our own convenience and clarification. What is, is. What we call those doesn't change that. We could call everything "it" as long as we described the features of "it" and still communicate. It would just be super annoying.
 
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