Question on Speciation [Archive]

Benguin

18th November 2004, 12:35 PM

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

18th November 2004, 12:46 PM

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

18th November 2004, 12:59 PM

Originally posted by 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.

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

18th November 2004, 01:05 PM

Originally posted by Benguin
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

18th November 2004, 01:08 PM

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

18th November 2004, 01:13 PM

Originally posted by 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.

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

18th November 2004, 01:15 PM

Originally posted by 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. 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

18th November 2004, 01:21 PM

Originally posted by wollery
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

18th November 2004, 01:39 PM

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

18th November 2004, 03:01 PM

Originally posted by Benguin
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

18th November 2004, 03:13 PM

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

18th November 2004, 03:52 PM

Originally posted by 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.

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

18th November 2004, 03:59 PM

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.

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

18th November 2004, 04:05 PM

Originally posted by Benguin
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

18th November 2004, 05:26 PM

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

19th November 2004, 01:32 AM

Originally posted by Soapy Sam

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 (http://www.science-frontiers.com/sf110/sf110p06.htm) 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

19th November 2004, 02:38 AM

Originally posted by Benguin
... 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 (http://www.nyu.edu/projects/fitch/courses/evolution/html/speciation.html#2MechanismsMaintainingIsolation) 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. :D

Dymanic

19th November 2004, 03:00 AM

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

19th November 2004, 03:22 AM

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

19th November 2004, 03:25 AM

Soapy Sam

19th November 2004, 04:46 AM

Current thought is that the two surviving chimp species separated from each other about 2 million years ago- though that's based on genetic clocks rather than fossils. The common chimp line and the human separated about 6 million years ago.

This is roughly a quarter of a million generations ago. Plenty time for gradual drift to accumulate the .5-2% DNA difference.

On this question of ratchets and "hopeful monsters"- It seems to me that while a sudden, major mutation in a germ line is most likely to result in the non viability of any generated offspring- ie they die before birth, or before sexual maturity, or are themselves infertile- there is another possibility. If there is a chemical or structural reason why a particular chromosome might split (for example) into two- and if that tendency results in a small percentage of individuals in a population carrying the mutation at any time, then
1. Those individuals may be unable to breed with "normals" of their own or previous generations, while appearing in every way
normal themselves.
but
2. They may be able to breed with other individuals carrying the same mutation.

If this happens even once in a population, the mutation may become sexually inherited in several members of the next generation. We now have two breeding groups in the same population, physically together, but genetically isolated.
In small, nomadic browsing populations, this could rapidly lead to the development of two separate species.

neutrino_cannon

19th November 2004, 09:55 AM

A good example of the essential arbitrarity with which the term "species" is applied to animals is the genus canis. To the best of my knowlege, there are no members of the genus which can't interbreed to form fertile young, so you could in theory mix a greyhound and a dingo if you were so inclined. According to some, certain dogs will even sucessfully interbreed with foxes (which belong to several genera).

In my bit of the states (and many others) the ubiquity of coyotes, wide open spaces and unleashed dogs often results in a beast called a "coydog", which can be almost any mixture of it's parentss traits. I personally suspect that the furtive beasts I see darting through the fields aren't necessarily "pure" coyote by any means.

RussDill

19th November 2004, 10:09 AM

Originally posted by 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?

That would probably be a big enough difference, but I think usually, its other possibilities. There is a species of bird that lives about the lattitude of canada, alaska, etc. If you start in alaska, you can find a slightly different breed of the bird in canada, and these two breeds can produce offspring. Same with canada to a breed in greenland, and you can go around and around the longitudes this way, util you get to serbia. There is a breed in serbia that cannot produce offspring with the breed in alaska. There isn't one difference that makes them unable to breed, but a series of changes.

Soapy Sam

19th November 2004, 10:15 AM

Dogs are an interesting example because for thousands of generations, their mates have often been selected for them by another species entirely. (Us). In that case, we would expect some strange results. And we've got them.

SGT

19th November 2004, 11:24 AM

An example of a case where speciation was observed in the laboratory:

From www.talkorigins.org

Speciation in a Lab Rat Worm, Nereis acuminata In 1964 five or six individuals of the polychaete worm,
Nereis acuminata, were collected in Long Beach Harbor, California. These were allowed to grow into a population
of thousands of individuals. Four pairs from this population were transferred to the Woods Hole Oceanographic
Institute. For over 20 years these worms were used as test organisms in environmental toxicology. From 1986 to
1991 the Long Beach area was searched for populations of the worm. Two populations, P1 and P2, were found.
Weinberg, et al. (1992) performed tests on these two populations and the Woods Hole population (WH) for both
postmating and premating isolation. To test for postmating isolation, they looked at whether broods from crosses
were successfully reared. The results below give the percentage of successful rearings for each group of crosses.
WH X WH - 75%
P1 X P1 - 95%
P2 X P2 - 80%
P1 X P2 - 77%
WH X P1 - 0%
WH X P2 - 0%
They also found statistically significant premating isolation between the WH population and the field populations.
http://www.talkorigins.org/faqs/faq-speciation.html (11 of 16) [31/8/1999 1:45:18 PM]
Finally, the Woods Hole population showed slightly different karyotypes from the field populations.

Paul C. Anagnostopoulos

19th November 2004, 03:14 PM

Soapy mentioned drift. Even if two environments were identical, random drift could ultimately cause speciation. For example, one population could undergo a harmless chromosome split that increased their chromosome count by one, but was otherwise neutral or beneficial. Except . . . it might prevent the two populations from mating.

~~ Paul

CBL4

19th November 2004, 03:33 PM

I believe that it would be possible to have speciation without mutation. For example, an isolated population could lose certain genes through genetic drift resulting in animals that become too small (or too large or too orange) to mate with the standard population. In other words, if all the genes that favor large animals are removed from a population, a pygmy species could result. It would no longer be able to interbreed even though no mutation has occurred.

CBL

RussDill

19th November 2004, 03:45 PM

Originally posted by CBL4
I believe that it would be possible to have speciation without mutation. For example, an isolated population could lose certain genes through genetic drift resulting in animals that become too small (or too large or too orange) to mate with the standard population. In other words, if all the genes that favor large animals are removed from a population, a pygmy species could result. It would no longer be able to interbreed even though no mutation has occurred.

CBL

I think this may have occured at the high school I attended.