Well yeah, but when talking trees, who better to ask than the mighty oak?
I completely disagree with your highly simplified example. You have compounded the problem by introducing two variables at once. But even if you break it down into two separate problems (disadvantages of B vs the original and disadvantaes of the original against A), you will not see the difference between selecting for and selecting against.
Selecting for implies that only positive (or advantageous) gene combinations are passed on. Selecting against implies that only negative (or disadvantageous) gene combinations are weeded out.
The concepts of selecting for or against are human inventions intended to describe what is being seen. Natural selction doesn’t sit there and say “oh I like this gene, let’s make sure it doubles it frequency in the population.” There is some random gene modification and it turns out that this modification increases its frequency. But, in order for that to have happened, the original must decrease its frequency. I.e. the original has been selected against. In this example, there is no difference between selecting for and against.
New example: there is some random gene modification that does nothing whatsoever for the individuals it is in. Unless this modification later becomes a disadvantage, its frequency in the population will increase over time. It is more correct to say that it has not been selected against.
Has the OP been answered yet?
But it is not simply a case of beneficial / benign traits remaining; it is the case that the beneficial traits are preferred. Again, those individuals who, as a result of mere variation, possess an advantage over others of their kind will have a higher than average chance to pass those beneficial genes on to the next generation. The frequency of such advantageous alleles thus tends to increase in the population over time. This process is often abbreviated to the phrase “selected for”.
Meanwhile, thoe traits which bestow a disadvantage will tend to be less favored than average, and individuals possessing them will tend to have a lower than average chance of reproducing. The frequency of those alleles will tend to decrease in the population over time. This is what it means for a trait to be “selected against”.
Those two statistical statements are what natural selection is all about. Natural selection operates in both directions, but it is the positive direction that leads to adaptation.
Were it simply the case that negatives were culled, then both the beneficial and the benign would thereafter be subject to random drift. This is not the case. NS is creative as well as destructive.
Except the gene/allele frequencies do not need to be compared to others in order to see trends. One need only observe that in generation T[sub]1[/sub], allele X is found in y% of the population. In generation T[sub]n[/sub], it occurs in z%. If z >> y, then it is most probable that X is being selected for. If z << y, then it is most probable that X is being selected against.
JustAnotherGeek, I suggest that you allow your ignorance to be fought. You pretty clearly don’t know enough about evolution to be making these arguments. Natural selection is a far more complicated, intricate mechanism than you give it credit for, and it certainly doesn’t give a good og-damn about your precious variables and how many there are at once.
As for the OP: advantages to “leaf flutter?” None. Zippo. Nada. The primary selection pressure behind needle leaves in conifers has to do with water loss. A broad leaf deciduous tree has a huge amount of leaf surface area. This allows for lots of efficient photosynthesis, but also massive amounts of evaporative water loss (we’re talking gallons per hour, here). During most of the year, this isn’t a problem, but during the winter most of the available water is locked up in ice and unavailable to plants. So deciduous trees shed their leaves, which prevents them from literally dying of dehydration. Conifers, on the other hand, have much less surface area through which to lose water, and can hold onto their leaves year-round. There is also less water at higher altitudes, which is why conifers predominate here, too. Incidentally, in addition to being able to tolerate smaller amounts of rainfall, conifers are also able to grow healthily in soil so devoid of nutrients that no self-respecting deciduous tree would tolerate. Deciduous trees (which happen to be flowering plants, known as angiosperms) haven’t figured out how to deal with these kinds of conditions, which is probably why conifers are practically the only gymnosperms (naked seed plants) who can compete with them under any circumstances (ginkgo, a gymnosperm that isn’t a conifer, is a notable exception). When you get to a certain size of leaf, then it is conceivable that there would be some pressure to not get any larger, but this may be for totally unexpected reasons, and fluttering in the wind certainly had nothing to do with the difference between needle leaves and broadleaves.
Incidentally, you don’t really see normal leaves under any kind of dry conditions. Though many angiosperm plants have figured out how to live in deserts, few of them are trees, and most don’t have anything resembling normal leaves. In fact, cactuses had no use for their leaves as photosynthesizers. It turned out that they worked better at defending the plants - in the form of sharp spines!
Don’t forget abscission. A properly fluttering leaf abscisses more easily in the fall.
Half the oaks down the block seem to forget that it’s healthier to lose the old leaves before winter winds hit.
This may be the first instance ever in which a question was answered quicker, and using fewer words, by a lawyer! 
I just have to say that I do find this type of statement incredibly condesending and anathema to this board.
How, oh wise-one, shall my ignorance be fought if I do not attempt to root out misconceptions by positing explanations and determining their validity? Shall I simply decide to believe everything that comes my way? :rolleyes:
Grelby, I can’t help but read the above as, “Kiddie, just shut up and believe what your betters are saying.” I don’t care how poetic you put it, but your suggestion to me is a poor attempt to argue from authority. You wanna actually try to be helpful, here? Or would you rather just jeer?
Darwin’s Finch:
Let me ask you then to explain to me how benign genes become widespread in a population?
Also: “advantageous gene” would imply that there is one that is not as advantageous, yes? How can you determine that one is being selected for and not the other against?
E.g. Let’s say there is a random mutation that gives a subset of a population of birds a better efficiency in digesting their food. Using the “selected for” argument, one would say that “the frequency of the birds with enhanced digestion will increase.” Using the “selected against” argument, one would say that “the frequency of the birds without the enhanced digestion will decrease.” How are these statements different? How would it be different if the random mutation gave a subset a lower efficiency in digesting their food?
In short, isn’t the phrase “selected for” a short-hand way to describe the winner that was not selected against?
I swear that this argument is not some random fluctuation in my brain. I can remember reading this concept before, but cannot, ATM remember where. When I get home and have time, I will try to run through some of my books to see if I can figure out where I read this in order to give you a cite to prove I am not 100% crazy. 99% maybe, but not 100%
Well, I thought that providing an example of a kind of selective pressure that could lead some trees to have broad leaves and others to have needle leaves was addressing the OP - one of the answers suggested that this was an adaptation to prevent leaves being torn off in high winds. While needle leaves have this benefit, prevention of moisture loss is a more likely reason for them to have evolved.
As for you, I’m sorry if I made you angry, but you posted something in answer to the OP that was patently false - namely, that natural selection works by selecting against negative traits, not by selecting for positive ones. You openly admitted that you know “little about evolution,” and yet when Darwin’s Finch came along and gently corrected you, you got defensive. Not understanding and asking for further explanation, I can understand. But arguing the point when Darwin’s Finch clearly knows what he or she is talking about, and you clearly do not? Bad form. People in GQ are looking for factual answers to questions. You provided an answer that was factually incorrect. I don’t mean to patronize, and I don’t mean to jeer, but yeah; I’ll appeal to authority in this case. Now, if we want to start whipping out citations, that’s another matter. I had a nice post that explained things more fully, but then the power went out. I didn’t want to re-write the silly thing, so I just kept it short, which may have resulted in the jeering tone you heard. My apologies.
When did I become a lawyer? I’m studying to be a scientist!
Random drift. There is pretty much an equal probabilty that a neutral allele will become fixed in a population, or that it will be eliminated from a population.
It’s not so much an advantageous gene as an advantageous allele (or, for those, like me, who don’t subscribe to the “gene as primary agent of selection” school of thought, simply a trait that provides a minute advantage over the average individual).
Variation results in very slight differences between individuals. Those variations which are heritable, and which bestow a small advantage to those who possess them, have a higher than average probability of being passed on to the next generation. The “average” version is not detrimental by any means (in that particualr environment), so it is not being actively selected against. It is the norm, especially in large populations. In such a case, even a very advantageous allele might have difficulty gaining a foothold.
Well, for starters, you can’t select against (or for, for that matter) a null. Lacking a trait is not a trait in itself. Therefore one cannot say that the quality of “not having enchanced digestion” is being selected against, any more than you can say the inability to breathe fire is being selected for. But you can say the positive trait of having enhanced digestion is being selected for.
Not really. It’s a short-hand for saying that a trait has a positive selection pressure (that is, the advantage provided is sufficient for the trait to have a statistically higher-than-average chance of being passed on). The average case is not being selected against because there is not an active pressure for the trait to decrease; were it not for the even-more beneficial variations, the trait would remain with high frequency. That average condition may be displaced eventually, but again, that’s not what is meant by “selection against”. Selection is an active force, if you will, not a passive one.
Naw, man. I’m the lawyer, and I touched on cactus spines/leaves in post #17.
When I discussed my posing of natural selection (i.e. as selecting against as opposed to selecting for) with a biologist friend, her response was that that was what had been drilled into her head, as well. OTOH, since I can’t find where I read this, and she can’t find where she read this, I will withdraw my earlier assertions. I will state that I believe that we are talking past each other. I am using (appearently unorthodox) terms to describe the observed phenominon; perhaps my argument is not as clear as I am trying to make it. And lastly, I swear I am not crazy; when I find out where I have gotten this idea from, I will create a GD thread and invite you all.
I agree: I don’t think you’re way off base, just using different words. Selecting ‘for’ or ‘against’ an allele can only happen in comparison with an alternative allele, so they’re always a relative comparison (i.e. if A is selected for over B, then B is selected against in preference to A).
Considering a very simple situation where there are a whole bunch of alleles, with a strict ranking in order of fitness. It seems that you agree that there aren’t really different mechanisms at work on the high and low ends of the fitness spectrum; you’re just considering all of the alleles except the very best one as ‘negative’, and therefore selected against. It’s also possible, and just as correct, to consider the worst allele as the baseline, and say that natural selection only really operates to select for the alleles that aren’t the worst ones.
In general, it’s probably most useful to set your baseline at the most common allele in the population, and consider better alleles to be selected for and worse ones to simultaneously be selected against.
Anyway, I don’t think you were out of line, and hope my comments didn’t come across as any kind of smackdown. We’re here to fight ignorance, especially our own, and there’s no higher calling for a Doper than to be willing to explore and extend their own knowledge.
So you did! Such a short post, I missed it ;).
I think I bit off more than I could chew by asserting that leaf shape and aerodynamics (or whatever) could not be affected by prevailing weather conditions - it makes a lot of sense to believe that there could be a selective pressure for certain leaf shapes based on wind conditions. At least we appear to agree that the primary driver behind differences in leaf size and composition between broadleaves and needle leaves has to do with transpiration, not wind!