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Author Topic:   Evolution Simplified
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 126 of 170 (311440)
05-12-2006 3:06 PM
Reply to: Message 117 by crashfrog
05-12-2006 12:21 AM


Clarifying sexual recombination
Crash writes:
That's sexual recombination.
Hi again Crash
Actually, what you are talking about is simply shared parental genetic representation in the offspring of diploid species.
Sexual recombination refers specifically to the rearrangement of genes on individual chromosomes that occurs during the formation of gametes in meiosis because of the phenomenon of 'crossing over'. Your sperm don't carry an exact single copy of each one of your chromosomes, but each carries a unique mix of alleles from the two copies.
The consequence of this process is called 'independent assortment' of alleles - genes are inherited independent of their (temporary) association with other genes on the same chromosome. It means that even though you have half the genes from your mother and half from your father they are all mixed up in completely different chromosomal combinations when the chromosomes of the gametes segregate, even though each allele retains its specific locations on a chromatid.
Violations of this general rule do occur, and 'two locus' effects can be detected when disportionate numbers of either 'coupling gamates' or 'repulsion gametes' can be demonstrated. This is refered to as a 'linkage disequilibrium' and was once hypothesized to indicate incipient 'supergene' formation.

This message is a reply to:
 Message 117 by crashfrog, posted 05-12-2006 12:21 AM crashfrog has not replied

Replies to this message:
 Message 128 by robinrohan, posted 05-12-2006 3:25 PM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 131 of 170 (311465)
05-12-2006 4:07 PM
Reply to: Message 128 by robinrohan
05-12-2006 3:25 PM


Re: Clarifying sexual recombination
Since mutation occurs randomly, albeit not with equal probability on all portions of the genome, the odds are that we all carry a number of mutations that occurred during the formation of the gametes that gave rise to us when they formed a zygote. Fortunately, most of these probably have little or no effect on our fitness or we wouldn't be here.

This message is a reply to:
 Message 128 by robinrohan, posted 05-12-2006 3:25 PM robinrohan has replied

Replies to this message:
 Message 133 by robinrohan, posted 05-12-2006 4:15 PM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 138 of 170 (311487)
05-12-2006 4:37 PM
Reply to: Message 133 by robinrohan
05-12-2006 4:15 PM


Mutations
They (AbE *heritable* mutations) can occur at any time in the development of a germ cell line(the cells giving rise to sperm and eggs), but they are most likely to occur during a cell division because this is when all the genes are copied. Any other type of mutation is termed a 'somatic' mutation and is not heritable (as kuresu correctly notes).
This message has been edited by EZscience, 05-12-2006 03:41 PM

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 155 of 170 (311698)
05-13-2006 6:01 AM
Reply to: Message 153 by robinrohan
05-12-2006 7:43 PM


Population increase
robin writes:
Wouldn't it depend on how long they've been increasing and what rate?
Not really, it depends more on the scale of your measurement.
A population might seem to be increasing one year, but usually decreases the next. If you take a more appropriate, longer term, scale (which hinges critically on the generation time of the species in question) then you almost invariably see that such fluctuations are temporary and oscillations occur around an equilibrium point that is a relatively steady, long term average (unless the population is in a tailspin due to human exploitation or habitat destruction). Hence we have the study of 'population dynamics', the analysis of the patterns of these fluctuations and the forces contributing to them.
robin writes:
Perhaps most species are increasing but haven't been doing so for very long, or perhaps the rate of increase is so small we don't notice it. Is that theoretically possible?
That would not be consistent with the majority of long-term data sets we have for changes in abundance of many species. More to the point, why would you expect to observe such increases? The fact is, most organisms produce more offspring than can possibly survive, especially the simpler, 'r'-selected organisms, and yet their populations oscillate around equilibrium points.
Differential juvenile mortality is typically what accounts for this, and this is why all species are evolving gradually: the survivors always possess a distinct subset of the genes of their progenitors. (And that's not to say they don't ocassionally evolve more rapidly, as well). Think of the number of acorns an oak tree produces every year, and yet the forest will seem to have a stable number of mature oak trees over the years.

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 Message 153 by robinrohan, posted 05-12-2006 7:43 PM robinrohan has not replied

Replies to this message:
 Message 156 by NosyNed, posted 05-14-2006 12:54 PM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 157 of 170 (311820)
05-14-2006 8:53 PM
Reply to: Message 156 by NosyNed
05-14-2006 12:54 PM


Re: Skirting the issue
Nosy writes:
Shouldn't the absolute need of an excess of births be shown first?
Superficially, that would seem to be the simple inverse of the argument. But you need to clarify your use of the term 'need'. What entity (unit of selection) has this 'need' of excess births?

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 Message 156 by NosyNed, posted 05-14-2006 12:54 PM NosyNed has replied

Replies to this message:
 Message 158 by kuresu, posted 05-14-2006 9:30 PM EZscience has replied
 Message 159 by NosyNed, posted 05-14-2006 11:51 PM EZscience has replied

  
EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 160 of 170 (311882)
05-15-2006 6:51 AM
Reply to: Message 158 by kuresu
05-14-2006 9:30 PM


Re: Skirting the issue
kuresu writes:
most young die.
Yes. This is pretty basic, and I think you and I and Ned can see that, however, we have to choose the words carefully as we try and explain it, for example:
kuresu writes:
In order to ensure that the population survives,
This is not a good choice of words because whether or not the population or species survives is not a selective force on the evolution of traits within it.

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EZscience
Member (Idle past 5154 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 162 of 170 (311898)
05-15-2006 7:41 AM
Reply to: Message 159 by NosyNed
05-14-2006 11:51 PM


Re: Selecting for excess births
Ned writes:
a problem with "assuming" that populations do reach some maximum value and may (or may not) then hoover around that
This is the concept of a 'population equilibrium', in terms of numbers of individuals as opposed to the frequency of a gene. The stability of this numerical equilibrium will vary considerably among species, but it is an inevitability for all species. However, change the ecological context or some force of selection and a new equilibrium point may be established. So the problem is often that when we sample a population, we don't know if it is actually at equilibrium, or in the process of approaching a new rquilibrium. The existence of an equilibrium is a theoretical extrapolation, but one which is quite supported by many long-term demographic studies. I'm going to provide an example, but first I want to finish my previous point. You said:
Ned writes:
...the absolute need of an excess of births
In your mind you are thinking that the 'population' needs excess births to sustain itself at an equilibrium, and this is true, but the population, as an entity, hasn't got any 'needs'. I know it may seem pedantic, but that is why I don't think we should use the term 'need' in reference to a population or a species. Only individuals have 'needs'. As you quite rightly point out:
quote:
The individual is the unit of selection.
At least the primary unit of selection. So it would be an improvement to state that individuals have been selected to maximize their fecundity up to the point where their fertility becomes limiting (produce as many offspring as is possible without compromising the quality, or 'survivability' of those offspring) and the latter is the key factor which varies greatly among species. That's why some animals produce thousands of offspring with survival chances of 0.0001 and others produce tens of offspring with survival probability 0.1 or of that order. So let's consider an example of population dynamics theory and natural selection in application.
It has recently been demonstrated that the average size of the many of the worlds most prized marine fish species is declining (having trouble finding the article on this from last month, but I'll add it be edit if I find it. Along with this, we know that the size of the stocks of many species like Atlantic cod have been declining numerically. What happened to our former equilibrium and why is it not being recovered?
So you might say, "Well we are simply removing more fish than the population can replace by overfishing", and that's part of the story. But there's more to it. We have, through artificial selection, actually driven these populations to new, lower, equilibrium values and even with extensive moratoriums on fishing of cod on the Grand Banks of NFLD for many years, these stocks show little sign of recovery yet. Why not? We have relaxed the selection?
The answer is, because for many decades we actually been selecting for a different life history in the fish population, namely earlier maturity at smaller and smaller sizes. This is because the net sizes for fishing have always been set so as to catch the largest fish.
But the problem is that fecundity scales with size very strongly in fish (bigger females produce way more eggs than smaller females). And it is the large females that really provide a disproportionate number of the 'excess births' at population level. So when we selectively remove the largest fish, we are selectively removing a very important contributor to the birth rate of the population. What we end up with is a lower population of fish that spawn at very early ages and smaller sizes because the odds of making to become large have been so dramatically reduced by the way in which commercial fishing is conducted. There are still 'excess births' in the population compared to the number of fish maturing, but we have forced the population to a new equilibrium of much lower numbers and much smaller body size. We now need to alter selection so as to favor larger body sizes (enforce modes of fishing that do not select so strongly against large fish), but even then, changes in life history trends in a population can take many years to evolve.

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