How much variation in a population


#1

There was a thread that was closed down because people were talking at cross purposes, but there was an interesting scientific issue at stake, so I will ask the question here.

I understand that evolution in sexually reproducing species takes place with the variation already present within the population, but for how long? That is, if there were no supply of new mutations also contributing to the variation, how far could evolution go?

For instance, could you get from the common ancestor of chimps and humans to humans and chimps without mutation? If so, how about from the common ancestor of lemurs and monkeys and great apes and humans to all the different species of primates that exist today?

My guess is that for the really dramatic changes of that sort mutation was necessary. At any given moment natural selection works with the variation present, but there has to be new variants supplied or variation for a given trait gets used up. I refer to the bottom of page 202 in Sean Rice’s book Evolutionary Theory Mathematical and Conceptual Foundations where he says " without mutation, selection continues to reduce heritable variation until fitness itself is not heritable."

I also have a copy of Smith’s Evolutionary Genetics, 2nd edition, where he says “phenotypic variation in morphological traits is generated by mutation at a rate of about 0.1 percent of the variation typical in natural populations.” To this layperson that sounds like selection operates on the variation currently present, but over the long run mutations adds to it.

My point in posting–I am not a biologist and would like to know what the current thinking is on the questions in my opening paragraphs. But I also want to point out that the previous discussion in the earlier thread raised an interesting question, but it got lost in the name calling.


(Stephen Matheson) #2

You are right. In fact, strong selection depletes variation, at least in the genomic region surrounding the gene(s) selected for. But variation is constantly being generated, so this is not typically a long-term hindrance to evolution.


#3

Wow, that was quick. But anyway, thanks. That was the impression I had picked up. Do you know if there is a quantitative understanding of how far you could evolve a population with a typical amount of variation? For instance, if you started with a collection of mongrels, how many might you need to have enough variation to produce Great Danes and chihuahuas? I don’t expect a serious answer to that particular question, but wondered if quantitative geneticists or animal or plant breeders could estimate how far they could push a given set of traits with a given sized breeding population with some specified amount of variation? Or is that the sort of question only a naive outsider would ask?


#4

For all I know, Sean Rice might have an answer to my question, but I have only looked at small parts of his book. I might get through it someday.


(Stephen Matheson) #5

I think the question is empirical, in the sense that it can be answered in principle but probably only in model organisms and simpler traits or characters. There are lots of papers about the consequences of selection (which includes breeding) on genetic variation, both at individual genes and in larger genomic regions. Whole review articles are written about these topics; here is a recent one that I have been reading:

Determinants of genetic diversity


(George Brooks) #6

@Donald

Something I mentioned in another thread is that the canine gene pool may well be an ‘outlier’, compared to un-domesticated species/breeds. In a population intensely managed by humans, the end result of every breeding project is an animal that presents some dramatic new phenotype . . . while retaining a high level reproductive compatibility with other breeds.

This is an inevitable “artifact” of the canine gene pool, because as soon as an individual is produced that shows resistance to producing fertile offspring with other mates, the individual is removed from the process.

In the natural world, where humans are not monitoring every new generation, reproductive compatibility is not artificially sustained or extended.


(Steve Schaffner) #7

I doubt anyone knows, but I’m skeptical about the possibility. The numbers are a little fuzzy, but probably more than have of all genetic differences between humans and chimpanzees are the result of mutations that have occurred since their common ancestor. The fraction is probably higher for functional variation, since diversity would start out lower in functional elements and would be depleted faster by selection. Where we have information about selection for new traits in very recent human history, we find a mix of selection on standing variation and selection on what seem to be new variants.


(Lynn Munter) #8

This raised my eyebrows in the other thread and they are still raised now. The defining characteristic of the canine gene pool is not that individuals are promptly removed for failure to reproduce. It is that individuals whose ability to survive is comparatively weak are given more opportunity to reproduce than they would have in the wild.

A pug is a highly compromised version of a wolf. Dramatic new traits are valued by humans even if they make the animal less physically rugged.

I assure you, chihuahua breeders do not care if their animals maintain reproductive compatibility with Great Danes or any other breed but chihuahuas.

Now, it is true that dogs breed more opportunistically and readily than wolves. But I think it is a mistake to think of the evolution of dogs here as guided mostly by breeders. I think it is more due to the fact that dogs’ survival is not particularly seasonal, unlike wolves who must time their litters carefully if they want them to survive, and although wolves tend to mate for life, that is not a driver of dogs’ survival or reproductive fitness either.


(Stephen Matheson) #9

Well said. I thought it was funny that someone would claim that selection against “resistance to producing fertile offspring with other mates” was associated with some kind of unique aspect of the “canine gene pool.” D’oh!


(George Brooks) #10

@Lynn_Munter

I am hoping we will avoid a semantic wrestling match. There are lots of factors that go into deciding who shall be mated and who will not.

I would certainly agree with you that with domestic breeds, individuals that wouldn’t ordinarily be very successful in the wild are given a chance to be successful (or more successful).

But if you have a weak animal who creates many offspring of the desirable phenotype … vs. a strong animal who creates 10% of the offspring that other mating individuals do, wouldn’t you agree that the offspring would have to be unusually special to justify keeping that adult individual in the mating program if other individuals produce more offspring?

And then if we discover that the 90% fewer offspring also do not produce many offspring themselves (as a further indication of poor reproductive compatibility) - - what would it take for you to really want to retain that difficult genetic strand in your breeding project?


(Lynn Munter) #11

@gbrooks9, I think you overestimate the importance of deliberate human choice in the mating of dogs. What mating program are you talking about? Most dogs in the history of our canine companions have not come from dog breeders as we are accustomed to thinking of today. They are village strays breeding without human supervision (but in a human-dominated environment) or pets breeding with the neighbor’s pet. Working animals being deliberately bred for attributes by humans is a distant third, and the modern obsessive preservation of ‘breeds’ is unique to the last century or two.

As @sfmatheson points out, what you have failed to take into account is the extent to which animals which are not good at reproducing generally reproduce less successfully, regardless of whether or not they are in a human-run “mating program.” Indeed, I think it would be just as easy to argue that an animal such as you describe, strong but producing only 10% of the number of offspring, may well experience more success in a breeding program than it would under natural selection.

You also assume that the goal of any human running a dog-breeding program would naturally be to produce as many dogs as possible, which again, I would suggest, does not necessarily track with the real world.


(George Brooks) #12

@Lynn_Munter

You and @sfmatheson apparently think I’m comparing the success of the “low producer” to the success of the average animal in the wild. And I can understand why you came to that conclusion.

No. I am not comparing the “low producer” to wild animals. I’m comparing the “low producer” to all the other dogs available to the breeder.

And I also encourage you to imagine the scenario I am describing as more likely at the very beginning of the human process to develop breeds. After centuries of this process, I would think the lion’s share of “homogenizing” the dog genome would be fairly well accomplished.

What I’m describing is the “opposite” of the Ring Species phenomenon. In the Ring Species scenario, different “breeds” emerge based on geography… and the ongoing high likelihood of one valley’s individuals mating (and exchanging genetic elements) with the individuals of a neighboring valley … and with a correspondingly low probability of exchanging genetic information with individuals from a very distant valley.

In this process, over time, “pockets” of compatible genetic interchangeability develop. And thus, as an inevitable outcome, the pocket of genetic exchange at one end of the species range is quite different from the pocket of genetic exchange at the other end of the species range.

You read the postings about the Alaska Rabbit vs. the Florida Rabbit, yes? Both ends of the rabbit’s North American territorial range are able to mate successfully with the Minnesota Rabbit “pocket”.

But! . . . when brought together by fate (or human ingenuity) the Alaska Rabbit and the Florida Rabbit are not reproductively compatible. They produce no offspring. What is happening at the genetic level for this to be possible? Somehow, somewhere, the genes most crucial to producing viable children have somehow drifted (or have been oppositely selected for) enough between these two geographic ends of the population that they no longer function with each other.

And yet, there is just enough similarity/compatibility with the Minnesota rabbits that reproduction is possible.

Now let’s turn to the Domesticated Dog… the Poster Child of “Phenotypes Galore” within a single species. We have giant dogs; we have tiny dogs. We have dogs that love the cold and those that do better in heat. And all of them, barring such physical limitations as a dachshund being unable to mount a Great Dane, seem quite able to produce gobs of fertile offspring. Alleles of great abundance course through the blood of any single mutt of the Canine species.

In fact, Dogs have become the Anti-Ring Species! By a constant mixing of dogs from all over the world, with myriad traits, the one thing we seem to be able to say with a modicum of confidence is that there doesn’t appear to be a dog that cannot breed with any other dog (at the embryo level anyway).

So if we can accept this semantic distinction . . . that dogs appear to be the very opposite kind of species compared to the Alaska/Florida rabbit species spectrum. . . what exactly is different? I don’t think it is the notorious diversity of the phenotypical alleles - - responsible for fur type, or for body size, or for hunting instincts or companionship - - which makes this possible. I would propose that it is the centuries of combining and recombining a specific constellation of genetic factors (the ones most responsible for reproductive compatibility) which has essentially turned the whole world into a Unified Valley of Dogs.

Nobody has ever proposed a Ring Species scenario for sub-groups living in close and easy proximity with each other. [I would think a Ring Species could still theoretically emerge if there are enough other factors at work. But for the moment, let’s not digress.]

In the dog world, physical proximity has been replaced by the eager zeal of human intervention.

Ladies, and Gentlemen, I present to you The Mutt: the average canine which has such robust combinations of alleles that facilitate biological compatibility that there is (virtually?) no corner of the world that is outside of its “Valley” of reproduction.

Lynn, doesn’t this seem like a perfectly reasonable way of explaining the existence of a Ring Species in the Rabbit world, and the appearance of the exact opposite in the Doggy world?

:smiley: :smiley:


(Lynn Munter) #13

Not quite. We think you are not comparing them, but you should be.

This does not make sense to me. Can you elaborate on what you picture as “the very beginning of the human process to develop breeds”? I will tell you what I picture, for the sake of contrast.

I think most ‘breeds’ started out as geographical distinctions, as humans and dogs spread across the world. Dogs have become less homogenized over time, not more, with the possibility of that homogenization increasing some over the past few centuries as people moved more between continents.


You say, “alleles of great abundance course through the blood of any single mutt” but I question your basis for this claim. For all of the phenotypic diversity of dogs, the genotypic diversity is quite low.

Yes, I read about (and watched the video on) rabbits as a ring species. I liked it, however, I think there is a much simpler explanation at hand for the difference between them and dogs.

Dogs split from wolves only 10-20 thousand years ago. The rabbits in Alaska and Florida (I just did a quick review here to try to figure out which species we were talking about, and it seems that they would be not only different species but different genuses as well—perhaps you could clarify?) At any rate, I think it probable that rabbits have spent more generations diversifying into different habitats than dogs have, so it makes sense that they would be more genetically and reproductively distinct than dogs, if not phenotypically.

I’d also note here that coyotes, which have spent far longer diverging from wolves than dogs, still interbreed with both wolves and dogs, despite being even more genetically different! So I think the idea you are proposing is a solution in search of a problem which doesn’t exist.


(George Brooks) #14

@Lynn_Munter

I raised the point about the early phase of the breeding of dogs because prior to that point, dogs would just be minimally different from wolves… with no particular “homogenization” factor uniquely applicable.

Under the right conditions, Wolves and early dogs would have been just as likely to contribute to a Ring Species episode as the North American rabbits - - at least until such time as we are able to mathematically determine what genetic events specifically led to the Rabbit Ring Species event of North America. Then maybe we could do comparative analysis of various populations… to determine proclivities to form Ring Species (under ideal conditions) vs. resistance to forming Ring Species.

The likelihood of finding a canine sub-breed, or perhaps just the occasional individual dog, which was reproductively not uniformly compatible with the average canine mate would in my view be higher in the beginning of the long domestication process than what we might find today in the total modern canine gene pool.

I thought it would be helpful if I brought in the Ring Species discussion. The whole thrust of my point is that dogs, as a breed, represents the very opposite of Ring Speciation.

Are you familiar with Ring Species? If Ring Species are logical to you … then anti-ring species scenarios should be logical to you as well…


(Stephen Matheson) #15

But that’s just not a useful comparison. There is no such thing as “ring speciation” and so there is nothing that could be its “opposite.” Dogs have been driven by selection to vary dramatically in morphology and other traits, but the forms (breeds) are reproductively compatible. Okay. And then there are examples of strong selection driving actual speciation. And then there are examples like ring species, in which the speciation has occurred, but stepwise, with the steps still visible.

Domestication is interesting because it is evolution at light speed, involving selection so strong and so potentially wacky that it would seem irrelevant to real evolution if it weren’t for the fact that it happens outside of human influence too. (Leaf-cutter ants are my personal favorite.) But there’s no point trying to create some new insight by speculating about dogs. @gbrooks9, I think your time would be better spent taking that Coursera course that @beaglelady has recommended.


(Phil) #16

Domestication of plants is also interesting, in that many plants we have selected and grow would never survive outside of a garden. Corn I think is a prime example, as we plant thousands of acres ( even excluding hybrids) but if we did not, it would be extinct in a season or two.


(George Brooks) #17

@sfmatheson,

You write that there is no such thing as “ring speciation” ?

That’s odd… there seems to be a lot of literature on the topic. I know you have a burning need to imply I’m making things up, but if you looked at my post to Linda, you would have seen the Wiki article on it (provided again below), as well as a few more links all on “Ring Species” … with footnotes showing some of the scholarship behind the idea.


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http://evolution.berkeley.edu/evolibrary/article/devitt_01
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http://www.zoology.ubc.ca/~irwin/GreenishWarblers.html
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https://www.jstor.org/stable/3893431?seq=1#page_scan_tab_contents
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Frankly, I think encouraging more discussions on Ring Species would help YEC’s understand how “real” Speciation really is.

For example, if some cataclysm suddenly wiped out the Minnesota Rabbits, what was once a “unifired” range of North American rabbits - - we would suddenly have 2 species, in two different geographies, that could not interbreed. I’m sure the YEC’s would object, but it would be an excellent teaching moment on two grounds:

  1. Speciation is not an impossibly difficult scenario; and
  2. Some populations are for practical purposes already at the verge of speciation and just require a loss of the “bridging” population to be officially so honored!

(Lynn Munter) #18

If anything was going to form a ring species, it would not be domestic dogs but a much larger group of canids. There just has not been nearly enough time for dogs to diverge that much.

If you read this, you will find many species which are capable of interbreeding. If there were one instance of, say, jackals and coyotes not being able to breed with each other while both could still breed with wolves, then we would have a canine ring species. Does that make sense to you?

Note: I think that jackals and coyotes have so far failed to interbreed due to lack of opportunity (living on different continents) but I could be proven wrong!

What? Why would you think this? In the beginning of the domestication process, you would be dealing with a small and fairly homogeneous population which had not differentiated much from wolves or from each other.


(George Brooks) #19

@sfmatheson

Oh, Stephen, seeing as how you didn’t even know Ring Species was “a thing”… maybe you and I could take the Coursera Course together, Yes?

And we could skype daily and share notes… and maybe have a few meals together via Skype.

I know you eagerly follow Ben’s lead on the theme that I’m some sort of dummy … but I really think you should give a good detailed look at what it was that Ben was blowing his stack about - - before you go down that route as well. Nothing good has come from it.

I learned my Allele lessons from Ben a long time ago. And I do everything in my power not to cross swords with Ben on Alleles. It must have bugged him terribly … otherwise why would he jump me in a discussion that was purely limited to some obscure aspects of mutations?

George


(Lynn Munter) #20

Before you get too much more wound up on this, I thought it was clear he knows ring species are a thing, he just doesn’t think ‘ring speciation’ is a verb. Since he has more biology expertise than either of us, don’t play the role you were just criticizing when it was nonlin and swamidass in the other thread.