"Ancestor's Tale" - Question(s)

[Donn]

I'm reading "The Ancestor's tale" by Richard Dawkins and (at the moment) I am trying to picture what a concestor is.
I have no education in these matters, simply curiosity, so I hope I don't ask a dumb question (in the wrong forum too!)

I gather a concestor is a creature from whom's immediate children came branches of the tree, one heading for humans, another for some other beasty.

Is it a case that a single creature (say concestor 1, the chimp-like one) gives birth to two offspring - one with the genes that set it onto the path towards humans, the other with genes to go towards modern chimps?

I mean, does it literally come down to a couple of siblings; two actual creatures? Or is it more complex than that?

I wonder about the other members of that species (con1) at that time. Where they not also breeding, what role did the many children of the many adults have in the split between the two branches?

I guess I am picturing some kind of Richard Attenborough show on TV where he kneels before a CGI image of con1 and says something like; "And here we are before the actual female who gave birth to the two creatures, one of whom became us! Quiet now, let's not disturb them, they have great things to become!"
(Please excuse the drama!)

I may have other questions too, hence the title!

 

[Dr Adequate]

Yes and no.

As far as I can figure it, there must have been at least one creature the offspring of which fall into two non-empty sets: those that only living humans descendants, and those that only have living chimpanzee descendants, plus one possibly empy set --- those that have no living descendants. Moreover, the "human-ancestral" set will be ancestral to all humans, and the "chimp-ancestral" set will be ancestral to all chimps.

(My reasoning goes like this, if anyone would care to pick holes in it. Let's identify each creature throughout time with the list of its surviving chimp and human descendants. As we move back through time, we are combining the lists: my mother's list is the combination of my list and my siblings' lists, etc. As chimps and humans have common ancestry, at some point we have some creature such that its list contains both chimps and humans without this being true of any of its children. This is a concestor.

Why do I say that the "chimp-ancestral" set of the concestor's offspring must be ancestral to all chimps? Suppose otherwise. Then we can divide chimps into at least two lineages, those which are descended from the concestor and those which aren't, which split before the time of the human-chimp concestor, and yet which both evolved into chimps. Which is ridiculous. The same applies to the "human-ancestral" set of the concestor's offspring.

Does that make sense?)

However, this does not mean that all our genes come from the "human-ancestral" set of the concestor's offspring --- presumably they did not breed with each other, but had partners. There could also have been more than one concestor.

 

[Dr Adequate]

Just to add: the reasoning that shows that the concestor is ancestral to all humans and all chimps applies equally well to show that each of the "human-ancestral" offspring must be the ancestor of all humans. So, yes, it would actually be possible to point to two siblings, and say, "One is the ancestor of all humanity and no chimps and the other is the ancestor of all chimps and no humans."

They might even have been identical twins.

 

[Donn]

Wow, Dr A, I admire your posts and your succint style. Thanks for the reply.

Even so, I have had to draw little doodles and read every line very slowly to understand your answer in this case, and I'm not sure if I nailed it.

It seems to me to say that one creature (con1 - a female) had at least 2 children A and B. A formed the Chimp-Set (no dispersion upon the Dr.) and B formed the Human-set.
All the "right" genes and whatnot where present in A and B to allow this to happen. Close?

I still wonder at this idea. What if a tree had fallen on her head (Monty python style) before she could birth A and B? It seems so fragile.

Also, what about her species - the other apes in her troop? I assume all their various descendants (going up the tree through time) all came to nought.

 

[Dr Adequate]

Wow, Dr A, I admire your posts and your succint style. Thanks for the reply.

Even so, I have had to draw little doodles and read every line very slowly to understand your answer in this case, and I'm not sure if I nailed it.

What can I say but "me too"?

It seems to me to say that one creature (con1 - a female) had at least 2 children A and B. A formed the Chimp-Set (no dispersion upon the Dr.) and B formed the Human-set.
All the "right" genes and whatnot where present in A and B to allow this to happen. Close?

No --- perhaps you missed the edit to my last post. They could have been identical twins. The "genes and whatnot" are the product of lots of mutation and natural selection subsequently. Imagine that A wanders west (apes marry out to reduce genetic load) to join one troop (troop A) and B wanders east and joins another troop (troop B). Each becomes the dominant male of the troop. The troops diverge in space over time until they're on opposite sides of the Rift Valley, their descendants find themselves subjected to different selective pressures... well, you get the picture. This is just one possible scenario, of course.

I still wonder at this idea. What if a tree had fallen on her head (Monty python style) before she could birth A and B? It seems so fragile.

No, because the genes for humanity and chimpdom did not have to be implicit in the genes of A and B. Without A and B, the selective pressures that produced humans and chimps would (highly probably) have operated on someone else's offspring. Someone else (to reuse the example above) would have been the dominant male of troop A, someone else would have been the dominant male of troop B --- and if you searched back far enough, the two of them would have had a common ancestor.

Also, what about her species - the other apes in her troop? I assume all their various descendants (going up the tree through time) all came to nought.

Not necessarily: others of them might also have been concestors (e.g. our concestor's mate); some might be ancestral to all chimps; some to all humans.

 

[Donn]

Okay, I'm gradually getting the hang of this.
I was fixated on this "genes for X" thing and I missed the very gradual nature of evolutionary change.

So, chimp B could be identical to A but it is the natural pressures on him that cause him to breed in the ways he does. His progeny might be some minute shade of an iota different from him, genetically, and so it goes on, quite haphazardly (via geography and selection etc) until there is a descendant (C) that is totally different to B (but not so different to C's own siblings).

So B heads for C heads for ... heads for Humans.

To re-query then:
The "concestor" is not neccesarily an individual that gave rise to the split but rather some member of a given species that did. It could be one animal that had both A and B, but it could also be that A and B were from the same troop but not the same parent.

Better?

 

[Dr Adequate]

The "concestor" is not neccesarily an individual that gave rise to the split but rather some member of a given species that did. It could be one animal that had both A and B, but it could also be that A and B were from the same troop but not the same parent.

But remember that A and B have common ancestry, so somewhere (somewhen?) in their ancestry, there is one individual whose direct offspring (call them A2 and B2) are split (see my first post). I wasn't trying to confuse you on this issue, but just to point out that evolution is not entirely contingent on the genes you start with. Of course, the same environmental pressures would work differently on, say, a squirrel and a proto-ape, because those are very different starting points --- but on two different proto-apes?

You seem to have got the rest of it --- the two branches can diverge as lineages long before they diverge as species. To take another example, the first tortoises to wash up on the Galapagos islands had diverged forever from the lineage of mainland tortoises: but they was nothing special about their genes --- they were the same species, at that point, as the mainland tortoises.

Moreover, there was nothing in particular in their genes that destined their descendants to become giant while mainland tortoises stayed small. If a completely different bunch of tortoises had been washed up, the environment would have had exactly the same selective effect on their descendants, and we'd still have ended up with giant tortoises on the Galapagos islands.

 

[Donn]

But remember that A and B have common ancestry, so somewhere (somewhen?) in their ancestry, there is one individual whose direct offspring (call them A2 and B2) are split (see my first post).

Ah. So if A and B were NOT siblings, then the concestor would have to shift backwards in time until there is a single creature who's progeny gave rise over time to A and B. So, the concestor is a single creature.

I read page 12 (in the Tale) again and this is a snippet:
"In a backwards chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, what I shall call their 'Concestor': the focal rodent or the focal mammal or the focal vertebrate, say."

Thanks, that's mostly cleared that up for me now.

 

[dann]

To take another example, the first tortoises to wash up on the Galapagos islands had diverged forever from the lineage of mainland tortoises: but they was nothing special about their genes --- they were the same species, at that point, as the mainland tortoises.

Moreover, there was nothing in particular in their genes that destined their descendants to become giant while mainland tortoises stayed small.

So far I think you're right! The population of Galapagos tortoises were now in an environment that favoured the reproduction of certain traits. But of course these traits had to either be there in the first place, as 'slumbering genes', whatever, or be caused by mutations. You should not forget this part of the 'evolutionary machine': The environment favours the reproduction of certain traits, but only if they somehow (= mutations) arise on their own. The environment cannot 'choose' between traits that aren't there in the first place!

If a completely different bunch of tortoises had been washed up, the environment would have had exactly the same selective effect on their descendants, and we'd still have ended up with giant tortoises on the Galapagos islands.

Based on what I just wrote, I would say no. You might have ended up with tortoises that did not produce the right kind of mutations. They might have become extinct or they might even have developed other traits that made them even more successful, reproductively speaking: Flying, nest-building tortoises? (That would probably take some time, though!)
The machine of evolution: 1) mutations, 2) natural selection. When you (nature) select, you can only choose between the qualities that you are offered. You cannot directly influence the development of certain traits, only select from the samples you are offered.

 

[Gwyn ap Nudd]

I have not read Dawkins' book and this is the first time I've come across the word "concestor," but I think I see where you are having trouble imagining the concept.

Just because the concestor is the last common ancestor does not mean that the two species divurged in the following generation. It just means that siblings, cousins etc. in the two lines did not happen to inter-mate before the groups separated and evolutionary forces ensured that they would not. *

But Sibling A and Sibling B did mate with other members of the "parent" species, as did their children, etc during this period. Any given pair (one in each line) of those mates can also be traced to a common ancestor, a "concestor." This means that while there is a very latest concestor, which might be proved, as per Dr A to be common to all chimps and all humans, she is not unique, she is the last only because she was the youngest and/or longest-lived of a larger group for which the definition of concestor holds.

An artificial example would be Thoroughbred race horses, which split off from Arabian horses. They are all descended from three individual Arabians, whose parents (or possibly grandparents) are "concestors" of both Arabians** and Thoroughbreds. Because the pool of concestors is so small, and there have been so many generations of thoroughbreds, all three lines are in the pedigree of everyone. So, any of these common ancestors could be considered the concestor.

* In fact it takes a number of generations as separate populations before the two gene pools divurge so much as to hinder interbreeding.

**The analogy is imperfect because Arabians and Thoroughbreds are not separate species, and it is not certain that all lines of Arabians that did not pass through these "concestors" died off.(Indeed, it is quite likely they did not.)

 

[dann]

I read page 12 (in the Tale) again and this is a snippet:
"In a backwards chronology, the ancestors of any set of species must eventually meet at a particular geological moment. Their point of rendezvous is the last common ancestor that they all share, what I shall call their 'Concestor': the focal rodent or the focal mammal or the focal vertebrate, say."

I don't see why the concestor has to be just one individual - or even a sibling! Mutations may occur at any time. Let's say that a couple of rodents has only one offspring and that is when the essential change takes place!
I don't get the idea of siblings at all!
I also don't see why the concestor has to be just one individual! But that mutations always take place at the individual level is clear. Two individuals developing the exact same mutation sounds so unlikely that I'd consider it impossible.

 

[dann]

I tend to agree with your line of reasoning, Gwyn.

 

[Dr Adequate]

Based on what I just wrote, I would say no. You might have ended up with tortoises that did not produce the right kind of mutations.

As I pointed out, we would expect selection to have a different effect on different species (so the original gene pool does of course matter in that sense) but why should it have a different effect on two tortoises of the same species in the same environment? The mutations will depend on chance, but not particularly on your choice of tortoise. And all tortoises have offspring which vary in size, so there's always going to be material there for natural selection to work on.

 

[Dr Adequate]

I don't see why the concestor has to be just one individual - or even a sibling! Mutations may occur at any time. Let's say that a couple of rodents has only one offspring and that is when the essential change takes place!

Again, you're confusing the point (actually, it's a slow process, but let it go) at which two lineages become different species, and the point at which the two lineages separate.

I also don't see why the concestor has to be just one individual!

There can be more than one. There was at least one.

 

[dann]

Again, you're confusing the point (actually, it's a slow process, but let it go) at which two lineages become different species, and the point at which the two lineages separate.

I don't see how I am.

There can be more than one. There was at least one.

Yes, and they don't have to be siblings!

 

[Donn]

There can be more than one. There was at least one.

Ok, my head is hurting a little. It doesn't hold trees very well!
Can you clarify that "more than one" bit?

 

[dann]

As I pointed out, we would expect selection to have a different effect on different species (so the original gene pool does of course matter in that sense) but why should it have a different effect on two tortoises of the same species in the same environment? The mutations will depend on chance, but not particularly on your choice of tortoise. And all tortoises have offspring which vary in size, so there's always going to be material there for natural selection to work on.

Well, sometimes a species or race simply becomes extinct. It isn't even true that the original gene pool matters much "in that sense", because very often the natural selection imposed upon species by the environment has surprisingly similar effects on very different species. Ever heard of convergence?
"Biology. The adaptive evolution of superficially similar structures, such as the wings of birds and insects, in unrelated species subjected to similar environments. Also called convergent evolution."
Still, marsupials don't become ordinary mammals, just because they are isolated and begin to fill out the different niches of nature.
You are right that very often size is what varies most when a population is isolated (on islands or continents). See the on-going debate about the homo-sap relative the "hobbit".

 

[Dr Adequate]

Ok, my head is hurting a little. It doesn't hold trees very well!
Can you clarify that "more than one" bit?

Suppose "the" concestor is monogamous. Then the concestor's mate is also a concestor. Suppose a proto-ape X has two offspring, C and D. C forms a breeding pair with A (from our original example) and D forms a breeding pair with B. Behold, X is a concestor.

 

[Donn]

Suppose "the" concestor is monogamous. Then the concestor's mate is also a concestor. Suppose a proto-ape X has two offspring, C and D. C forms a breeding pair with A (from our original example) and D forms a breeding pair with B. Behold, X is a concestor.

Okay. This has to do with the sexes I suppose. Dawkins' concept of a concestor doesn't speak of the sex. So there would be the two concestors in the male and the female right off. Then their would be (X) the (at least 2 again) concestors of C and D. C and D having opposite sexes to A and B.
But could there be more than the 4 concestors we have now identified?

 

[Dr Adequate]

Well, sometimes a species or race simply becomes extinct. It isn't even true that the original gene pool matters much "in that sense", because very often the natural selection imposed upon species by the environment has surprisingly similar effects on very different species. Ever heard of convergence?
"Biology. The adaptive evolution of superficially similar structures, such as the wings of birds and insects, in unrelated species subjected to similar environments. Also called convergent evolution."

Astonishingly, I have heard of convergence. I think this rather makes my point, though. If similar environmental challenges bring about similar adaptations even in different genera, then what effect should we expect exactly the same environment to have on members of exactly the same species?

Still, marsupials don't become ordinary mammals, just because they are isolated and begin to fill out the different niches of nature.

The question is whether we should attribute that to:

(1) The environment of Australia.
(2) The particular species that were stranded on Australia.
(3) The chance that certain mutations took place elsewhere, but not in Australia.
(4) The particular choice of individuals within species that were stranded on Australia.

How much does (4) really matter? Would things have been terribly different if one early marsupial had found itself on one side of the divide, and its litter-mate on the other, rather than vice versa? I'm guessing that this would have a negligable effect compared to the other factors.