Natural Selection for Mike T. W.

This is for Mike the Wiz. I hope this will help answer some of your questions on Natural Selection. I will try to keep it to layman terms and short as much as possible, and any definitions I think are needed to clarify what I am talking about will be at the bottom of the post.The two main causes of Microevolution are genetic drift and Natural selection. A change in a populations allele frequencies due to chance is called genetic drift. If you have a small population, due to some disaster possibly or geographical isolation, you have little chance that this population is representative of the original or whole populations gene pool. Some allele’s will be overrepresented among the survivors while some will be underrepresented or eliminated. Genetic drift will continue to change the gene pool over time and many generations until eventually sampling errors will become minimal.
Natural selection is differential success in reproduction. Some individuals will leave more offspring than others. This results in Alleles being passed down to the offspring in numbers disproportional to their relative frequencies in the present generation. A healthy Cheetah that runs faster and can catch prey better than his rivals may be able to produce more viable offspring. Some of his offspring may attain this ability that he has and are themselves able to produce healthier young, etc. Or white moths on the grey bark are less able to elude predators and therefore produce disproportionately less offspring than a grey moth. This differential survival effects the frequency of the alleles in the moth population or cheetah population. Natural selection accumulates and maintains favorable genotypes in a population. These are the two main causes of changes in allele frequencies, though there are others such as mutation and gene flow.Definitions:Allele : Alternate versions of a gene.Genetic drift : changes in the gene pool of a small population due to chance.Gene flow : the loss or gain of alleles in a population due to the migration of fertile individuals or between gamete populations.Mutation: A rare change in the DNA of a gene ultimately creating genetic diversity.Microevolution : A change in the gene pool of a population from generation to generation.FYI Macroevolution : Evolutionary change on a grand scale, encompassing the origin of new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction.I hope this is not too long that it loses you but long enough that you can begin investigating your question. Feel free to ask more questions as I am sure this is not comprehensive.

Bumpity...

Wow, there's a lot there. I think I get some. I will be back to read it about ten times (that usually works) though I definately understand natural selection more now.So genetic drift makes the evolution possible ? So it's a forced change because of survival, and the genetic drift is helpful. Being chanceful.Thanks for all this DBlevins my questions may seem a bit silly but I will read this again tomorrow.Mike TW? Have you been at internet infidels, or is that coincidence?[This message has been edited by mike the wiz, 02-03-2004]

Populations which don't interbreed will tend to become genetically distinct and this is part of the way we get new species. They may become incapable of interbreeding and will likely develop some unimportant physical differences.What genetic drift can't do is explain adaption - we still need natural selection to explain how species develop abilities useful to their envionment

So Natural Selection is when a trait causes the organisms that possess it to be reproduced more then organisms in the population that don't have it.The words which require special attention are "more then", Natural Selection is a relative theory. In stead of "reproducing more then the other" Natural Selection should read "reproduce or not reproduce". This becomes apparent when you consider extinction, or an organism coming into a new environment by a new trait.Extinction simply falls outside the scope of relative Natural Selection. Where obviously you might note that an endangered species are becoming less fit, this use of fitness is not applicable in relative NS, because all members generally become equally less fit. The relative fitness doesn't change, eventhough the real fitness goes down.An organism going into a new environment by it's new trait also largely falls outside the scope of relative Natural Selection. Consider nylon eating bacteria for instance. The ancestors can't eat nylon, only the mutants can. There is no one population anymore, the population has split into two. To make a comparison between them would be like comparing elephants and ants. Sure we can say that elephants reproduce less then ants, as we can say that nylon eating bacteria reproduce more then their ancestor. But statements like that have no meaning, as the saying goes that is comparing apples with oranges.The point should be to describe the relationship of the organism to the environment in terms of reproduction. This is what you get when you define selection as reproducting or not reproducing, selected for, or selected against.regards,
Mohammad Nor Syamsu

Careful. As PaulK noted, genetic drift is non-adaptive. In the case of genetic drift, we can honestly say it's pretty much a random process. It is NOT natural selection. In addition, random sampling of geographically isolated populations was mentioned (by Loudmouth). These are related, but two different concepts.Let's see if I can come up with another analogy. Picture a bowl of marbles - 80 black and 20 white. Close your eyes, and grab a handful. Statistically, four out of every five will be black. The sampling effect happens because the single handful you just took is not necessarily going to be a perfect representation of the distribution in the bowl. Maybe you picked up two black and three white. This is going to skew the distributions in your new marble population - making it not reflective of the original distribution. This is the so-called "sampling error" effect. Basically what this means is that when we're dealing with living populations (say, a flock of bats that gets blown to a distant island during a hurricane), the survivors that colonize the new area are generally not going to be representative of the full gene pool of the original source population.Genetic drift, on the other hand, deals with random changes in a gene pool of a population due to purely stochastic (statistical) effects. Going back to our marbles: let's say we've got 15 marbles - 12 black and 3 white due to perfect sampling. Each marble is capable of producing one same-color marble each generation. In the first generation, several black marble nests are destroyed when the tree they were in topples, and a predator eats two others, leaving only 7 black. Everybody reproduces once, then dies. We now have 7 black and three white marbles - an evolutionary change (genetic drift) in the marble gene pool because the distribution of marbles is now 2.3:1 instead of 4:1. Carrying this further, genetically speaking, eventually chance alone may eliminate certain members out of proportion to their numbers in the population. In such cases, the frequency of a particular trait may begin to "drift" toward higher or lower values in what's called a "drunkard's walk". Ultimately, the trait may represent 100% of the gene pool or, just as likely, disappear from it. Drift produces evolutionary change, but there is no guarantee that the new population will be more fit than the original one. Evolution by drift is aimless, not adaptive.What natural selection does is different. The emphasis here is on the "selection". Unlike drift, which is random, selection means that population change (i.e., the frequency of specific traits in the population) is driven by adaptive response to what are known as "selection pressures". These selection pressures in the organism’s environment tend to favor the survival of individuals with certain characteristics, primarily by weeding out those individuals who DON’T have those characteristics. (Just like in the old joke where two guys were being chased by a bear. One sees the other putting on tennis shoes and says, Why are you doing that? You can’t outrun a bear. The other responds, I don’t have to outrun the bear — I only have to outrun you.) What does this weeding out imply for overall change in characteristics within a population? It means that eventually you’re going to see a population where certain characteristics — the ones that gave a survival advantage in bear-racing, for instance — are going to come to predominate in the population. IOW, if being eaten by bears is a significant selection pressure, eventually everyone will be wearing tennis shoes, because the folks that didn’t have them have all been eaten — a fairly significant morphological change. I want to emphasize that this process is quintessentially non-random. It depends almost entirely on what works best for the given population in its given environment.

So, genetic drift is chance affecting the gene pool, statistical, but as you say, natural selection is not random because of the
" bear racing ". - Selection pressures dictating the selection.Some of the wording I'm not use to but I think I know the differences.
Genetic drift - the random affects on the gene pool.
Natural selection - the selective pressures "fittest survives" e.t.c.
Isolated populations - The distributions are not representative of the original distribution. When the populations are forced elsewhere. I'm trying!

Got in one! *Applause* Almost there. What you should remember about drift is that it is a seperate mechanism from natural selection. You won't see it much (or the effect is negligeable) in large, well-distributed populations. Where genetic drift - i.e., the vicissitudes of fate, if you like - has its greatest effect is in small, isolated populations, because in general large populations can have enough resiliency to bounce back from some kind of local disaster. At the genetic level, any population of organisms is going to have a whole bunch of common alleles (different variations of a particular gene), a few uncommon ones, and a very very few rare ones. A small population is more likely to lose the rare and uncommon genes through chance. Alternatively, a small population may be (through sampling error) composed of mostly rare or uncommon (in comparison to the source population) alleles, and hence through drift the once-common alleles are the ones that vanish. (Obviously, natural selection will have something to say about it, as well). In any case, drift refers to frequency changes - either increase or decrease - through what is essentially random acts. Pretty much (although I've always hated the phrase "survival of the fittest" - typical journalist hype IMO). Anyway, it's enough to go on for now. The details are a bit more complicated ( ) Well, yeah. Except that isolated populations merely have a statistically less liklihood of maintaining the distribution of the source population - not that simply being isolated means they're not representative. Think about it this way: you have a sock drawer with 10 pairs of black, 8 pairs of brown, and one florescent orange pair (given to you for Christmas by Aunt Martha, who still thinks you're 3). You have to catch a 4 am flight. You stumble around in the dark pulling socks at random out of the drawer and throwing them into a suitcase. What are the odds you'll have mostly brown and black socks? Pretty good. However, it's not impossible you'd accidently pick up the orange ones as well. (And wouldn't that be embarrassing during your presentation?)"Isolated populations" as a term simply refers to some subset of the main population that's cut off from the herd for generations, allowing variation and natural selection to take its course independent of what happens to the main group. And doing pretty bloody well. Keep it up!

Just to add a little to Quetzal's excellent post.We can see the effects of genetic drift in human populations. Beyond advantageous characteristics, such as darker skin in tropical zones, there are facial characteristics that humans can pick up on. I know we have discussed this in another thread sometime back, and the consensus was that human races are not completely independent of each other. That is, if we made a bell curve for each race they would overlap. However, certain genes and characteristics seem to be more concentrated in certain geographic areas. In other words, populations have somewhat isolated themselves and in each area certain genes have become more prominent in and other genes have become less prominent. This can be seen as mild genetic drift.I can't cite any actual papers, but my Zoology professor in college produced two sparrows, one from Europe and one from the Americas. Previous to the European discovery of the Americas this species was not native to the continent (I believe these sparrows had been in America for about 250 years). In the time since its introduction certain characteristics had changed, such as the spangle pattern on the chest and darker coloration. These were very small changes, but apparent.Hope these examples helped. Just wait until we get into sexual selection which can actually have the same effects as genetic drift. Think bright coloration in birds. Fun stuff, but we should hold off on this till later.

Wow, you guys know your stuff!!Quetzal: That's the clincher for me. Good job of explaining. So because a large population will also obviously have more numbers?
So bouncing back is more likely I suppose, for example - safety in numbers. Am I totally guessing if I say:The bear with the rare alleles got too horny with Martha bear who was Harold bears girl, so Harold bear torn the bear with the rare alleles to pieces and this would NOT help in the small populas whereas it wouldn't be too much of a worry in the large one. A lot of re-reading is taking place because of my lame short term memory. So it's not necessarily a population forced to move on, they could have moved on and genetic drift (as you established well) is more likely for the reasons of smaller populas.Loudmouth: Again showing genetic drift is more prominent in the isolated cases.Thanks guys, I may have made a mistake or 10 But I'm feeling/testing the ground, and trying to use my bonce.[This message has been edited by mike the wiz, 02-04-2004]

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The bear with the rare alleles got too horny with Martha bear who was Harold bears girl, so Harold bear torn the bear with the rare alleles to pieces and this would NOT help in the small populas whereas it wouldn't be too much of a worry in the large one.

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Nearly. There's one further factor. Absolute numbers. Suppose the rare allele bear is probability 1%. In a population of 1,000, that's 10: killing one kills 10% of the rare allele population. That's serious. In a population of 1,000,000, that's 10,000. Killing one kil1s .01%, That's trivial.So, in a small, isolated population rare alleles can be eliminated simply by the workings of chance. It's postulated that cheetahs went through such a reproductive isolation period a while ago, since cheetahs are about the most genetically identical large animal in the world. It's also a problem in trying to revive near-extinct species: with few founder members, often related, there's little genetic variation.Here is a Power point presentation on the topic.

Hi Mike,Mark outlined the only bit you haven't latched on to yet - absolute numbers. Looks like you've got it otherwise. Well done.Any other aspects you'd like us to expound on?

To be honest Quetzal I think you've answered everything, and very well too. I certainly am more informed about Natural selection.
Ofcourse when it comes to numbers it's - bed time Mike. Those analogies were excellent by the way, everyone has been very helpful - thanks for the input!

I apologize that I wasn't able to reply to you as promptly as I would have liked. I want to thank everyone for helping Mike with his questions on natural selection, and Mike for being curious and trying to understand this subject. I hope this has helped you Mike and please feel free to keep asking questions of me or anyone here. It is refreshing to see this. Again I feel bad that I wasn't able to respond and please don't feel i was ignoring you. I ran into problems in my lab research, which still need to be resolved( I hate chromatography sometimes ) but I will respond as fast as I can otherwise.ps. I would just like to reinforce for Mike that natural selection can amplify or diminish only heritable variations. It is also more of a process of editing than a creative force.