A quick question for the biological gurus at this board: Assuming that an arbitrarily-sized group of humans was isolated from the rest of the population, would a new species be formed? If so, how long would this take? I remember seeing a television program a few years ago, that featured a similar situation, where a number of biologically isolated environments existed (in South America, I think).
This message has been edited by AdminNosy, 06-26-2005 01:47 AM
To get this topic off to a good start(and I think it is a pretty good one) could you pick a better title?
How about something like "Human speciation due to geographic separation"?
There are real biolgists here who will be able to point you to the answer to this question.
If you change the title then I think this should be promoted. It might also be a good idea to suggest what boundaries you would like on the discussion. For example, can this talk about the example of other non-human groups or are you intending to restrict it a lot.
Thanks Nosy! Just to make it a bit clearer...If a certain number of people- say 100,000- were to be isolated in a remote part of the planet...how long would it take for this isolated group to become unable to interbreed with Homo sapiens? Is it even possible?
I've promoted this even though you didn't exactly follow the suggestions.
To "fix up" a proposal you can edit the opening post (OP). Generally it is bad form to edit (in a substantive way) a post that others have had a chance to read but the OP of a PNT is set up to allow you to edit it untill it is ready for release.
This message has been edited by AdminNosy, 06-26-2005 01:49 AM
It's a very interesting question, but I don't think anybody can even come close to answering it right now.
Most speciation scenerios involve a large amount of genetic drift. The initial form of genetic drift would be sampling error. If a small population were to be isolated from a larger one, possibly by a geographic barrier, what are the chances that the allelic frequencies of the smaller population would be representative of the larger one? This is called "the founder effect", and could be considered "instant evolution."
This isn't even close to speciation, though. Next, random mutations would accumulate in the small population, further changing the allelic frequency of the new population. When the new population increases (a genetic bottleneck), it will have an allelic frequency dependent of the founder effect and these newer mutations. These new mutations may or may not have phenotypic effects. In order for speciation to occur, there needs to be some prezygotic or postzygotic mating barrier that would prevent inbreeding with the parent population. This can result from the random mutations, but a timeframe would be hard to pin down.
Natural selection may also play a large role in speciation. This will cause the new population to become adapted to new environmental conditions. After time, through pleiotropic effects, this may result in reproductive incompatibility with the parent population. Again, a timefrime would be hard to put down.
The situation you describe will be very different from the classical view of speciation I described above (allopatric). For instance, I'd imagine that a 100,000 person sample would include much of the genetic diversity of the parent population, essentially eliminating the "founder effect." Also, random mutations in the new populations aren't likely to effect the overall allelic frequency because the bottleneck effect is dependent on their being a small initial population. Next, humans are such a technologically innovative species that I think we would alter the environment to suit are needs before any serious effects of natural selection could take place.
This message has been edited by JustinC, 06-26-2005 02:39 AM
This can result from the random mutations, but a timeframe would be hard to pin down.
A single mutation can result in a post-mating barrier; if, for example, the mutation is a translocation mutation that would disrupt proper sorting of the genome at chromosome pairing. (Robertsonian translocations in Alpine mice may be an example of this sort of speciation event).
At the opposite end of the spectrum, there are populations separated at the continent level for millions of years that remain reproductively compatible. (A number of plant species exist in both North America and Asia that have existed at isolated populations for 6-8 million years, but still are capable of interbreeding).
So, to answer the question of the opening post:
Speciation takes sometime between "a few generations" and "never" to occur.
Speaking of founder effects and population bottle necks....what if there was simply a "social isolation" instead of a geographic one. Couldn't we study the Amish or Mennonite or any other socially isolated group that essentially has been inter-marrying for a couple hundred of year. I believe it was "60 Minutes" a few weeks ago that ran a show about this, where several of the children in these communities are developing several severe and rare mental disorders.
In this case, would you think that the founding population was too small to have a viable gene pool for the long run, especially with what is probably very limited immigration into the population?
IIRC, and I'm old and forgetful so allow me some slack, there have been several studies on such founder effects. IIRC, one of the features supporting the multiple immigration events into the new world is the geographic distribution of blood types. As you move further south you find that the native populations consist of only one of the three possible blood types, while in north america, two of the blood types are found. This seems to indicate at least two migration events, one where the pilot population consisted of only one blood type, a later immigration that was either all of a second blood type or of a mixture of the two types.
I think JustinC has given you a very good overview of the the relevant processes. Populations can only diverge to become separate species through accumulation of sufficient genetic differences. Population-level genetic differences can come about through various mechanisms, but most will be very muted in this case.
The founder effect will be reduced because of your relatively large initial population size (few alleles will be absent that are present in appreciable frequencies in the ancestral population).
If your population has technology to assist survival, natural selection will be a weak force for altering allelic frequencies as a function of local ecology and environment, wher it could be important for normal animals lacking technology.
We are left with really only two plausible scenarios for generation of a reproductive barrier: mutation and genetic drift. As pointed out by Sasquatch a single 'critical' mutation could accomplish the reproductive barrier immediately - but first it would have to rise to fixation within the population (become the dominant allele or genomic form). This means it would have to confer some strong survival or reproductive advantage. Not impossible, just a very low probability event and extremely difficult to predict if, when, or how long it would take to occur.
One the other hand, genetic drift is a sort of slow, inexorable process - isolated populations just gradually become genetically different purely by the chance survival /loss of alternative alleles. The speed that this effect will result in meaningful genetic changes between two isolated populations will be a function of various factors, particularly mutation rate, effective population size, and generation time. Two large populations with low rates of mutation and long generation times (like humans) could well take thousands of years to diverge in any way that could result in reproductive incompatibility - which isn't to say that it couldn't happen either.
The issue of cultural isolation brought up by clpmini is also an interesting point, because it is a far more plausible barrier to gene flow than geography in modern human society. Nevertheless, we know that only very low levels of gene flow are required to prevent genetic drift between populations, so the culturally isolated population could tolerate very few, if any, defectors and accept very new initiates for them to continue to drift away from the rest of humanity in genetic composition.
As pointed out by Sasquatch a single 'critical' mutation could accomplish the reproductive barrier immediately - but first it would have to rise to fixation within the population (become the dominant allele or genomic form). This means it would have to confer some strong survival or reproductive advantage.
A quick clarification: In the case of chromosomal rearrangment mutations, fixation doesn't require the same sort of strong fitness advantage to reach fixation. Instead, you've basically split one chromosome set into two types of chromosome sets; BUT, they both contain the same genome/genetic information.
So it's all a matter of segregation - generally, offspring who are heterozygous for the original and rearranged chromosomes generally do not survive or have severe reduction in fitness. Thus, if some offspring happen to get a complete set of the 'new' chromosomes, they may look identical and have the same fitness level as other in the population with the original chromosome set, but they are now reproductively incompatible because their genome is incapable of combining to form a genome that will segregate properly during cell division.
So 'fixation' is a more a matter of luck, and is likely to happen rather quickly or not at all.
I'm not sure that I've explained this well, and it is a bit complicated with bits of chromosomes jumping about - it may be helpful to search out "Robertsonian translocations" on the web or pubmed if you are interested in the subject. "Robertsonian translocations" are such rearrangments in mice, and have been fairly well-studied, including at the level of their role in speciation.
Yes. I recognized the distinction between chromosomal rearrangements and conventional point mutations (hence my reference to 'alleles or genomic forms') without a full understanding of their implications. Radical chromosomal alterations, provided they still produce a viable individual, could be potentially the fastest route to effective speciation. As I understand it, polyploidy events have been implicated in 'instantaneous' speciation within various plant lineages. However, these species are typically either self-compatible or capable of vegetative propagation. Correct me if I'm wrong , but I would expect the problem with chromosomal alterations taking hold in a human population is the need for genetic compatibility between sperm and egg. The alterations would (1) have to appear at appreciable frequencies in both sexes and (2) segregate normally, both low-probability scenarios. True?
This message has been edited by EZscience, 06-27-2005 01:55 PM
quote:I'm not sure that I've explained this well, and it is a bit complicated with bits of chromosomes jumping about - it may be helpful to search out "Robertsonian translocations" on the web or pubmed if you are interested in the subject. "Robertsonian translocations" are such rearrangments in mice, and have been fairly well-studied, including at the level of their role in speciation.
A quick question about Robertsonian translocations. Are both of of the original centromeres of the telocentric chromosomes present in the the new metacentric chromosomes?
Well, we now have some remains of Homo floresiensis, which has been given species status - though it would be pretty difficult to say for sure that they couldn't interbreed with either erectus or sapiens. The papers on the find suggest that Flores man may have been isolated for as long as 800,000 years, though I understand that they are waiting on more finds and more dating to pin that down.
Interesting point. The ancestral Homo erectus gave rise to several distinct species in several geographic regions: H. florensiensis in Indonesia, H. sapiens in Africa, and H. neanderthalensis in Europe.
quote:Interesting point. The ancestral Homo erectus gave rise to several distinct species in several geographic regions: H. florensiensis in Indonesia, H. sapiens in Africa, and H. neanderthalensis in Europe. Interesting point. The ancestral Homo erectus gave rise to several distinct species in several geographic regions: H. florensiensis in Indonesia, H. sapiens in Africa, and H. neanderthalensis in Europe.
Don't forget those borderline cases, Homo rhodesiensis, Homo heidelbergensis...
But since you brought up floresiensis, I recall seeing Peter Brown's seminar a few months ago. I think he himself now doubts that floresiensis was decended from the Indonesian erectus. He showed that they have similarities to ...Homo georgicus.
Another theory is that floresiensis were derived from earlier Homo species, probably habilis-types or even australopiths. But this is still a black box for everyone.
...and Prof. Teuku Jacob's team is still studying that pygmy village in Flores. Too bad they haven't shown any pictures though.