Evolution is gradual ?

Discussion in 'Biology & Genetics' started by Rick, Apr 3, 2005.

  1. Rick Valued Senior Member

    Messages:
    3,336
    Hi,

    i was wondering wether evolution was really a linear path,but occured by sudden cataclysmic events over a period of time? which were noticable,like a meteor strike etc.
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. James R Just this guy, you know? Staff Member

    Messages:
    39,397
    What do you mean by "linear path"?

    For example, monkeys, cats and human beings all share a common ancestor, but humans did not evolve from monkeys, and they didn't evolve from cats, either.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. Rick Valued Senior Member

    Messages:
    3,336
    i mean in terms of graduality, was evolution phenomenon not exponential due to some cataclysmic events occuring over a period of time?
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. spuriousmonkey Banned Banned

    Messages:
    24,066
    I would say that evolution is in general gradual, but major events can cull a large part of and variation within species. It can have a huge influence on the nature of the collective biosphere or local biosphere, but does it shape new form? I would say that gradual evolution works more efficiently in this area.

    Maybe,...
     
  8. Thersites Registered Senior Member

    Messages:
    535
    There's a lot of debate. Evolution/descent with modification/survival of the fittest happens continuously of course. There is a theory of "punctuated equilibrium" [google] that argues that major evolutionary changes only occur now and then when there are drastic environmental changes causing mass extinctions. We- the human race- seem to be one of those changes, so, if we survive, we may see proof of the theory.
     
  9. Maddad Time is a Weighty Problem Registered Senior Member

    Messages:
    251
    We have punctuated equilibrium because species are stable in large populations for long periods. Evolution is an adjustment to pressures that make a specie less than ideal for its environment. The correlary is that if the specie is well adapted then there's no reason for it to evolve. The specie changes when its environment changes.

    Often that change is continuous, like a predatory cat getting bigger and faster while its lunch does the same. However, it may also be that the species are stable as they currently are. In that case you con't get any change for a while, and then when you do, both may change relatively rapidly.
     
  10. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    As usual, I arrive late for the party. Anyway, here's my bottle.

    This is an excellent question, with no simple answer. Others have told you that sudden cataclysmic environmental change can somehow lead to a corresponding "cataclysmic" evolutionary change. It is not at all obvious how this can be.
    We assume that mutation rates are not, in general, affected by environmental change. Let's assume a uniform rate of mutation of...er... 4 x 10<sup>-9</sup> per generation. Let's also assume that a species is perfectly adapted to its ecological niche (that's bold!), then mutations will be detrimental or neutral. Providing the ecological niche reamains the same, the detrimental mutations will be quickly removed by selection, and if they are neutral, some may become "fixed" (by a process I can describe if you're interested) but most won't.
    The point I'm trying to make is that it is very difficult to see how a resevoir of diversity can stably exist in a population of such a form and in such a way that, given cataclysmic environmental change, there will be a genetic pool capable of exploiting it.

    Nevertheless, as others have said, punctuated equilibria seem to to be the way evolution works (think of a staircase, but don't take the "upness" of it literally!). There are models, I have one which I expounded here not long ago. But all the evidence, fossil and other, says that this is the way it goes.

    So, the answer to your question is that, there may have been periods of gradual evolutionary change, mostly these changes have been rather abrupt.
     
  11. Rick Valued Senior Member

    Messages:
    3,336
    could you please ellaborate a little more for better grasp of the same...?

    thank you.
     
  12. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Well, I'll try, but it might be better if you just asked questions.
    Let's start with a hypothetical population perfectly adapted to its ecological niche, and let's say that niche is unchanging at the moment. We must assume that new mutations are occurring at a certain rate. As our population is perfectly adapted, these mutations cannot improve fitness in this niche - they must either reduce it or be neutral. Those mutations which reduce fitness will be removed from the population by selection. The question is, how do the neutral mutations become fixed. Classical theory says they are removed by dilution except where they are subject to a process called "fixation by random genetic drift". Ask if you want that explained.

    Now let there be a sudden change to our population's niche. In its present form, our population is ill-adapted to its new environment. So where are the individual members who carry the mutations which will allow them to survive in their new circumstances? We have explicitly eliminated all those except the minority that was fixed by drift. It is not at all obvious, under classical theory, that this process is sufficient to allow the accumulation of enough genetic variation for a sub-population to survive sudden and dramatic evironmental change.

    I hope that at least explains the problem.
     
  13. J.B Banned Banned

    Messages:
    1,281
    OK, so could it be possible that as the world got smaller and more people of more cultures became more in contact with each other, that humans as a whole made greater strides in our own evolution?
     
  14. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Hmm...I'm replying to myself - is this a first?

    There is something I didn't make clear because it's obvious to me, but may not be to others. So....

    Selection acts only on the phenotype. This means that the only effect that selection can have is to change allele frequencies in our population, and only ever in the generations that follow that which has had selection operating upon it.
    And second, for selection to have any effect there must be - must - pre-existing phenotypic and genotypic diversity in the population. That's why I asked the question "where does the genetic diversity come from?"
     
  15. NeonBlack Registered Member

    Messages:
    28
    So the rate of mutation and rate of evolution are different. The rate of mutation will be fairly constant. During periods of equilibrium, any beneficial mutations will be mixed into the population and evolution will proceed rather slowly. But during a period of stress on the population, the beneficial mutations have a strong advantage over the non-mutated and the rate of evolution increases. Am I on the right track here?
     
  16. Ophiolite Valued Senior Member

    Messages:
    9,232
    A couple of points, not as absolutes, but as plausibles.
    The process you have described in simplified form relating to punctuated equilibrium is that an environmental change occurs in which different genetic characteristics will afford an advantage. However, where did those changes come from?
    I suggest that many did not arise till after the envionmental change. (Don't panic - I'm not going down the Lamarckian route.) The magnitude of change in organisms seems roughly porportional to the change in environment. That change is often associated with not only a reduction in species numbers, but seemingly a reduction in population numbers. When the change is small the reduction in number is small, when the change in large the reduction is large. What seems to me to occur is that within this reduced and stressed population, that is no longer in balance with its environment, mutations occuring at the normal rate now have an opportunity to produce a change that is beneficial in this new enviroment. Secondly, you noted that
    I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. In some settings the differences between two options may be so small that neither comes to dominate, yet in a changed environment one or other may deliver a substantial survivial benefit. These cryptovariants then provide a further source for the genetic diversity required for selection pressures to operate on within a changed environment.
    Together these two mechanisms seems to me to solve the problem.
     
  17. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Yes. Mutation occurs at a constant rate (as far as is known), evolution requires phenotypic diversity and evironmental change or migration (plus selection)
    Hmm...our population is ideal- it is perfectly adapted- so "beneficial" mutations (don't like that term, but we all use it) don't exist. Of course if our population were not perfectly adapted, then beneficial mutaions will confer a survival advantage, and our population will become better adapted.
    You are, but my point was, in a perfectly adapted population, where is the genetic variation required for rapid evolutionary change?
     
  18. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Well, either you are stating the obvious - that change in the environment gives rise to selection pressure- or you are indeed being ever so slightly Lamarkian!
    Yes, well. I was being sloppy here. I was using the terms population and species interchangeably. Large-scale population reductions are called a "bottle-neck", and give rise to some quite interesting effects. We Europeans, for example, are almost certainly the result of some sort of bottle-neck. Hmm. I'd like to say more on that, but won't just now.
    Yes, dear boy, that's the standard model.
    Yes, my population was hypothetical. The point I was addressing was that, in gradually changing environments, it is possible to imagine that a mutation rate of 4 x 10<sup>-9</sup> is sufficient to keep pace with slow environmental change. But it may not be so when environmental change is abrupt.
    But you haven't explained how these "cryptovariants" (ugh) stably co-exist with the rest of the population.
     
  19. Gambit Star Universal Entity Registered Senior Member

    Messages:
    317
    Evolution is humans reason for questioning itself as a collective conciousness.

    I believe that the fact we recognise our evolutionary path provides reason to share and create with other entities. The fact that the "known" universe is apart of our lives shows just how far ahead of ourselves we could be, but not just for human reasons.
     
  20. Ophiolite Valued Senior Member

    Messages:
    9,232
    I was looking to engage in an interesting discussion not in a game of patronisation.
    I have to my satisfaction "I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. In some settings the differences between two options may be so small that neither comes to dominate, yet in a changed environment one or other may deliver a substantial survivial benefit. " You seem to imply that a phenotype of restricted character always reflects an equally restrcited genotype. I await the opportunity to be educated.
     
  21. CharonZ Registered Senior Member

    Messages:
    786
    That's bold indeed. But even if we assume that such a perfect state exists, one mustn't forget that even detrimental mutations are unlikely to be eliminated completely, if the given pehnotype is recessive.
    That is, the detrimental allel might survive very long, even if it is selected against. Thus diversity in the gene pool is likely to exist even in the perfect state,possibly providing the basis of rapid adaptive radiation.
     
    Last edited: Apr 7, 2005
  22. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Hey! I'm sorry if you found my "dear boy" offensive. Here in the UK it is regarded as a (rather dated) form of paliness.
    And I agreed that I was talking hypothetically, and that in practice, perfect adaptation is unlikely to occur (and not only for the reasons you gave). I'm trying to build a model, not describe the real world!
    But I'm afraid I don't understand this comment. What is a restricted genotype? Are you in fact saying that in the population genetic homogeneity may not result in a corresponding phenotypic homogeneity? Or maybe that just because a population is phenotypically homogeneous that doesn't mean it is genetically homogeneous? Yes, that looks better. And if so, you are right, of course. But I don't see how it helps. Remember that selection acts on the phenotype, and it's only affect on genotypes is to change allele frequencies in the population.
     
  23. QuarkHead Remedial Math Student Valued Senior Member

    Messages:
    1,740
    Depends what you mean by "very long". Remember a population (mine here is, at least) is always in Hardy-Weinberg equilibrium, even when the recessive homozygote is lethal (or strongly selected against - it amounts to the same thing). This pushes the allele frequency down rather rapidly, in evolutionary terms.
    Look, I'm not trying to be controversial, I am simply trying to point out that yes, punctuated equilibria are the name of the game, but they are not that easy to accomodate in the standard neo-Darwinian model.
     

Share This Page