Question on Speciation

[Benguin]

I was reading the article on Darwin in National Geographic and stumbled on a bit that confused me ...

There was a description of how, when a group is isolated by time or space from its main population, it will vary from that population over time. Generally become more adapted to the particular environment.

The article then says this may eventually reach a point where they are so diversified they can no longer interbreed and, voila, a new species is born.

I couldn't quite see how the species variation and ability to interbreed were necessarily related? Is there a point at which morphological and other characteristics have so significantly altered that the genetic makeup changes too?

... and my apologies if I misrepresented the article in paraphrasing it, I left the mag at work.

 

[wollery]

All characteristics are related to genetics, not the other way round.

Genes mutate over time, altering the physical characteristics of a species. Advantageous or neutral alterations survive in the population, disadvantageous characteristics tend to get bred out.

Keep two different groups of the same species separated and there will be different mutations in the groups. If they are in different environments then different mutations will be advantageous to each group. Give it some time and the number of mutations will be enough to prevent breeding between the two groups. That's speciation.

 

[Benguin]

All characteristics are related to genetics, not the other way round.

Genes mutate over time, altering the physical characteristics of a species. Advantageous or neutral alterations survive in the population, disadvantageous characteristics tend to get bred out.

Keep two different groups of the same species separated and there will be different mutations in the groups. If they are in different environments then different mutations will be advantageous to each group. Give it some time and the number of mutations will be enough to prevent breeding between the two groups. That's speciation.

Oh OK, I see I think. Thanks for that

So if, speculating wildly, the environment was such that two seperated groups found similar mutations advantagous and evolved, in isolation, in similar ways would that mean they'd take longer to speciate to the point of being unable to interbreed?

I know it's a daft hypothetical scenario ... I'm trying to learn something, not do a kumar!

 

[wollery]

Oh OK, I see I think. Thanks for that

So if, speculating wildly, the environment was such that two seperated groups found similar mutations advantagous and evolved, in isolation, in similar ways would that mean they'd take longer to speciate to the point of being unable to interbreed?

I know it's a daft hypothetical scenario ... I'm trying to learn something, not do a kumar!

No, that's a fair question, and not being an evolutionary biologist I'm afraid that the best answer I can give is;

Well I guess so, that kinda makes sense.

Umm, any evolutionary biologists in the house?

 

[Yaotl]

IANAEB, but the two environments would have to be perfectly identical and the original genes of the two seed groups would also have to be identical. Otherwise, that one little thing in either would effect change in the genetic makeup.

 

[Benguin]

IANAEB, but the two environments would have to be perfectly identical and the original genes of the two seed groups would also have to be identical. Otherwise, that one little thing in either would effect change in the genetic makeup.

Yes, I can understand the principle there. As I said it was a hypothetical thingamy.

So what kind of differences make for an actual genetic mutation? I'm always surprised by how wildly different domesticated animals are and yet still able to produce viable offspring, even with some of their wild cousins.

 

[wollery]

IANAEB, but the two environments would have to be perfectly identical and the original genes of the two seed groups would also have to be identical. Otherwise, that one little thing in either would effect change in the genetic makeup.

I don't think even that would prevent speciation, remember that the genetic mutations are largely random. Whether or not they stay in the population depends on how advantageous they are.

 

[Yaotl]

I don't think even that would prevent speciation, remember that the genetic mutations are largely random. Whether or not they stay in the population depends on how advantageous they are.

I figured, but there's more of a chance it would prevent speciation than not. Not that I'm saying it would definitely mind you, but there's always a chance.

Originally posted by Benguin
So what kind of differences make for an actual genetic mutation? I'm always surprised by how wildly different domesticated animals are and yet still able to produce viable offspring, even with some of their wild cousins.

I have no idea what part of DNA would cause that. Way beyond me.

 

[MRC_Hans]

Bear in mind that speciation is a matter of definition. The usual definition, and distinction between race and species, is the ability to produce viable offspring, but that does not always hold water. Probably the inability for interpecies breeding is more an evolutionary trait than anything else. If two species that could interbreed shared the same habitat, they would mix, and there would only be one species. Where habitats are separated by other means, it is not unusual that species can interbreed. Many species of fresh-water fish, normally living in separate habitats, can actually interbreed if mixed.

Hans

 

[Xeriar]

Oh OK, I see I think. Thanks for that

So if, speculating wildly, the environment was such that two seperated groups found similar mutations advantagous and evolved, in isolation, in similar ways would that mean they'd take longer to speciate to the point of being unable to interbreed?

I know it's a daft hypothetical scenario ... I'm trying to learn something, not do a kumar!

Possibly. Humans and chimps are more closely related than horses and donkeys, but no known incident (can I say thank god or goddess here or something?) has resulted in a half-human/half-chimp.

The thing is mutations themselves are random. If chromosomes duplicate in one population, speciation is likely, and it can happen anytime.

 

[RussDill]

I think it has to do more with mutations of largely unrelated genes. Many genes mutate constantly with little or no effect (like hemoglobin). A series of random mutations that didn't really have an advantage or disadvantage could prevent the two new species from producing offspring.

 

[Benguin]

I think it has to do more with mutations of largely unrelated genes. Many genes mutate constantly with little or no effect (like hemoglobin). A series of random mutations that didn't really have an advantage or disadvantage could prevent the two new species from producing offspring.

I was going to ask why that had never happened with the domesticated cats, dogs, cows etc. But I suppose they'd just be regarded as infertile as they couldn't breed with the other animals and forgotten about.

 

[John Bentley]

So what kind of differences make for an actual genetic mutation? I'm always surprised by how wildly different domesticated animals are and yet still able to produce viable offspring, even with some of their wild cousins.

If I understand your question correctly, the genotype (DNA makeup) of the animal would have to produce one of three different scenarios, or a combination:
1. The phenotypic (how the animal looks) differences would have to be so different that the genital organs could no longer function together. They just don't fit.
2. The behavioral differences produced by the genotype would render the animals sexually unattractive to each other. They don't turn each other on.
3. The genotypes are so different that the gametes can no longer merge into a single nucleus. This is why human/chimp matings don't work, even though 98.8% of our DNA is essentially identical. A lot of the DNA is the same, but the genes are in different areas of the chromosomes, and chimps have more chromosomes than we do (or is it the other way around? I can never remember.)

I suppose you could also throw in the "lethal gene" combos which produce phenotypes incompatible with cellular function.

You also have to bear in mind that domesticated animals are wildly different because of selection by man for certain desirable traits. You could say that man has selected certain mutations as desirable, whether or not they are advantageous to the animal. Hence we have a Great Dane and a Chihuahua that are the same species, but wildly different in appearance. We also have a Siberian Husky and a wolf who look very similar, but are different species.

This also points out, incidentally, MRC-Hans point that the ability to interbreed is not necessarily the criterion for whether the species is the same. Huskies and and wolves are different species, but can interbreed (if forced to mate, or artificially inseminated by man).
As a final confusion, some animals are more genetically "malleable" than others. Dogs can and have been bred for really strange appearances. Cats have been bred for different traits, but their basic genetic "stock" is very stable. Despite every effort of man, most cat breeds are still pretty hard to tell apart (at least harder than Great Danes and Yorkies).

 

[John Bentley]

I was going to ask why that had never happened with the domesticated cats, dogs, cows etc. But I suppose they'd just be regarded as infertile as they couldn't breed with the other animals and forgotten about.

It does happen, and the results aren't just forgotten about. They just require the intervention of man to keep going. Probably most famously, mules (and the lesser known hinny) are the result of interbreeding between donkeys and horses. If you breed a male donkey to a female horse, you get a mule. If you breed a female donkey to a male horse, you get a hinny. Either way, the offspring is sterile.

 

[Soapy Sam]

Benguin- Hans has it right.

Imagine two populations of monkey, separated by a river. The main population is on one side, a splinter group on the other having crossed a fallen tree , which is then washed away. The smaller breeding population allows mutations to spread rapidly.
Maybe the plants are different on that side, so they develop a different diet and behaviour; that causes selection of different characteristics- teeth, gait etc.

After a few generations, the two groups start to look a little different, maybe smell different. Move on a few dozen generations, or a few hundred. The river changes course and the two populations meet. They may be genetically capable of interbreeding, but they are not interested. They are not attracted to each other. The ancestral (larger) group has not changed much, but the smaller group has. They look odd. They act odd. Heck- they are odd. Genetically, they may differ by only a small amount, but then so do chimps and humans.

At this point, along comes a zoologist and names them as two separate species, on the basis of their breeding behaviour.
Species is a human label. Nature doesn't care.

 

[Benguin]

At this point, along comes a zoologist and names them as two separate species, on the basis of their breeding behaviour.
Species is a human label. Nature doesn't care.

Ah yes, but chimps and humans could physically interbreed, and I'm sure the odd event has happened. As far as I know, copulation could not result in an offspring at all, let alone a viable one. I think scientific consensus is against Oliver's claim at this time.

It was that particular nuance I was interested in, though I probably was mixing it up with what seems to me to be a fairly flexible definition of what is a new species.

Presumably at some point our ancestors (and those of chimps) were able to interbreed and not enough of it happened so the populations gradulally diverged. It is the intermediate steps I'm interested in learning a bit more about.

Mutation would suggest that these genetic incompatabilities move in ratchet steps, meaning the mutated offspring would be incompatible with their own parents, and would be reliant on other simultaneously mutated offspring in the population to continue to thrive. That seems to me to be flawed in all sorts of ways, which suggest I am not understanding the concept correctly ... !

 

[Dragon]

... snip ...

Mutation would suggest that these genetic incompatabilities move in ratchet steps, meaning the mutated offspring would be incompatible with their own parents, and would be reliant on other simultaneously mutated offspring in the population to continue to thrive. That seems to me to be flawed in all sorts of ways, which suggest I am not understanding the concept correctly ... !

A mutation which entailed incompatibility with the rest of the population would not survive because the odds against another individual being available with the same mutation would be astronomical. So, you don't get a new species in one generation, it takes a gradual build up of differences between the gene pools of populations.

Here's a link from New York University which might help.
Like Sam said - Species is a human label. Nature doesn't care.
Look up "ring species" if you want to be further confused.

 

[Dymanic]

Originally posted by Benguin

Mutation would suggest that these genetic incompatabilities move in ratchet steps, meaning the mutated offspring would be incompatible with their own parents, and would be reliant on other simultaneously mutated offspring in the population to continue to thrive. That seems to me to be flawed in all sorts of ways...

It is. Ratchet steps wouldn't mean that. Ratchet steps would mean the differences would be very slight, so that at any step, there would be no barrier to mating with either parents or offspring. That's why the species distinction is a judgement call on the part of the taxonomist, and it is the basis for the creationist's macro/micro argument (we see tiny steps, but never one big enough to cross a species boundary). Look again at what MRC_Hans said:

Bear in mind that speciation is a matter of definition. The usual definition, and distinction between race and species, is the ability to produce viable offspring, but that does not always hold water.

Keep in mind that not only can it be difficult to establish sterility with absolute certainty, there really isn't any such thing as perfect fertility either -- embryonic development is a touchy process, and it fails in the early stages surprisingly often, even between breeding partners who may be successful on another attempt (an apparently normal menstruation is very often actually a miscarriage).

Taxonomists consider two groups to be separate species when there is no gene flow between them for whatever reason -- there might be a geographical barrier (as in the monkey example above), a mechanical barrier (e.g., a Great Dane and a Chihuahua), a behavioral barrier (wrong mating dance, whatever), a temporal barrier (we might have been able to mate with Neandertals after all), or a genetic barrier.

Some taxomomists (the 'lumpers') are more conservative, and withhold the distinction as long as there appears to be any chance of interbreeding, while others (the 'splitters') are quicker to declare separate species. You might say there are two different types of taxonomist; those who like to split groups into two different types, and those who don't.

The best analogy I've heard has to do with observing the division of a cell; at the beginning, it would be easy to say: "here is a single cell" and, at the end, to say: "here are two cells". But anywhere partway through the process, it would not be possible to declare in any meaningful way precisely how many cells there were.

 

[Benguin]

Thanks guys, this is making sense to me.

So with the ratchet step thing ... I understood the difference between us and chimps (preventing interbreeding) to be down to having a different number of chromosomes. Is that incorrect? Are they just indicators of genetically which one is which or would you need a matching set of pairs to reproduce?

 

[Xeriar]

Thanks guys, this is making sense to me.

So with the ratchet step thing ... I understood the difference between us and chimps (preventing interbreeding) to be down to having a different number of chromosomes. Is that incorrect? Are they just indicators of genetically which one is which or would you need a matching set of pairs to reproduce?

Horses and donkeys have a different number of genes too (72 and 74? I forget)

Plants don't give a hoot at all. One species 24, another 8? No problem. Animals are a bit fussier, it seems.