Evolution Simplified

in the sense of needing the right partner to pass on the trait, yes, it has nothing to do with the partner.

Ok, we can conclude the following:We need two ingredients for evolution to occur:1. a non-neutral trait must appear in the phenotype.
2. the genetic make-up of the individual with the non-neutral trait must be such that the non-neutral trait can be passed down to the next generation.Neither of these are absolutes. But they are probabilities.

Wouldn't it depend on how long they've been increasing and what rate?Perhaps most species are increasing but haven't been doing so for very long, or perhaps the rate of increase is so small we don't notice it. Is that theoretically possible?Are you going to answer this question or not? I already asked you twice.This message has been edited by robinrohan, 05-12-2006 06:43 PM

as to your second point not being absolute, if the new trait is dominant, it will be passed down. If recessive, it might not be passed down.a third ingedient would be a new environment, as that changes just what traits are preferable. Ohterwise, the population remains fairly stable.

Not really, it depends more on the scale of your measurement.
A population might seem to be increasing one year, but usually decreases the next. If you take a more appropriate, longer term, scale (which hinges critically on the generation time of the species in question) then you almost invariably see that such fluctuations are temporary and oscillations occur around an equilibrium point that is a relatively steady, long term average (unless the population is in a tailspin due to human exploitation or habitat destruction). Hence we have the study of 'population dynamics', the analysis of the patterns of these fluctuations and the forces contributing to them. That would not be consistent with the majority of long-term data sets we have for changes in abundance of many species. More to the point, why would you expect to observe such increases? The fact is, most organisms produce more offspring than can possibly survive, especially the simpler, 'r'-selected organisms, and yet their populations oscillate around equilibrium points.Differential juvenile mortality is typically what accounts for this, and this is why all species are evolving gradually: the survivors always possess a distinct subset of the genes of their progenitors. (And that's not to say they don't ocassionally evolve more rapidly, as well). Think of the number of acorns an oak tree produces every year, and yet the forest will seem to have a stable number of mature oak trees over the years.

EZ, I'm not sure that the answer to RR's question is being made clear. Your post is on track but we need to show reasons why an assumption of long term excess of births is a good assumption.The sizes of clutchs for dinosaurs for example. We can show that a population that did an exact replacement 1 child per adult can NOT survive by doing the math etc. Shouldn't the absolute need of an excess of births be shown first?Then the impossibility of a sustained pop growth from that. We've all seen the flies calculation (at observed fecundity how long to equal the weight of the earth) for example.When that is in place your discussion of pop dynamics fits in.Maybe RR can help by clarifying what he is looking for or why he is asking what appears to be a question with an obvious answer. (It makes sense to ask all the questions in a thread laid out like this one though)

Superficially, that would seem to be the simple inverse of the argument. But you need to clarify your use of the term 'need'. What entity (unit of selection) has this 'need' of excess births?

the answer is simple--most young die. In order to ensure that the population survives, an astronomical number of zygotes must be created.

The individual is the unit of selection. I'm not sure why there is a hang up at this point in the step by step explanation but it seems to be a problem with "assuming" that populations do reach some maximum value and may (or may not) then hoover around that for some number of generations.Involved in this is the idea of "excess" births -- that is more than enough born to replace the two (in sexual organisms) parents. If individuals each produced, at birth, only one child then the species would rapidly go extinct. If someone needs to have that shown then we can go into more details. It is why the replacement level of north american humans is given as (I think) 2.2 children per couple.

Yes. This is pretty basic, and I think you and I and Ned can see that, however, we have to choose the words carefully as we try and explain it, for example: This is not a good choice of words because whether or not the population or species survives is not a selective force on the evolution of traits within it.

I'm trying to figure out what is meant by a "fact" as used in the OP.Also I'm tyring to figure out if the OP is talking about the present day or in historical terms.I'm trying to find out if these facts are inevitable.

This is the concept of a 'population equilibrium', in terms of numbers of individuals as opposed to the frequency of a gene. The stability of this numerical equilibrium will vary considerably among species, but it is an inevitability for all species. However, change the ecological context or some force of selection and a new equilibrium point may be established. So the problem is often that when we sample a population, we don't know if it is actually at equilibrium, or in the process of approaching a new rquilibrium. The existence of an equilibrium is a theoretical extrapolation, but one which is quite supported by many long-term demographic studies. I'm going to provide an example, but first I want to finish my previous point. You said: In your mind you are thinking that the 'population' needs excess births to sustain itself at an equilibrium, and this is true, but the population, as an entity, hasn't got any 'needs'. I know it may seem pedantic, but that is why I don't think we should use the term 'need' in reference to a population or a species. Only individuals have 'needs'. As you quite rightly point out:

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The individual is the unit of selection.

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At least the primary unit of selection. So it would be an improvement to state that individuals have been selected to maximize their fecundity up to the point where their fertility becomes limiting (produce as many offspring as is possible without compromising the quality, or 'survivability' of those offspring) and the latter is the key factor which varies greatly among species. That's why some animals produce thousands of offspring with survival chances of 0.0001 and others produce tens of offspring with survival probability 0.1 or of that order. So let's consider an example of population dynamics theory and natural selection in application.It has recently been demonstrated that the average size of the many of the worlds most prized marine fish species is declining (having trouble finding the article on this from last month, but I'll add it be edit if I find it. Along with this, we know that the size of the stocks of many species like Atlantic cod have been declining numerically. What happened to our former equilibrium and why is it not being recovered?So you might say, "Well we are simply removing more fish than the population can replace by overfishing", and that's part of the story. But there's more to it. We have, through artificial selection, actually driven these populations to new, lower, equilibrium values and even with extensive moratoriums on fishing of cod on the Grand Banks of NFLD for many years, these stocks show little sign of recovery yet. Why not? We have relaxed the selection?The answer is, because for many decades we actually been selecting for a different life history in the fish population, namely earlier maturity at smaller and smaller sizes. This is because the net sizes for fishing have always been set so as to catch the largest fish.But the problem is that fecundity scales with size very strongly in fish (bigger females produce way more eggs than smaller females). And it is the large females that really provide a disproportionate number of the 'excess births' at population level. So when we selectively remove the largest fish, we are selectively removing a very important contributor to the birth rate of the population. What we end up with is a lower population of fish that spawn at very early ages and smaller sizes because the odds of making to become large have been so dramatically reduced by the way in which commercial fishing is conducted. There are still 'excess births' in the population compared to the number of fish maturing, but we have forced the population to a new equilibrium of much lower numbers and much smaller body size. We now need to alter selection so as to favor larger body sizes (enforce modes of fishing that do not select so strongly against large fish), but even then, changes in life history trends in a population can take many years to evolve.

What do you mean by a "fact," as used in the OP? Is it a probability or a certainty?Edited by robinrohan, : No reason given.

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Statement of basic position: I am a nihilist, which means, in my sense of the word, that life has no objective purpose. This entails a lack of belief in God. My beliefs are tentative.

By "fact" I mean that these things have been observed.

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"We must respect the other fellow's religion, but only in the same sense and to the extent that we respect his theory that his wife is beautiful and his children smart."
-- H. L. Mencken (quoted on Panda's Thumb)

I see that this is an old thread, but it has been briefly been referenced in a new threadI haven't read through all of the replies here, but I want to challenge the original post:

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7. Conclusion: A corollary of 6 is that as generations pass, the number of organisms with "good" traits will increase, while the number of organisms with "bad" traits will decrease, until eventually all individuals in the species will have the "good" trait and the "bad" trait will disappear altogether.

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10. Conclusion: From 7, 8 and 9 we can conclude that a species will slowly "improve" with time, as new helpful traits appear and as the organisms with these traits are better able to survive and produce offspring with these traits.

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These two conclusions are not entirely true, and do not follow from the previous facts. The further conclusion to 7 and 10 is that evolution would lead to "perfect" species. This sort of reasoning led to the eugenics movement. This reasoning is simply not true- often evolution is quite happy with an organism that is just good enough to survive. Points 1-6 only say that evolution selects towards organisms that are good at surviving, not good at living in general. If an organism is good enough to survive, it survives- there is not neccessarily a trend towards perfection. Take vestigial structures as an example of a "bad" trait that hasn't dissappeared. I'd really rather not get into a debate about whether vestigial structures will eventually dissappear but haven't yet, I just want to point to the fact that the road of evolution accomodates neutral appendages.Another related problem- it is hard to define a "good" trait that is not relative to an organisms environment. Granted, there are some, like a functioning heart, but those traits are relatively fixed. Many changes we observe in organisms are caused by response to environmental factors. An organism changes environments, and the list of "good" traits changes. Organisms do not become "better," just more adapted.This point is more than semantics, thinking about good or bad structures is really inconsquential to many evolutionary arguments, and often leads to misunderstanding about evolutionary predictions. Species adapt, its important not to think of those adaptations as good or bad, they simply are changes in an organism that lead to speciation.