How does ISOLATION factor in Evolution?

Discussion in 'Biology & Genetics' started by Eflex tha Vybe Scientist, May 8, 2002.

  1. paulsamuel Registered Senior Member

    Reply to Northwind

    When you say,

    "However, diversity of traits does not in and of itself equal greater evolution,"

    it is confusing. Increase in genetic variability is evolution, decrease in genetic variability is evolution, changes in allele frequencies is evolution. Greater evolution??? Don't know what that means. What is greater evolution?

    Your comment on an "evolutionary dead end" is controversial. Small groups that become isolated have tremendous decreases in genetic variation as a consequence of what are termed 'population bottlenecks.' With no genetic diversity these groups are less able to adapt to environmental change, but this is true of any group with low genetic variability and is not necessarily indicative of isolation. Theories of speciation all use isolation as a requisite (exception: some evidence for sympatric speciation). Isolated populations (such as island populations) can become over-specialized in some ecological aspects, but this does not necessarily mean that they are in an evolutionary dead end.
    Last edited: May 11, 2002
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  3. paulsamuel Registered Senior Member

    Reply to Adam

    Although Morris' book was important in popularizing the topic of human evolutionary biology, he is not really taken seriously by human evolutionary biologists (in fact, he is derided). Please don't accept his book as any authority.
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  5. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

    Why is that? Personally I don't consider his veiw of human development gospel, since the fact is nobody was around to record what went on. But it is a nice way to visualise the evolution from monkeyboys to astronauts, and to start people thinking about such things. Why exactly is he derided?
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  7. paulsamuel Registered Senior Member

    reply to Adam

    I was trying to say, I think Morris' book was important for generating interest in human evolution for the general public. However, human evolution was a thriving science before Morris. Morris also mis-interpreted evolutionary theory in his book to explain some human behavior and general human evolution. Some think he did it on purpose to sensationalize his book, and sell more copies.
  8. oedipus I enjoy fecal matter Registered Senior Member

    hmm after you said that it made so much more sense
  9. Lesion42 Deranged Hermit Registered Senior Member

    Makes tremendous sense to me.

    Please Register or Log in to view the hidden image!

    Couldn't say it any other way.
  10. scilosopher Registered Senior Member

    I've been busy [and apparently built up quite a sciforums post - sorry about the length of my rant] and seem to have missed an interesting and animated discussion. It seems to me there has been a lot of confusion over language. I'm going to post with the context of the discussion in mind, but not attempt to reply to any one persons comments directly as that would just be confusing ...

    Evolution is surprisingly just defined as genetic change of a population (as pointed out by paulsamuel). People often use the term implying some type of progress. As no good measure of "genetic progress" exists change is the only requirement we place on it. The most satisfying aspect of this definition is that without change you can be sure no evolution of any sort or progress is being made. The unsatisfying part is that I highly doubt there are any populations which are balanced such that there is no change in gene frequencies over time (even if they are fairly stable fluctuations are unavoidable).

    Is change inherently beneficial in such a way that one can expect it to lead to progress? Again one falters on what exactly to define as the measure for progress. There are so many possibilities I shudder to go down that road. To approach it in a different way one could instead ask the question: is any species perfect? While this approach has the same problem of defining perfection I think most would agree that none is. The one further complication is that our environment is dynamic and what is perfect certainly is context dependent.

    In terms of definitions, a gene is considered to have a positive contribution to fitness if its frequency is increasing over time. Again this seems like a bit of a dodge, but the complexity makes any explicit measure beyond this highly arbitrary. At the same time here are documented instances in pairs of alleles whose frequencies oscillate seasonally, particularly in species with life spans under a year. Always evolving, but making no more progress than sysyphus.

    At any point in time there are certain organisms which can survive. Certain compositions survive more on average and increase in frequency. The fact that some stick around is important as in the case of the cycling as changes in the environment place can easily bring a gene or set of genes back into vogue. If they were eliminated completely the decline of the one subset would translate into a decline of a whole species (or subpopulation of a species). In the extreme extinction.

    So it would seem maintaining a pool of variation is of a benefit to a population. A species however can only increase it's variability to a certain point and then problems of incompatability in certain combinations will begin to limit further variation. Population size also places practical limits on variability. Selection also removes a lot of variability, in the extreme instance dominant lethal genes. Variation is only good in the long run, not in the short. It is the constant change in environment that helps maintain and over time select for organisms that can maintain variation.

    I would like to distinguish between two different types of variability - there a specific combinations of genes that are rare and there are certain alleles that are rare (allele = version of a gene). If you mix two populations where the set of alleles in each subpopulation are different then for sure there will be an increase in the total number of allele variants and possible combinations in this mixed population. However the total number of alleles in the species remains unchanged so this increase variability is only relative to the previous two subpopulations. The existence of new combinations that didn't exist before is a clear increase in the existence of specific variants that exist. By the same token a simple exercise in statistics will demonstrate the fact that the number of realized variants that occur in the combined population will decrease relative to that of the two independent populations taken together. Combinations seem to be very important in evolution in higher eukaryotes.

    This can basically be explained by thermodynamics. If you have two chambers - one which contains a vacuum and one which contains air separated by a stopper. Once you remove the stopper you will never see either side become vacuum. Similarly if you take a two populations of 100 urns - one that contains 70:30 mix of green:red chips and one that contains the reverse 30:70 green:red. If you take each and mix it each "generation" by pairing up the urns dumping one into the other in each pair then resplitting the chips between them and count the number of variant combinations it will be more than if the populations are combined. While both are very different from the dynamic state of things in biology they hold much of the insight necessary to understand the decrease in variation upon subpopulations mixing.

    Regarding small populations and evolution, there are two things that can allow rapid change. First in generating the small population sampling errors or fluctuations occur which can lead to populations with a distinct skew (while one might think on average they should be a good representation there is the clear point that at a certain size sample in which the total variability of a population can't be representation and before this point is reached there will typically be a threshholding loss of variability). Also there is a reduction in inertia which will allow alleles to pass through the population much more quickly (this can be added to by geographic spread of the species as well). While there are many reasons such populations are at risk, if they happen frequently enough in some instances they will make it and the altered circumstances can alter their "evolutionary trajectory" or general dynamics going between the composition of the gene pool and the environment (and remember the environment includes ALL the other organisms gene pools).

    If one is to consider local minima (or maxima depending on the measure used) of evolution. To take contrived examples changing bone structure from a human to a bird (a very unlikely path, though if one accepts time reversibility [and given the thermodynamic consideration of evolution this is quite possibly not 100% kosher], evolution, and speciation ther IS a path of living genetic intermediates between ANY two organisms [including you and me]). Obviously this was not a straight geometric morph. At all points it had to be a body structure that could protect itself and function to feed and support itself. The path is hard to imagine and in truth they could be entangled such that it could never be linearized. Especially if you think about them all at once. With spatial separation that allows potentiations and gross overall differences that make adding a new organism to the ecosystem is calamitous.

    I have to agree about us not understanding it, but that's not to say we don't understand any of it. Arguing where exactly we are on that continuum is an exercise in futility and much less fun than trying to figure it out. I like it when people toss out ideas rather than facts. Evolution a bunch of "ideas" bounced of the world and eachother for millennia.
  11. paulsamuel Registered Senior Member

    Reply to scilosopher

    You said,

    "Variation is only good in the long run, not in the short. It is the constant change in environment that helps maintain and over time select for organisms that can maintain variation. "

    This will depend on how and how often the environment is changing. Consider MHC genes. These are genes that code for cell receptor molecules (proteins) responsible for binding antigens for T-cell recognition. The more variable MHC genes are, the more forms of antigens that can be recognized. This is an obvious benefit to the individual confronted with various disease causing organisms, but also to the population at large (it would prevent large scale epizootics that could threaten population extinction on a localized scale, as seen in some mono-cultures of agricultural products). This appears to be a short term benefit, even a constant benefit, as well as a long term benefit.

    Re: increases in variation of newly isolated populations. Ken Kaneshiro and Hampton Carson have together and separately published on Hawaiian drosophila radiation. One of their hypotheses is that there is a relaxation in sexual selection in newly arrived colonists (probably for a few generations) resulting in an increase (founder flush hypothesis) of genetic variation. So, perhaps a third mechanism for rapid differentiation.
  12. scilosopher Registered Senior Member

    By short run I meant under constant conditions. In reality this never happens, but conceptually I think it's illustrative of an important facet of evolution.

    Consider a frozen environement with a certain mix of organisms of given unchanging genetic backgrounds. There is 1 best combination of genes for a single organism to have to maximize the probability of survival (there are probably a bunch that are real close, but assuming continuous functions there will be one combination that is the best with almost probability 1). That was all I meant.

    In reality you're right. Even as an approximation one should determine the time scale over which appreciable change in the environment occurs for determining the meaning of short run. I should have been more explicit about that.

    I've also thought a bit about the genetic path between any two organisms. I think that time forward paths and time reverse paths have different properties such that while an interesting note biologically, it's evolutionary relevance may be lacking.
  13. oedipus I enjoy fecal matter Registered Senior Member

    but is there ever, or could there ever be a immortal life form, by survival what species could survive for ever, would they be eventually out dated?

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