Does evolution require selection?
(context can be found in this comments thread on Sandwalk)
Go!
Update 16.11.09: I've mentioned this in the comments, but it's kind of a new question so I'll repost it here:
An evolutionary system, regardless of its medium (biological, linguistic, cultural, etc), has finite resources, and must abide by laws of physics. The former necessitates some form of positive selection eventually, the latter invokes negative selection right from the start, provided variation exists. Some forms will end up being unable to self-propagate, regardless of which system we are dealing with. In the biological system this is very obvious - fuck up DNA replication, and the organism's lineage ends there.
My question was invoked partly by the commonly accepted statement that evolution fundamentally requires three things: Heredity, Variation, Selection. As much as I like fighting commonly accepted statements, this one seems strong thus far. Also, I've heard strong proponents of Neutral Theory use these preconditions as fact, so it's not in any way conflicting with a more neutral view of evolution. So I referred to it in the discussion on Larry Moran's blog, and the selection component, to my surprise, was shot down entirely.
So I often hear that evolution is change in allele frequencies (or their equivalent in non-biological systems) over time. Can this change occur without positive selection? Well, yeah, we've got genetic drift. But what about negative selection? Surely, for allele frequencies to shift, the population size must be finite; else the proportions would remain the same (ignoring positive selection). Stuff must die. Even if we assume no selection at that level, the new variation must be 'proof-checked' via negative selection, especially considering many changes are actually deleterious.
I guess if we remove all mutation, then we could have a system devoid of any selection, where stuff just randomly dies and then the allele frequencies would drift, and evolution can be said to occur.
But then, would weather patterns be an evolutionary system? They can be argued to be heritable in a non-discrete sense. Say we have each sq km being rainy, sunny, foggy or whatever. It inherits its next weather state partially from its previous condition (and obviously influenced by neighbours; lets call it LGT). The percentage of sq km in a given population experiencing rain or shine changes randomly selection-wise (although non-randomly if we consider it from the physics perspective; but the same applies to biology).
I'll argue here that, unless I've missed something, the weather system is akin to the biological system minus selection (the variation and heredity are still there). Does it still evolve?
(and thanks, this discussion is really helping me wrap my head around certain things! I don't learn well by just listening and digesting; I have to constantly prod at stuff until I'm ready to accept it!)
Absolutely not. Futuyma defines evolution as the change in gene frequencies in a population over time. No need for selection. People confuse evolution and adaptation.
ReplyDeleteI need to go back and re-read later when time permits, but I caught this right away:
ReplyDeleteWe know that populations evolve, not individuals.
What about horizontal gene transfer as an argument against that concept?
Just to clarify, I don't mean adaptive processes necessarily. Do neutral processes (which can result in increased complexity, etc) also require selection?
ReplyDeleteOr perhaps to reword the question a bit:
ReplyDelete"Is selection necessarily inherent in an evolutionary system?"
@Oroboros: HGT would still be meaningless if the organism receiving the gene fails to pass it on. (sex isn't necessary, btw)
ReplyDeleteWhat did you have in mind as an argument against evolution of populations?
IT'S A TRAP!
ReplyDeleteEverything depends on how you define each concept. But let's imagine we agree that evolution means just change and that adaption means some dies and some survives for a reason different than randomness. Although randomness is always there.
I'd start thinking of what would happen if selection didn't happen....
But is that possible? In a world where predators and preys exists, selection will happen.
If you want no selection, you'd need no competition for limited resources nor feeding from each others. But that's a selection by itself! Cause you need something to prevent creatures to start feeding from each other. And you'd need to select your creatures in a way that the won't kick each others from the area they live and to not eat all the food causing others to starve.
You can't avoid to change and mutate either. Unless you are an elemental particle but I also doubt of that, each time the universe breaks a symmetry particles get split and the composition of the universe changes dramatically.
I think there's not a dependency among those phenomena (change and selection), but both things happen to be unavoidable and you can't avoid interaction among them. Selection happens. If selection is not killing all your friends, you could survive forever, but if it gets enough stressing on the population, everybody will die. Unless they evolve! Which means that new forms with worse or better chances of be selected can appear.
Selection could kill all of us with enough time if the mutation rate isn't fast enough and if we just don't get the mutations we need to survive cause the intelligent designer doesn't answer our praying.
.........
I think your question would be taken as: Is selection driving the evolution? Like, is selection causing the change?
I'd put this example: Imagine a population where no one competes with any other for food, energy, space nor anything, and nothing can kill you and everyone is immortal. Let's say that reproduction is possible. No selection. If a couple suddenly breeds a mutant with a faster reproductive rate and with a different color let's say they are purple. Then the purple kind would get more numerous than the rest of the population. Is that selection? I won't call that selection, but you'll start with a 100% green population and some years later the 80% of the total population will be purple and the 20% will be green. What's going on there? Is that evolution? Looks like a passive selection. You are purple, you don't kill the green ones, you don't stop them to reproduce, but you still end up being more than them, in proportion. Is this selection??? Looks like it. Instead of affecting the 'losers' reproductive rate making them less than the winners, it just provided the winners with a higher reproductive rate. It's the equivalent for what we usually call selection, but instead of putting more pressure on one population to affect the reproductive rate, the reproductive rate changed by itself.
So, in this case was the opposite!!! IT IS EVOLUTION CAUSING A SELECTION EFFECT!
Does selection require evolution, then? Hehehe.
Ok, I'm done for now.
HGT would still be meaningless if the organism receiving the gene fails to pass it on. (sex isn't necessary, btw)
ReplyDeleteI agree with Kaexar's assertion that it's a trap. If evolution is the process by which a sub-population in one species becomes another distinct species that is no longer capable of interbreeding (and I'm a noob and probably going to make an elementary mistake here real soon if I haven't already), why can't we consider a case of an individual bacterium becoming an archaeon via HGT as an example of evolution (where the sub-population size is 1).
What did you have in mind as an argument against evolution of populations?
I really just meant that you can still contend evolution does happen to individuals in some cases.
The only reason I even think this way is because of some arguments with creationists against evolution. It seems like HGT where a single organism becomes a member of an entirely different domain is obvious proof of the concepts (even if it isn't the norm).
Retaking my previous post, what is selection about?
ReplyDeleteSelection seems to care only when it affects the reproduction rate or ability to reproduce.
Actually, the problem is death.
If we eliminate death, then you can have evolution without selection. Then you could also have creatures that don't need to replicate of only one individual. But then we are getting out of the concept of life. Life IS REPLICATION! If it wasn't then everything would be considered alive. :P
Question: Is life about REPLICATION in general or only SELF-REPLICATION? (viruses and cars would be life forms them, even memes)
.........
Anyway, my point is: Don't care about deleterious selection, what matters is the reproductive skills!
So for evolution we'd only need reproduction. Change will happen by itself. And if Death is around, it will only keep some life forms out of existence. But it won't stop variability to happen.
In the end it's not to adapt, it's to reproduce more, because reproduction is the shape immortality adopted among us.
Kaexar, you've kind of mentioned what I was vaguely thinking of. Namely, an evolutionary system, regardless of its medium (biological, linguistic, cultural, etc), has finite resources, and must abide by laws of physics. The former necessitates some form of positive selection eventually, the latter invokes negative selection right from the start, provided variation exists. Some forms will end up being unable to self-propagate, regardless of which system we are dealing with. In the biological system this is very obvious - fuck up DNA replication, and the organism's lineage ends there.
ReplyDeleteMy question was invoked partly by the commonly accepted statement that evolution fundamentally requires three things: Heredity, Variation, Selection. As much as I like fighting commonly accepted statements, this one seems strong thus far. Also, I've heard strong proponents of Neutral Theory use these preconditions as fact, so it's not in any way conflicting with a more neutral view of evolution. So I referred to it in the discussion on Larry Moran's blog, and the selection component, to my surprise, was shot down entirely.
So I often hear that evolution is change in allele frequencies (or their equivalent in non-biological systems) over time. Can this change occur without positive selection? Well, yeah, we've got genetic drift. But what about negative selection? Surely, for allele frequencies to shift, the population size must be finite; else the proportions would remain the same (ignoring positive selection). Stuff must die. Even if we assume no selection at that level, the new variation must be 'proof-checked' via negative selection, especially considering many changes are actually deleterious.
I guess if we remove all mutation, then we could have a system devoid of any selection, where stuff just randomly dies and then the allele frequencies would drift, and evolution can be said to occur.
But then, would weather patterns be an evolutionary system? They can be argued to be heritable in a non-discrete sense. Say we have each sq km being rainy, sunny, foggy or whatever. It inherits its next weather state partially from its previous condition (and obviously influenced by neighbours; lets call it LGT). The percentage of sq km in a given population experiencing rain or shine changes randomly selection-wise (although non-randomly if we consider it from the physics perspective; but the same applies to biology).
I'll argue here that, unless I've missed something, the weather system is akin to the biological system minus selection (the variation and heredity are still there). Does it still evolve?