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Common ancestor question

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Common ancestor question

Postby Kenny » Tue May 25, 2010 5:05 pm

Is "Common ancestor" a species or an individual when talking about a common ancestor of two species?
In other words, does the separation from one to two species origin from a group of the earlier species or from a single individual from that species? For ex. common ancestor of humans and Chimpanzees.
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Postby plasmodesmata11 » Tue May 25, 2010 11:33 pm

I guess it would be a species. First, the definition of what constitutes a species is debatable. There would be an individual from which it would stem, but there would be a need for a group, because the mutations would need to accumulate over time in that population of species. It is improabable that one individual would harbor all those differences. If that were true, it could easily be reproductively isolated and its genes not passed on. A good mehcanism for evolution is allopatric speciation.
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Re: Common ancestor question

Postby Lenoxus » Wed May 26, 2010 12:45 am

Short answer: Yes, it would be a species — although that comes down to numerous individuals, each of whom can fully claim to be a common ancestor.

The tricky thing to remember about ancestry is that any given organism's line of descent has a good chance of spreading throughout an entire population, so that eventually, all members of the population are descendants of that particular organism (http://en.wikipedia.org/wiki/Most_recent_common_ancestor). We tend to describe kinship as though it were linear, and focus strictly on patrilineal or matrilineal lines (because we want to establish things like surname-taking, inheritance, and royal succession). But because every instance of reproduction involves the fusing of two lines of ancestry, which themselves are at some point fused in the past, the actual picture gets much messier than that.

If you wanted to go back in time to, say, the Bronze Age and visit each of your ancestors alive then, you wouldn't just visit a Grandma and Grandpa Kenny; you would have a very busy itinerary. (Consider that you have two parents, four grandparents, eight great-grandparents, and it kinda-sorta doubles every generation back. The reason it doesn't double indefinitely is that at some point, one of the ancestors of your mother is also an ancestor to your father, making for fewer total great-great-etcs. It's nonetheless a reasonably big number the further you go back, assuming that your ancestors didn't all come from some sort of bottleneck.)

Go back far enough in time, and every single living person who happened to have a line of descent that survived to your birth would be one of your ancestors. (That's not the same as every single person; for example, as far as we know, though William Shakespeare had six grandchildren, not one of them had children, and thus the Bard has no living descendants.)

This is why you only have to go back some eight thousand years or less to find a person (the "MRCA") who is a common ancestor to all currently living humans. This person lived with thousands of other humans, most of whom also have numerous living descendants, and many of whom have nearly every living human as a descendant; the only special claim of the MRCA is that s/he just happens to have all of us as descendants. The MRCA need not have had an enormous number of children, just a reliably growing line of descent, each child having, on average, at least two children. And in no sense was the MRCA the "first human". In another thousand years from now, the privilege of being MRCA will go to someone else. In fact, if your own line of descent never dies out, you yourself are guaranteed to one day be the MRCA of all the humans at some time in the distant future! (The chances of a line not dying out start out low and increase with every generation, until there are pockets of descendants all over the place.)

This also works backward in time. The MRCA-to-current-humans was part of a worldwide human population which in turn had an MRCA living thousands of years before that. This "proto"-MRCA would also have lived with a large population of people who would look just as human as you and I. (The scale of divergence from our common ancestor with chimps doesn't become noticeable until over a million years have gone by.)

So to make a looooong story short, the common ancestor of humans and chimps (to take just one example, of course), is best understood as a species, although thousands upon thousands of members of this species could, if I'm not mistaken, rightly claim to be common ancestors. Not all, though; some lines die out, which is part of how natural selection works.
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Re: Common ancestor question

Postby Kenny » Wed May 26, 2010 7:20 am

Thanks for this detailed explanation.

If there is a mutation in a particular organism's genome. Couldn't that in the long run lead to a new species and therefore make that single organism the common ancester for at least two species in the future? (the species that started from the mutation and the species without that mutation)?

In case of disruptive selection or as a result of a change in the ecological system, then a specific group would have the advantage to survive and reproduce and maybe develop to a whole new species. In that case the common ancestor for future generations would be that group (or members of that group) and not (necessarly) a single organism.

Tell me if I'm correct.
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Re: Common ancestor question

Postby SelfishGene » Mon May 31, 2010 6:26 pm

Kenny wrote: If there is a mutation in a particular organism's genome. Couldn't that in the long run lead to a new species and therefore make that single organism the common ancester for at least two species in the future? (the species that started from the mutation and the species without that mutation)?



In actuality the most recent common ancestor(s) (MRCA) of any two organisms (whether of different species or not) is either going to be a a specific SINGLE organism (if that ancestor is a part of a asexually reproducing species) or a SINGLE PAIR (if that ancestor is a part of a sexually reproducing species).

However, when describing long-term evolution, there is often no way (and no reason) to identify the specific organism(s) whose descendants diverged into their respective evolutionary pathways (ie species). So when we say "species X is the MRCA to species Y and Z", what we are really saying is, "an organism or mating pair a part of species X is the MRCA to all the organisms of species Y and Z". We know statistically that it is very rare to find a fossil of an ACTUAL MRCA between 2 modern species. The fossil labelled as "MRCA to Y and Z" are usually of an organism who belonged to the SAME SPECIES as the ACTUAL MRCA but is not the ACTUAL MRCA itself.

Lenoxus I believe has misunderstood the wikipedia article. Think of a family tree, however big, (asexual or sexual - it makes no difference), select any two individuals in that family tree, trace them back to there MRCA. it will always be only 1 organism (for asexual) or 1 mating pair (for asexual). Of course all the parents of the most recent common ancestor will also be ancestors but not the MOST RECENT ancestor(s).

So, the answer to your question I quoted above is a very confident YES. However, you didn't include "mating pair" as an alternative to "single organism" as the potential MRCA, so your statement would technically be confined to asexual species.

Also, speciation isn't caused by a single "mutation" it is caused by a long series of mutations. There is no definite generation when speciation occurs, it is a gradual process, just like the maturity of infant humans to adulthood.

Kenny wrote:In case of disruptive selection or as a result of a change in the ecological system, then a specific group would have the advantage to survive and reproduce and maybe develop to a whole new species. In that case the common ancestor for future generations would be that group (or members of that group) and not (necessarly) a single organism.


No, The actual common ancestor will always be either a single organism or a mating pair. Whole species don't all speciate together to the same species. They branch off and the unsuccessful branches are cut off by natural selection.

It's also worth noting the unit of selection in natural selection. Natural Selection selects individuals when evolution is perceived from a generation-to-generation perspective, but when taking a long-term-evolution perspective it selects genes. Group selection (used to explain altruism) has lost most of its support. Clade selection deals with the selection of classes of organism (species, genus, family) and is sometimes interpreted as group selection but has nothing to do with altruism. It has to do with the "evolution of evolvability".

Side note:
The only rebuttal I can think of to "any two organisms having only one or a mating pair of ancestors" would be to consider retroviruses as ancestors, but that's completely different topic and I don't want to extend this post any further.

I also wonder if it is possible for daughter bacterium to receive it's genome from more than 2 (sexual) parents.
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Postby JackBean » Mon Jun 07, 2010 7:48 pm

SelfishGene: didn't you read Dawkins? :lol:
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.
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