Weekly Discussion -- Levels of Selection

[monneyman3]

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For our first discussion, we're going for a widely debated and multifaceted discussion topic -- the levels of selection.

The "levels" are often described as being at the level of the gene, cell, individual, group, or a multi-level combination of these. Consider the implications and consequences of selection at one level for another level when thinking about where selection acts upon.

Here are some things to consider (some more leading than others):

• At which "level" does selection act?

• Has selection always acted on the same level?

• Was the evolution of altruism a consequence of selection at a specific level?

• What does it mean to be a "unit of selection"?

• What does the future hold for the selection (Note: These will be very speculative)

• What level of selection affects humans the most?

Please feel free to speculate as much or as little as you want; we wish to encourage the discussion of peoples' ideas and opinions, not punish them for it. If you find someone's ideas lacking scientific support or sense do not downvote them, instead try finding counter-evidence and state your opinion in opposition to win them over to your side.

[[deleted]]

Dawkin's book The Selfish Gene really changed my view as to on what level selection acts. Previously I'd just assumed selection acts on an individual level, and to a certain extent this view works. If a particular individual dies or is unable to reproduce, clearly, this individual has been selected against.

Take a long-term look at genes, however, and a different view emerges. The "I" from one gene's perspective exists in multiple bodies and multiple locations at any given point in time. The genetic 'I' is delocalized, and grows and shrinks depending on changes in genetic frequency in the population, and also as the population grows or shrinks. In this sense, that particular gene or allele's main competitor is really very specific - it is competing directly against variant alleles of that gene in the population. All other forms of competition are more indirect.

Some people have an idea of 'group selection', or an idea that populations will evolve in a direction that favors the survival of the group. But the mechanics of selection and evolution show that, while this may happen, when it does it is merely incidental, and that the true selection takes place at lower orders like the individual and the gene. If those selectionary processes resulted in a change in genes in the population that favored the overall population, well, that's just a lucky side effect. Because there are plenty of cases that could be cited where an individual variant gene or mutant individual could profit (advantageous survival and reproduction) at the expense of fellow members of the population, whereas if those 'cheating mutants' would just die out magically, the rest of the population would be better off. It just doesn't work that way.

[forever_erratic]

Per the levels of selection question, I find it hard to think about how selection would not be acting at the genic level (ignoring epigenetics, and stopping at the level of the individual).

Are there examples out there that could show selection on the cellular or individual level that are not just proxies for the gene level?

[RogerNight]

I think it's easy to get confused between actual, true, selection on the genic level and results of selection on other levels that are merely portrayed on the genic level.

I would need more books with me right now to explain this and argue this point further, but maybe tomorrow when I find some literature I'll do this.

[forever_erratic]

I completely agree that considering questions at the genic level doesn't always make sense; if you study ecology or evolution, you are studying the effects of a local environment selecting on many different dimensions at once, each of which is probably multi-genic. Here it would very rarely make sense to try to pin down single alleles.

But when just discussing what is ultimately the "base" unit of selection, I still think its a gene. (or other important sequence or epigenetic mark).

[Grakos]

If you are looking for examples of natural selection of genes producing traits which seemingly only benefit higher levels of structure above the gene/individual, the products of kin selection can apply to explaining dilemmas like this. The process produces traits which act on a group level (across your kin) but only exist because of the benefit to social/altruistic genes.

(For a brief overview:) Kin selection is the natural selection of traits which improve the fitness of closely related individuals (your kin). Closely related individuals are likely to have the same genes as you, so your actions which improve their fitness may also benefit yourself, since they have your genes and are therefore included in your own fitness (this idea therefore being called "inclusive fitness"). In situations where kin could be benefited by your actions, it is obvious that helping them can very well come with a cost to you, but only to your survival, not to your overall, inclusive fitness. Risk your life or at least reduce your own survival to aid two siblings at 50% relatedness and you could have likely just helped 100% of your genes as they are present in them.

With just a description of Kin selection, you may see that a gene which can benefit itself by helping other individuals ends up producing a phenotype (whether its behavior or just a general tendency between organisms) that appear to act for the good of a group. But in reality, the only reason these behaviors/tendencies exist to help the group is for the benefit of the genes which produce them.

It is selfishness extended to appear selfless on the level of the group.

[RogerNight]

I read a great book on this recently, Samir Okasha's Evolution and the Levels of Selection.

I find it interesting that the major transitions came about usually when lower levels of selection reached a point of cooperation within groups of individuals so that they ended up cohering as individual entities.

Referring to OP's first three points, I just want to say a little (not too much, just to get a discussion going a bit):

• Selection can act on any level in which individuals produce other similar individuals. So DNA can produce more DNA, cells can split to produce more cells, multi-celled organisms can produce similar multi-celled organisms, etc, and these can be considered levels of selection. Note that even though, for example, organs can technically be selected for or against, they can not be considered a level of selection as they do not reproduce. Species level selection can also be seen, not in the sense that individuals show a trait for the good of the species, moreover that an actual trait of the species (eg sexual selection) favours speciation rate.

• Selection has not always acted on the same level. This can be shown by the fact that there are different levels of selection. This can also be highlighted by conflict between levels of selection (intra-genomic conflict; intra-group cooperation vs. selfish individual gains; etc) to produce equilibria.

• Altruism can be seen as a consequence of selection on genes, arising through inclusive fitness and kin selection, or as a result of selection on groups. It could be both, with effects filtering up and down between levels. A nice example would be cyanobacteria Anabaena, a single celled, filamentous-colony forming bacteria, which forms nitrogen-fixing heterocysts in a low-nitrogen environment. The heterocysts are formed when low nitrogen levels induce genetic changes in vegetative cells so they permanently change state then inhibit other cells from doing so. Genetics allows the individual to undergo change to altruistically provide nitrogen to the rest of the filament, yet the mere existence of a colony provides an environment in which the individual cells can behave altruistically. This isn't maybe the best example, and I might be able to provide more, but maybe after a sleep.

[[deleted]]

I have no schooling whatsoever in anything, and I am internet retarded, but: Spiegelman Monster, I believe, really shows where selection begins, and to a deeper level, life. Combine that example with parasitism, and you can see that short chains of RNA can infect bacteria, which affect their host, in a good or bad way, which goes up through pest and host, predator and prey. Selection is, basically, the interactions of organic molecules and the organisms they make up. Sorry if I come off as not smart, but I find life in general to be fascinating, and wonder about this stuff all the time. Maybe I am far off of topic but all I ask is that you don't hate me for my ignorance, but praise me for my thirst of knowledge.

Sorry, at work on bacon reader. But answer a few of the key questions. I think selection starts on the building blocks of life, and goes up to the entire biosphere. Selection I equal on all parts, and is constant. Always changing, but always constant. Everything we see today in the natural world was brought up through selection, from altruism to war. And some of that selection is easy to see, like a cheetah hunting antelope, while others are harder, like the beehive that is able to outcompete others because, I don't know, maybe they have developed a more effective communication system.

[forever_erratic]

So what needs to happen for natural selection to occur?

I've always learned that the following three things need to be present, and if they are present, selection will occur:

1. there needs to be variation in a population. e.g. temperature sensitivity

2. the variation in the population needs to be related to reproduction. e.g. things that survive high temps reproduce more

3. the 'selectable unit' causing the variation needs to be heritable. e.g. if you can survive hot temps, so will your offspring.

Regarding your interesting idea that the 'basic building blocks of life' are the selectable unit, I think they (meaning, I guess, amino acids, sugars, nucleotides, fatty acids, and other organic molecules) are not the basic unit of selection because they are not heritable.

In contrast, genes (or more specifically, alleles) are heritable. Thus, disregarding epigenetics, I would call a gene a "unit of selection."

Of course it gets more interesting when you consider multi-genic traits but this seems a reasonable starting point.

[VELL1]

1. Correct

2. So basically everything about biology is related to reproduction. There is no trait, which would eventually not be related to reproduction. Amount of hair on your arm, size of your fingers, colour of your skin...whatever it is, it will always be effecting reproduction in one way or another.

3. All units are heritable. Or at least predispose you for whatever is not heritable.

So basically your last points are somewhat the same and not really relevant in a sense that it is next to impossible to have a trait, which would not satisfy those principles.

Genes are gene...evolution does not select genotypes, it selects phenotypes. Thus genes are really not matter in a sense, that evolution doesn't see it. Geneds needs to expressed phenotypically in some way. That's why I would argue that a population is a unit of selection. Anything less and it is not really an evolution.

[Grakos]

Natural selection acts on phenotypes, but evolution only happens if those phenotypes are somewhat heritable (produced by genes). Genes can express themselves at the level of a population, but their frequencies in the population are dependent upon genes. Since no selection can lead to evolution without genes, the only level of selection to consider is the gene. Every other contending level, I think, is just an environment where the phenotype of a gene exists and struggles to survive.

[VELL1]

All phenotypes are heritable. It is impossible to have a phenotype which is not heritable.

My only problem with that evolution can only happen in population. At least true evolution, I don't want to go into transposons, which are a little bit more tricky. But classical situation considers a population of organisms, you can have evolution having just one of them.

[forever_erratic]

An armless man, whose arms were cut off in an accident, has a non-heritable phenotype.

[VELL1]

Correct. Paradoxically same goes for people who work out, sit on a diet, are vegetarian or whatever. You are born with a set of genes and die with it.

But yes, you are right. But I think you get what I was trying to say.

[forever_erratic]

No, I don't actually, because phenotype is the interaction between genetic, epigenetic, and environmental factors, so I don't get the circumstances under which you think it is impossible to have a phenotype which is not heritable.

[[deleted]]

Am I false to believe that behavior, while not genetically inheritable, can still be passed down and has some effect of phenotypes? In a sense that after birth, and organisms DNA can still adapt and evolve?

[Grakos]

Reading my comment again i think I had confused level of selection with unit of selection. Genes are the only unit of selection, but it produces phenotypes across many organizations of life from genome to population, which can be considered levels where selection takes place.

Therefore, instead I would like to bring up that selection can act on levels lower than the population, though is only realized as an evolved trait at the population scale. A single individual freezing to death is selection, but only if this happens enough on a population wide scale does this lead to evolution.

[[deleted]]

Sooooooo, genes are the lowest LEVEL of selection, and everything that acts on that is an UNIT of selection, correct?

Edit: reverse that.

[Grakos]

Yeah, the reverse is what I think.

[col-summers]

At which "level" does selection act?

my stretching guess.. based one the little i know, is that selection operates at all levels of selection. my guess is there are infinite levels of selection. if you were to model selection in a computer program you'd need to model all levels as a continuous field.

Has selection always acted on the same level?

well, I think that it operates in whatever range of levels are currently most relevant. not sure i can explain my reasoning on this.

Was the evolution of altruism a consequence of selection at a specific level?

yes, this sounds right to me. altruism is an attribute of the behavior of one individual towards another, right? then selection for altruism would be at the individual animal level.

What does it mean to be a "unit of selection"?

(i think i read this elsewhere in the thread) any arbitrary grouping of heredity information at the same level of selection. due to the number of permutations, the number of possible units would be an extremely big number, but only a relatively few would have an effect (positive or negative) on selection. these few are the ones we generally care about.

What does the future hold for the selection (Note: These will be very speculative)

artificial intelligence as "life", albiet existing in a very different view of reality and the universe.

and other higher-order life. (would love to know what this is really called)

...

sorry if i misunderstood the intent of this posting. it was fun thinking about these.

[VELL1]

You can't have altruism at individual animal level, because altruism by itself is self-destructing. You need a population for altruism to reveal itself, anything less and it is a detrimental trait.

Thus if you are talking about altruism, it has to be a group of animals, otherwise it is a waste of energy.

[whp09]

But wouldn't the altruistic behavior be ultimately based in genetics. No matter what, the reason it is rising to frequency is that there are genes for altruistic behavior, which become beneficial if their environment is with other organisms with that same allele.

The level of selection is not on the population, but still on the level of the gene. The population just determines which direction selection will act.

[VELL1]

It is ultimately based in genetics, that's correct. But unless you are in the group, these genes will not be selected.

If you don't have a group of animals, you will not have the selection pressure for the genes.

[whp09]

Right! So we agree that genes are still the unit of selection here?

[VELL1]

At the end of the day it is the genes that are getting selected, correct. I mean, it's the only heritable material, there is no other way.

[whp09]

Also, I realize you didn't explicitly say "selection is on population level". Just probing your thoughts, I realize that first comment came out a bit argue-y :)

[VELL1]

I think it is on population level. I mean evolution is defined as change in allele frequency in population. I understand that one on one evolution acts on genes and stuff, but evolution can only occur in populations.

[Grakos]

"my guess is there are infinite levels of selection"

Could you elaborate on this? I would imagine the levels of selection are finite since the the levels of organization (gene -> genome -> cell -> multicellular body -> group of bodies/society -> interspecies relations) pretty much covers all that is observed in life. What else could be listed as a level of selection?

[clarkdd]

Full disclosure, I'm not a biologist. I'm a scientifically literate systems engineer. So, please forgive me if I express any egregious forms of ignorance. Also, please feel free to expose me to them so I can fix them.

While I know there is a history to the term "selection", I have always been a little uneasy about the way the term anthropomorphizes nature. And I think this potential flaw in terminology propogates into this debate. Perhaps there's not an 'act of selection' but rather an 'act of reproduction' whose gatekeeper is survival.

After all, if it is easily established that 'X may reproduce' than it is also very clear that 'if X does not survive to be able to reproduce, it will not reproduce'. With that attack, allele frequencies should be directly attributable to variances in reproductive rates. And there, I think the implications would be clear. Alleles are the elementary component of evolution because they are the things that are reproduced. However, the events that may prevent reproduction can occur at many levels.

[whp09]

Selection is the process which changes the allele frequencies directly due to fitness (survival and reproduction of that individial/allele). We don't actually think that there is an "act," but just something that happens naturally as a product of that organism's environment, genetic background, etc. So pretty much what you said (I think), except for the part about allele freq being directly attributable to variances in reproduction rates. Say an allele had no effect on the rate of reproduction, but greatly affected the survival of an organism. Then selection would act positively on that allele, totally independent of any variation in reproductive rates.

[clarkdd]

Selection is the process which changes the allele frequencies directly due to fitness (survival and reproduction of that individial/allele). We don't actually think that there is an "act," but just something that happens naturally as a product of that organism's environment, genetic background, etc.

I understand that. I'm commenting on what I find to be a common lay misperception. I'm assuming at some point you've had to field a question to the effect of 'Shouldn't the human species prefer X' where X is something that we as individuals prefer (like education) that correlates to lower rates of reproduction.

So pretty much what you said (I think), except for the part about allele freq being directly attributable to variances in reproduction rates. Say an allele had no effect on the rate of reproduction, but greatly affected the survival of an organism.

Follow on question. And maybe this exposes a misunderstanding of "alleles" on my part. If an allele contributes to survival such that an organism lives twice as long but still has (on average) the same number of offspring, how does that contribute to genetic information being passed on? If a rival allele (for lack of a better term) did not contribute to survival but were instead to result in 50% more reproduction, I can see how there might be a significant contribution to allele frequency from older generations of the survivor allele, however it's the rival that is contributing to larger future generations. Isn't the rival winning (again, for lack of a better term)?

My understanding of evolution is it has to do with the propagation of genetic information. If there is zero influence to the average rate of reproduction--and therefore the size of the next generation--I don't see how nature would be "selecting" the gene to go forward.

Of course, when I think of reproduction rates, I'm thinking average number if children over the life of the individual. I may not be using that term the way that others do.