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Author Topic:   Evolving Populations, and Speciation
Zawi
Member (Idle past 1794 days)
Posts: 126
From: UK
Joined: 12-02-2004


Message 1 of 8 (286381)
02-14-2006 9:14 AM


How does a population evolve?

Does it work like this:

A parent gives rise to a child who happens to be in posession of a beneficial mutation. This mutation proves to be so useful that the child manages to reproduce with no problem. If its children also posess the beneficial mutation, then that family line will soon dominate the population, until all members of the population posess the mutation.

Or does it work like this:

An environmental change occurs, 'encouraging' mutations across the population. The mutations might all be different, but all are beneficial as far as survival in the environment is concerned. Over time, the beneficial mutations are pooled together to create new species.

(Please forgive my crude way of explaining things; I'm not familiar with scientific terminology, but I'm eager to learn!)


Replies to this message:
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AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 8 (286397)
02-14-2006 9:48 AM


Thread moved here from the Proposed New Topics forum.
  
crashfrog
Inactive Member


Message 3 of 8 (286409)
02-14-2006 10:07 AM
Reply to: Message 1 by Zawi
02-14-2006 9:14 AM


It's more your first option. It's very rare in organisms that they respond to changes in the environment by having more mutations than they normally would, but it does happen. But even in these cases the environment doesn't drive the outcome of mutations; mutations are always random influences on the gene pool.

Over time, the beneficial mutations are pooled together to create new species.

It's important not to conflate mutation with speciation; a single species could concievably experience drastic physical change over time without actually being a new species; rather, the origin of new species is a situation where a formerly large gene pool becomes bifurcated when a portion of the population's individuals are split off from the whole (by geographic distance, for instance; really, anything that would prevent the individuals in the offshoot population from reproducing with individuals from the original population). This sub-population accrues mutations that do not find their way back into the large original population, until a point of genetic incompatibility is reached and the sub-population could never again reliably interbreed with the large one. At that point, we consider it a new species.

In large populations, there's a kind of genetic "homeostasis" that occurs, where their large gene pool is resistant to the fixation of radically new alleles. (Fixation is when one allele of a gene outcompetes all other alleles and becomes the only allele for that gene that any individual possesses.) So drastic morphological change seems to be much more likely to occur in very small populations, which explains the patterns of the fossil record, where species seem to continue largely unchanged for great periods of time subsequent to the rapid appearance of radically different organisms. This is something that we call "punctuated equilibrium."


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New Cat's Eye
Inactive Member


Message 4 of 8 (286417)
02-14-2006 10:30 AM
Reply to: Message 1 by Zawi
02-14-2006 9:14 AM


more like the first one
Its more like your first example. Mutations happen all the time and mostly have no affect on reproductive success or any non-negligible affect on the allele frequency of the population. The environmental affects come after the mutations and determine which mutations ‘succeed’ or ‘fail’.

then that family line will soon dominate the population, until all members of the population posess the mutation

Even if this happened it would not be speciation. Speciation requires a separation from the parent population, either geographically, by some morphological difference that prevents reproduction, or something else. If the mutated population is not separated, but ends up dominating the parent population, then that population has evolved but no speciation has occurred.

An environmental change occurs, 'encouraging' mutations across the population.

Mutations happen all the time, randomly. I think it is possible for the environment to encourage mutation, by nuclear radiation or something, but I don’t think this is something that really promotes speciation.

Over time, the beneficial mutations are pooled together to create new species.

No, some kind of seperation is needed.


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ramoss
Member
Posts: 3110
Joined: 08-11-2004
Member Rating: 8.2


Message 5 of 8 (286663)
02-14-2006 7:33 PM
Reply to: Message 1 by Zawi
02-14-2006 9:14 AM


When it comes to speciation, all you need is two populations that do not
have a genetic interchange. This can be because of geological reasons. It can be also because of diet.

Although I can not find the source, there was an article about a species of insect that in one location , had two seperate populations that do not interbreed. The reason for the difference was one starting having a diet from one source, in a tree, and the other one stayed onthe ground and ate something else. They are still the same species.. yet the two populations don't interbreed, based on their diet. This is going through the same process that it is believed the birds in the galopagos islands did.


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Nuggin
Member (Idle past 657 days)
Posts: 2965
From: Los Angeles, CA USA
Joined: 08-09-2005


Message 6 of 8 (286752)
02-15-2006 3:21 AM
Reply to: Message 1 by Zawi
02-14-2006 9:14 AM


Good Question - let's muddy up the water
I agree with the three previous posts in that the first one is a better description.

However, remember that this is dynamic. There could be hundreds of mutations within the original population, additionally new mutations are popping up with each new generation.

For clarity it's fine to assume we are talking about a specific mutation and tracking it. But I think people often fall into the trap of thinking about a population as a set of genetically identical beings.


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Speel-yi
Inactive Member


Message 7 of 8 (287258)
02-16-2006 9:48 AM
Reply to: Message 6 by Nuggin
02-15-2006 3:21 AM


Re: Good Question - let's muddy up the water
quote:
Fixation is when one allele of a gene outcompetes all other alleles and becomes the only allele for that gene that any individual possesses.

Not exactly. Competition implies that there is some increase in fitness for an organism that bears an allele.

Fixation is more likely to occur in small populations where the selection is weak. Genetic drift in small populations will overcome such weak selection and as a result the offspring may actually be less adapted to the environment than the preceeding ancestral population.

For example, a population may have 100 breeding pairs that produce 100 more breeding pairs in the next generation. In the first generation you may have an exact 50/50 mix of a certain allele. The chance that you will see the same 50/50 mix is 1% as is the chance that you will get a 100/0 and a 0/100...all of the potential percentages have an equal chance of occuring. But any deviation away from the 50/50 could have a drastic effect on the following generation. So if you have the next generation with a 40/60 mix of the allele in question, the chances that you will get a 50/50 mix is quite a bit less than 1% and the chances that you will get even less than 40/60 is even greater.


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Speel-yi
Inactive Member


Message 8 of 8 (287296)
02-16-2006 10:50 AM
Reply to: Message 1 by Zawi
02-14-2006 9:14 AM


quote:
A parent gives rise to a child who happens to be in posession of a beneficial mutation. This mutation proves to be so useful that the child manages to reproduce with no problem. If its children also posess the beneficial mutation, then that family line will soon dominate the population, until all members of the population posess the mutation.

IMO, there is way too much attention paid to mutations and not enough to the novel combinations that existing genes may be shuffled into new forms.

The basic textbook definition of evolution is: "The change in gene frequencies in a population over time." It says nothing about mutations accumulating or even if they occur at all.

You could also have a novel combination of existing alleles that would result a new species. Consider the African Elephant that has had its population severly impacted by poaching for its ivory. The result we now have is that there is an increase in the percentage of tuskless males. We also see an increase in the percentage of surviving "loner" males. (These are males that seem to prefer to live away from the herd.)

The end result is that the most likely males to survive to breeding age are tuskless/"loner" males. Both of these alleles already occur in the population and they occur independently. The novel combination would be to have the genes for tusklessness and for males to live away from a herd. These two strategies both work, but in different ways. But given enough time, we will have a genetic bottleneck.

If we were to jump forward a million years and look back at the fossil record, we could possibly see a sudden change in the morphology of elephants and from that it could be assumed that a new species had evolved even though no mutations had actually occured in the population. Evolution had happened with the same gens that had always been there.


This message is a reply to:
 Message 1 by Zawi, posted 02-14-2006 9:14 AM Zawi has not yet responded

  
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