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Author Topic:   Interweaving Evolution & Hybrid Vigor
herebedragons
Member (Idle past 857 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


Message 12 of 19 (771793)
10-30-2015 12:05 AM
Reply to: Message 5 by RAZD
10-21-2015 9:40 AM


Re: Moderator Question -- the basic question for debate
The basic idea behind punctuated equilibria, as I understand it, is that a smaller relatively isolated population undergoes rapid evolution, possibly to adapt it to a slightly different ecology. Then it returns to the parent ecology which has been in stasis (continually readapting to a static ecology) and has an advantage for survival or breeding, and then it takes over and displaces the parent population.
I think you may have a slight misconception about punctuated equilibrium. The basic idea behind PE is that organisms remain relatively stable for long periods of time and then make a sudden evolutionary leap. The idea was proposed to help explain why the fossil record looks the way it does, with organism appearing suddenly, hanging around a while virtually unchanged and then disappearing from the fossil record to be replaced by a derived version. It is often thought of as being in opposition to gradualism, but in reality it is a form of gradualism.
I like the way Allen Orr describes it. The image below is a model that Fisher developed and Orr expanded on.
Travelling towards the center of the sphere is adaptation toward an optima. The red line is the fitness path a hypothetical organism might follow. Sometimes the path doesn't lead directly to a most optimal level, but makes a big change not directly towards the center. Some changes are quite small, but others (one in particular) are rather large. The large step (the third layer in from the outside) is what Gould would have identified as punctuated evolution. Another observation Fisher/Orr made was that as the fitness gets closer to optimum, the steps begin to get smaller. So, there is not likely to be large leaps when the population is close to optimal fitness. I think Orr has a good balance between PE and pure gradualism (including neutral theory).
The question I am raising on this is whether full speciation of this daughter population is necessary, or would not a new varietal phenotype that can interbreed and form hybrids that join the best features in a mosaic of phenotypic traits be a valid or better explanation.
The phenomenon you are describing is called reinforcement.
This appears to be an excerpt from Jerry Coyne's book "Why Evolution is True" and is a good article about reinforcement.
HBD

Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.

This message is a reply to:
 Message 5 by RAZD, posted 10-21-2015 9:40 AM RAZD has replied

Replies to this message:
 Message 13 by RAZD, posted 10-30-2015 7:51 PM herebedragons has replied
 Message 14 by RAZD, posted 10-31-2015 6:44 PM herebedragons has replied

  
herebedragons
Member (Idle past 857 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


(2)
Message 15 of 19 (771919)
10-31-2015 11:25 PM
Reply to: Message 14 by RAZD
10-31-2015 6:44 PM


Re: The Hardy-Weinberg ratio
The Hardy-Weinberg ratio explains quite easily why mutations or rare alleles are more likely to be spread in a small population than a large one:
H-W is one of the most basic principles in population genetics and yet is one of the most often misunderstood. H-W by itself does not address changes in allele frequency but describes what genotypes will be present at any given allele frequencies after one round of mating.
Let's say we have an allele with a frequency of q = 0.10 so that p = 1 - q = 0.90. What H-W says the genotypes would be AA = 0.81, Aa = 0.18, aa = .01. But notice that p still equals 0.90 and q still equals 0.10, no change in allele frequency. Without some force that changes the allele frequency, the frequency will remain q = 0.10.
Here's where H-W gets useful; let's say we find the population has an allele proportion of q = 0.10 and p = 0.90 but the measured genotype frequency is AA = 0.828, Aa = .162, aa = 0.028. We recognize this to be out of H-W equilibrium and so one of the 5 assumptions must not be valid
1. mating is random
2. population is infinitely large
3. no migration
4. no mutation
5. no selection
In this case, mating is not random but shows signs of inbreeding (the proportion of heterozygotes is reduced). We can estimate the inbreeding coefficient by F = (HO-HE)/HO where HO is heterozygotes observed and HE is heterozygotes expected. In this case F = 0.10 which means 10% of the population is autozygous.
Now I was a little confused at first by the "reached after a single generation" comment, until I realized that this applies to each generation and changes with selection acting on the relative fitness of p and q.
Right, so we start with an allele frequency and determine the genotype. The population may then be subjected to selection which will remove an uneven proportion of alleles, ie. one allele will increase in frequency, the other will decrease. Selection models are kinda involved and I won't take time on it here, but let's say that the genotype aa is advantageous and the allele q increases from 0.10 to 0.15 because of selection. We now need to do the H-W calculation again to determine the genotypes of the next generation AA = 0.723, Aa = 0.255, aa = 0.022. So, an increase of 50% in allele frequency resulted in a 120% increase in 'aa' genotype frequency.
But certainly for a new mutation allele the q will be necessarily small, and thus it's relative proportion within the population will be overwhelmingly swamped by p in very large populations but not so much in very small populations.
Definitely true. Unless q confers a large fitness advantage, I would expect drift would be the primary process that would increase its frequency in a small population. Mutations can move to fixation in small populations rather rapidly, especialy when they have even a small fitness advantage.
If you are interested in a really good, easy to understand book on population genetics, I highly recommend A Primer of Population Genetics. We used it in my Evolutionary Biology course and it is an excellent introduction to population genetics. Even if you have a more advanced understanding of the material, it is still a really good book.
HBD

Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.

This message is a reply to:
 Message 14 by RAZD, posted 10-31-2015 6:44 PM RAZD has replied

Replies to this message:
 Message 18 by RAZD, posted 11-01-2015 11:52 AM herebedragons has not replied

  
herebedragons
Member (Idle past 857 days)
Posts: 1517
From: Michigan
Joined: 11-22-2009


(2)
Message 16 of 19 (771920)
10-31-2015 11:47 PM
Reply to: Message 13 by RAZD
10-30-2015 7:51 PM


Re: punk eek and interwoven evolution \ hybrids
... but was the old population replaced or is it a hybrid population, mixing the best adaptations from the isolated population and the static population? Certainly the derived new population is a mosaic evolution from the static parent population with some new traits and some old traits,
Ahhh, I see where you were going with this. I certainly agree that evolution is not a straight-line linear progression, but favors a mosaic, as you say.
For instance here are a pair of different interpretations of pelycodus and copelemur evolution:
I don't think I have seen the Copelemurs on the charts before. Interesting. Is there a cladogram for this group? Whether the trend appears gradual or punctuated depends on where the branch is connected to the main trunk. What I don't like is that the author connected the red line to the mean of the different groups, but just made a basically straight line for the gradualism chart.
In both interpretations the first branch appears to die out or get reabsorbed, and I (not surprisingly) favor the reabsorbed with new mutations added back into the main population. Also I see that the whole population shifted to the left as this occurred, which is another reason why I see reabsorbtion as a viable option here.
I think you make a good case for that.
One of my dad's pet comments was that evolution in general and human evolution in particular displayed mosaic evolution, essentially where parts evolve rather than the whole individual. He thought it was underplayed in the field.
I think it is just a difficult and complex thing to nail down. We are just getting to the level where we can handle the kind of data needed to understand population, community and genetic networks. It's an exciting time to be a scientist!
Or am I smoking some kind of crack?
Naw, Medical marijuana maybe?
HBD
Edited by herebedragons, : No reason given.

Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.

This message is a reply to:
 Message 13 by RAZD, posted 10-30-2015 7:51 PM RAZD has replied

Replies to this message:
 Message 17 by RAZD, posted 11-01-2015 10:17 AM herebedragons has not replied

  
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