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Author Topic:   One evolving species vs speciation.
jar
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Message 1 of 48 (430771)
10-27-2007 11:38 AM


I see a lot of discussion here at EvC that seems to revolve around looking at critters two different ways. I believe an example can be seen in this post.

As an admittedly somewhat uneducated lay person looking at a great big pile of data and trying to make some sense out of it, it appears to me that when we look at the fossil record we see two different things going on.

The first is that critters evolve, and over periods of time separated sufficiently from one another there will be enough of a difference between two critters to call them different species. However, if we could look at all the intervening steps, it would likely be difficult to tell much difference between adjacent critters. This is due to the selective filtering of traits across a population. Gradually some trait is added or subtracted from the population as a whole.

In that case, it is only when looking at samples separated by long periods of time that the changes are significant enough to say, "These are different species."

A second method is one species splitting into two species. The most common example of this is when a population becomes geographically separated into two or more isolated groups. In this case each group evolves slightly differently and we find a point in the record where we see two or more related yet unique species, both existing concurrently.

In the former case we see but one species at any given point in the record, but multiple species when examined over longer periods of time, each species occupying an ordered position along the timeline.

In the later we see one species at one point in the timeline but multiple species at a more recent point. (It can also go in reverse where we see a division and then one or more of the daughter populations disappearing)

When it comes to humans, it seems to me that when looking forward down the path of Human evolution, we will only see the former until we leave earth and colonize other worlds or possibly (and I give this a very low probability) unless there is a major catastrophic event.

Is there a reasonable scenario for Humans to become split and isolated over a period of time long enough to result in two separate species other than our moving off world?

Edited by jar, : fix awkward sentence


Aslan is not a Tame Lion

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Message 2 of 48 (430776)
10-27-2007 12:13 PM


Thread moved here from the Proposed New Topics forum.

  
Taz
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Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 3 of 48 (430788)
10-27-2007 2:13 PM
Reply to: Message 1 by jar
10-27-2007 11:38 AM


jar writes:

Is there a reasonable scenario for Humans to become split and isolated over a period of time long enough to result in two separate species other than our moving off world?


I think the most likely scenario of this happening so far is presented in the movie The Time Machine remake. The moon breaks up and falls on us. A part of the population goes under ground to escape the disaster while the other part remains on the surface. Life for both groups undoubtedly becomes very harsh.

After centuries of living under ground, the first group has adopted well enough that they don't want to go back to the surface. The second group has also survived and begins to thrive again on the planet surface.

After 800 thousand years, we effectively have 2 different species of humans.

As the vampire-like under ground human sub-species tells our time traveler, "Who are you to question 800 thousand years of evolution?"

Other than this scenario, I can't really think of any other way we could prevent gene flow from happening for long enough for the human species to split into two.


Disclaimer:

Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.

He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!


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Fosdick 
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Posts: 1793
From: Upper Slobovia
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Message 4 of 48 (430813)
10-27-2007 7:02 PM
Reply to: Message 3 by Taz
10-27-2007 2:13 PM


Flow or drift?
Taz, isn't the scenario you decribe as gene flow really random genetic drift ?

—HM


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Taz
Member (Idle past 1609 days)
Posts: 5069
From: Zerus
Joined: 07-18-2006


Message 5 of 48 (430833)
10-27-2007 8:56 PM
Reply to: Message 4 by Fosdick
10-27-2007 7:02 PM


Re: Flow or drift?
I don't know. I was just using a couple of words I remembered from my high school biology text book hoping I could impress people.


Disclaimer:

Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.

He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!


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 Message 4 by Fosdick, posted 10-27-2007 7:02 PM Fosdick has not yet responded

  
Quetzal
Member (Idle past 4190 days)
Posts: 3228
Joined: 01-09-2002


Message 6 of 48 (430906)
10-28-2007 8:09 AM
Reply to: Message 1 by jar
10-27-2007 11:38 AM


Is there a reasonable scenario for Humans to become split and isolated over a period of time long enough to result in two separate species other than our moving off world?

Given the requirement for some type of isolation to arise within a population before speciation can occur (at least in the vast majority of multi-cell organisms), I'd say that your two scenarios were about the only way it could occur in humans. Emigration off-world or some type of catastrophe that geographically separates huminity into isolated fragments would be about the only way speciation could happen. With the trend today being quite the opposite (homogenization rather than isolation), I'm not sure how else it could happen.

Edited by Quetzal, : No reason given.


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PaulK
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Message 7 of 48 (430908)
10-28-2007 8:20 AM
Reply to: Message 4 by Fosdick
10-27-2007 7:02 PM


Re: Flow or drift?
I don't think you've read very carefully. Taz's scenario postulates an absence of gene flow betrween the two populations. That permits drift to carry them in different directions but it also gives selection more opportunity to encourage divergence.

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jar
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Posts: 31656
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.5


Message 8 of 48 (430923)
10-28-2007 10:58 AM
Reply to: Message 6 by Quetzal
10-28-2007 8:09 AM


on catastrophes
Is there a catastrophe you can imagine that would keep the populations separated long enough for speciation to occur?

Would some other factors need to be thrown in such as increased radiation levels?


Aslan is not a Tame Lion

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Fosdick 
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Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 9 of 48 (430926)
10-28-2007 11:13 AM
Reply to: Message 7 by PaulK
10-28-2007 8:20 AM


Re: Flow or drift?
PaulK writes:

Taz's scenario postulates an absence of gene flow betrween the two populations. That permits drift to carry them in different directions but it also gives selection more opportunity to encourage divergence.


You're right about the scenario that Taz postulates. He describes one population drifting into two. Stiil, I don't see why gene flow needs special mention. The evolutionary force in his scenario is drift, not flow. Gene flow might happen eventually, however, after all the drifted alleles are fixed in each of the new populations.

—HM


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Quetzal
Member (Idle past 4190 days)
Posts: 3228
Joined: 01-09-2002


Message 10 of 48 (430934)
10-28-2007 12:10 PM
Reply to: Message 8 by jar
10-28-2007 10:58 AM


Re: on catastrophes
My guess would be that increased radiation would not be required. The normal mutation rate would be sufficient if given long enough time spans. Of course, an increase in the level of ionizing radiation would likely increase the mutation rate - but probably at the same time decreasing the probability that anyone would survive.

As to what type of catastrophe, my guess is it would have to be pretty global in scope and decimate the current population. I wouldn't want to hazard a guess as to how much, 'tho. It doesn't have to be an extraterrestrial "dinosaur killer" of course, or a global nuclear war. A nice airborne hypervirus could theoretically do the trick.


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jar
Member
Posts: 31656
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.5


Message 11 of 48 (430936)
10-28-2007 12:28 PM
Reply to: Message 10 by Quetzal
10-28-2007 12:10 PM


Re: on catastrophes
It doesn't have to be an extraterrestrial "dinosaur killer" of course, or a global nuclear war. A nice airborne hypervirus could theoretically do the trick.

The key is time IMHO. Could even a hypervirus keep populations apart long enough for speciation?


Aslan is not a Tame Lion

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Quetzal
Member (Idle past 4190 days)
Posts: 3228
Joined: 01-09-2002


Message 12 of 48 (430937)
10-28-2007 12:57 PM
Reply to: Message 11 by jar
10-28-2007 12:28 PM


Re: on catastrophes
Depends, I suppose, on how far down the population gets reduced. The only example we have of human speciation is whatever caused the split between Homo sapiens and H. neanderthalensis, assuming they had a recent common ancestor. My guess is that it would require reduction to the point where you have isolated populations on each continent that have lost so much of the cushioning effect of "civilization" (for lack of a better term) that thousands of generations pass before reconnection occurs (if any). Especially given the fact that any surviving population of modern humans is likely going to have a pretty wide cross-section of the available human alleles due to the current effects of globalization.

However, there are a couple of other factors to also consider in any such scenario. The most obvious is at what level would our species become non-viable. I don't know if anyone has done any modeling on a Minimum Viable Population for H. sapiens. If there is any correlation between us and, say, bonobos, then an MVP1000 would be 5-10,000 individuals in a freely inter-breeding population. Or more. Some models suggest long-term viability (i.e., greater than MVP1000) would require 50,000 or more individuals in the starting population due to potential loss of alleles, etc.

Another factor that might be important when we consider H. sapiens is that our species seems to have something of a wanderlust. IOW, given that it appears there have been at least one and possibly two "out of Africa" migrations in our lineage, it strikes me as entirely possible that there wouldn't be sufficient time to allow isolation and natural selection to "do their thing" before gene flow between catastrophically isolated populations is re-established.

The science here seems to be pretty soft, if you ask me. I'm not sure how realistic any of the above might be when you look at our species. It just might be that the only realistic scenario for human speciation would be the one where utterly different selection pressures operate on a totally or mostly isolated population for a really really long time (i.e., the "other planet" scenario).


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jar
Member
Posts: 31656
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.5


Message 13 of 48 (430940)
10-28-2007 1:12 PM
Reply to: Message 12 by Quetzal
10-28-2007 12:57 PM


Re: on catastrophes
Another factor that might be important when we consider H. sapiens is that our species seems to have something of a wanderlust. IOW, given that it appears there have been at least one and possibly two "out of Africa" migrations in our lineage, it strikes me as entirely possible that there wouldn't be sufficient time to allow isolation and natural selection to "do their thing" before gene flow between catastrophically isolated populations is re-established.

Exactly. And we also have evidence that isolation of even tens of thousands of years is simply not enough; consider the dispersal of humans and the lack of "ring species".

The science here seems to be pretty soft, if you ask me. I'm not sure how realistic any of the above might be when you look at our species. It just might be that the only realistic scenario for human speciation would be the one where utterly different selection pressures operate on a totally or mostly isolated population for a really really long time (i.e., the "other planet" scenario).

Agreed. Unless we incorporate some additional factors, I see no other possibilities. If though we reduced the ozone layer at the same time, so that we could incorporate isolation with increased radiation, then I think a likely scenario might be imagined.


Aslan is not a Tame Lion

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PaulK
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Posts: 15567
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Message 14 of 48 (430952)
10-28-2007 2:33 PM
Reply to: Message 9 by Fosdick
10-28-2007 11:13 AM


Re: Flow or drift?
Gene flow needs to be mentioned because it is a force that prevents populations from diverging by drift. That is why it is the ABSENCE of gene flow that is relevant. And if the populations do speciate there will NOT be any gene flow between them in the future.

I'm sorry to say this but it seems that you are just picking up on the terms without any sign of understanding what they mean.


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Wounded King
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Posts: 4149
From: Edinburgh, Scotland
Joined: 04-09-2003


Message 15 of 48 (430966)
10-28-2007 3:46 PM


Argument from another thread
This post is a repsonse to Hoot's critique of a article posted by Schraf in the Good Scientists Gone Bad -- Dr. Watson and Dr. Pauling thread.

Schraf posted an extract...

Let's look at the major claim: that humans will subspeciate. I can't think of anything less likely among a species that has major gene flow between all its populations on a scale of thousands of generations. Species aren't formed by selection for differing adaptive traits within a population, but by the interruption of the gene flow that is caused by migration or invasion between populations. Take a look at human gene flow over the past 10,000 years - massive amounts of interbreeding and invasive gene flow. Not a hint of the sorts of isolation required for a mammalian species to speciate in sight. Not even the Tasmanian aborigines, who were isolated for about 10,000 years. Not even the San or M'buti. Nada.

And Hoot said ...

Hoot writes:

Just a question of clarification: Do you mean to say that you agree with the sentence I've highlighed in orange? I'd say the author has demolished only his own credibility.

Why not try and make a reasoned argument Hoot.

No one conversant with current evolutionary theory would consider this to demolish the authors credibility, they might disagree with it, or with some of its particulars, but it is not beyond the pale of current evolutionary theory. to some extent it depends on how you read the statement.

Taking the most extreme interpretation that adaptive selection played no role, there is still some currency for the idea that non-adaptive neutral drift can play a part in speciation, even though it may not be accepted and its importance is often considered minor. A review by Douglas Futuyma of two books on speciation mentions points both for and against a role for drift.

Futuyma writes:

Very different kinds of data, ranging from DNA sequences to correspondence between RI and ecological divergence, support natural selection, but there is hardly enough evidence, in my opinion, to support Coyne and Orr's strong conclusion that “at least one important debate has been settled: selection plays a much larger role in speciation than does drift”

Coyne and Orr appear to adopt selection as the null hypothesis for speciation, whereas drift is generally taken as the null hypothesis in much of evolutionary genetics, for the simple reason that drift operates at all loci in all finite (i.e., real) populations, whereas selection need not. The burden of demonstrating that selection is not responsible for an evolutionary event (i.e., demonstrating a negative) is, of course, far heavier than the burden of demonstrating selection; indeed, Coyne and Orr do not address the difficult question of what would constitute evidence for drift. Having, perhaps, stacked the deck, Coyne and Orr find almost no evidence that drift has contributed to speciation in nature, but conclude that there is “considerable evidence” that selection has done so

Assuming that experiments with laboratory populations can be validly extrapolated to natural speciation processes, founder-effect speciation may indeed be a moribund hypothesis, but I do not believe long-term genetic drift can yet be ruled out, and cannot agree that this “important debate has been settled”

This article also bring up another good example which may be considered a suitable basis for considering 'selection for differing adaptive traits' not to be the foundation of speciation, sexual selection. It is debatable whether the products of sexual selection coud be considered adaptive in the usual sense. For myself I would consider them so, as adaptations to that element of the environment consisting of other members of the same species, but others might take a differing view that sexual selection is a distinct process .

That aside I think his argument falls down on as the mere fact that human have not speciated and show some signs of differing traits associated with more isolated geographical populations in no way makes a coherent argument with the proposition that species cannot be formed by 'selection for differing adaptive traits', merely that this has not occurred in the case of H. sapiens considered as one extended population.

And lets not forget he is talking about within a population, or sympatric speciation.

At its best interpretation all the author is claiming is that reproductive isolation is required for speciation, the most acceptable evolutionary claim imaginable, although exceptions may be posited. Even in a sympatric case speciation is considered to require reproductive isolation, the inital basis of the RI may be due to adaptive selection on variation in the population but the speciation itself need not be driven by this.

So what exactly is it you object to, how do you understand the statement and what about it makes you think it demolishes the authors credibility?

It seems to me that such offhand glib unsupported statements argue more against your own credibility.

TTFN,

WK


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