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Author Topic:   what is the evolutionary advantage of...
mick
Member (Idle past 4986 days)
Posts: 913
Joined: 02-17-2005


Message 1 of 12 (186590)
02-18-2005 3:21 PM


Hello everybody,
I have noticed a very widespread misunderstanding of evolutionary theory on this board, so I would like to set up this thread to discuss it and hopefully correct it.
There are currently a number of threads in the Biological Evolution board that ask "what is the evolutionary advantage of trait X". For example, we have "what is the evolutionary advantage of guilt", "what is the evolutionary advantage to religion", and we have an "evolution of behaviour" thread which includes the questions "how is murder, rape, cannibalism etc. justified?" which I take to mean "what is the evolutionary advantage of murder, rape, cannibalism, etc".
I just want to make something very clear. It is extremely important that those who wish to understand evolution realise that evolutionary processes do not necessarily result in evolutionarily advantageous traits. Proponents of evolution on this board seem to think that describing the putative benefit of some trait is helping to support evolutionary theory. It is not! Similarly, opponents of evolution try to think of apparently maladaptive traits like murder or whatever, with the challenge "so how could this have evolved?". This is irrelevant! Processes of natural selection frequently result in maladaptive traits, traits that harm the direct fitness of their bearers, or at least provide no benefit whatsoever.
I will give some examples of traits which have evolved through selection or other evolutionary processes but do not benefit their bearer. I can provide references on all of these if you wish, but I'll wait to see if the thread gets started and if there's any interest before I go through my endnote library!
1. population size and mutational meltdown. Small population size results in the accumulation of harmful mutations, because the efficiency of natural selection is directly proportionate to population size. These harmful mutations spread through small populations due to genetic drift - an evolutionary process. Recent mathematical models suggest that in certain circumstances mutational meltdown can result in extinction of a population.
2. transposable elements and "junk dna". Transposable elements are stretches of DNA that code for their own replication. They are self-splicing chunks of DNA. When they get translated into protein by the cell, they immediately cut the DNA from which they've been translated and insert a new copy of themselves. You can think of them as a kind of nuclear virus. They fulfil no function whatsover to the benefit of the individual that carries them. In fact they probably carry a considerable cost - about half the human genome consists of millions of copies of various sorts of transposable elements. In the newt this gets up to about 80-90% of the genome (!). I gather there is a considerable efficiency cost in terms of dna replication when much of what we're replicating is selfish genetic elements. But natural selection favours genetic elements because they are good at replicating themselves, not because they confer any advantage on their bearers.
3. Cancer (and other genetic diseases)
Cancer is caused by uncontrolled cell growth and division. It's a pretty negative trait, causing many deaths worldwide. Nevertheless recent physiological work suggests that genes causing rapid cell growth are very useful to have when you're a fetus. It's only when you get middle aged that these genes start to cause problems. This is an example of a trait that is adaptive and beneficial at one life stage but negative at another. Genes don't have a "value" independent of the context in which those genes are expressed. Cancer genes are good for you when expressed in a placenta, bad for you when you expressed in a lung.
4. Evolutionary hangovers.
The appendix was useful when it first evolved in the mammalian lineages. In human beings it no longer has any function whatsoever. It probably represents a small cost in terms of tissue production and repair throughout the lifetime of the organism. The fact that genes involved in producing the appendix confer no evolutionary advantage does not mean that they did not evolve by natural selection.
There are many other examples. The important thing to bear in mind that traits do not require "evolutionary advantages" in order for them to become fixed within a species. The value of a trait in terms of fitness depends on the context in which it is expressed - no trait has an absolute value. Fitness is measured in terms of the degree to which genetic material is replicated, not in terms of the number of children one has. There are a whole bunch of evolutionary processes that explain how "maladaptive" or cost-imposing traits arise in populations and become fixed as the dominant form.
We should be very wary of asking questions like "what is the evolutionary advantage of trait X". How can we tell if a trait is adaptive? The usual approach is comparative analysis. We find a similar trait that has evolved repeatedly in different species or pupulations, and test to see if it could have become fixed in these populations or species in response to equivalent environmental pressures.
When somebody asks "what is the evolutionary advantage of guilt" we may not be able to give a valid answer. Species with the social complexity of human beings have only evolved once, so we have no other species with which to test our hypothesis comparatively. When we talk about the evolutionary advantage of human cultural traits we are almost invariably combining a poor understanding of biology with conjecture.
I didn't want to put this post at the end of each offending thread on the board, so I thought I would set up a new thread to discuss issues of "maladaptive traits" and the notion of fitness, if people think it would be useful.
Any biologists out there might like to provide other examples.
Best wishes!
Mick
This message has been edited by mick, 02-18-2005 15:26 AM

Replies to this message:
 Message 6 by Mammuthus, posted 02-24-2005 5:30 AM mick has replied
 Message 7 by macaroniandcheese, posted 02-24-2005 3:09 PM mick has replied
 Message 11 by Brad McFall, posted 03-01-2005 11:08 AM mick has replied

  
AdminNosy
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From: Vancouver, BC, Canada
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Message 2 of 12 (186636)
02-18-2005 8:06 PM


Thread moved here from the Proposed New Topics forum.

  
caligola2
Inactive Member


Message 3 of 12 (186664)
02-18-2005 11:35 PM


I agree
Imperfection is exactly what we should expect from evolution to create..
another good example is wisdom teeths which serves no function, or female maidenhead..
To both maidenhead and wisdom teeths there is no possible function.

  
Gary
Inactive Member


Message 4 of 12 (186668)
02-19-2005 12:30 AM


In a thread that I made, Truthlover reminded me of the Vitamin C gene, which is deactivated in primates. Other mammals are able to produce their own Vitamin C, but primates, including humans, cannot.
I too think it is important to look at the big picture and realize that sometimes natural selection fails to weed out maladaptive traits. Many times organisms can still live long enough to reproduce, even if they don't have the best genes. If they have a mixture of good and bad, it isn't hard to imagine an organism with strengths that outweigh its weaknesses. If an early primate can't produce its own Vitamin C, but has other strengths, such as physical strength or traits attractive to the opposite sex, it might be able to pass on its genes even in a population that can largely still make Vitamin C.

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mick
Member (Idle past 4986 days)
Posts: 913
Joined: 02-17-2005


Message 5 of 12 (186793)
02-19-2005 2:57 PM
Reply to: Message 4 by Gary
02-19-2005 12:30 AM


imperfections of the vertebrate eye
Thanks for the examples! Another good one is the vertebrate eye. The vertebrate eye is built backwards. Photoreceptor cells are at the back of the eye and aimed backwards, away from the light source, and the nerves that carry signals from photoreceptor cells pass in front of the photoreceptor cells, impeding the incoming light. The eye lids of the subterranean mole rat are completely fused and the eye therefore can't be used even when the animal is above ground. This makes sense in the light of evolution, in which modifications are built on top of what existed previously, with no foresight in the process of evolution.
Darwinism has often been accused (by its own supporters) of producing "just so" stories where each adaptation is meant to be optimal for some purpose. But the fact that evolution can account for such anomalies is very important.
How would somebody who believes in an intelligent designer account for these design failures? Was the designer stupid?

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Mammuthus
Member (Idle past 6475 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 6 of 12 (188035)
02-24-2005 5:30 AM
Reply to: Message 1 by mick
02-18-2005 3:21 PM


quote:
2. transposable elements and "junk dna". Transposable elements are stretches of DNA that code for their own replication. They are self-splicing chunks of DNA. When they get translated into protein by the cell, they immediately cut the DNA from which they've been translated and insert a new copy of themselves. You can think of them as a kind of nuclear virus. They fulfil no function whatsover to the benefit of the individual that carries them. In fact they probably carry a considerable cost - about half the human genome consists of millions of copies of various sorts of transposable elements. In the newt this gets up to about 80-90% of the genome (!). I gather there is a considerable efficiency cost in terms of dna replication when much of what we're replicating is selfish genetic elements. But natural selection favours genetic elements because they are good at replicating themselves, not because they confer any advantage on their bearers.
That is not quite true i.e. that there is no advantage. For example Syncytin 1 and 2 in humans (A and B in mice) are HERV/ERV env genes that are critical to the formation of the placenta. Without them, there is no chance for reproduction in primates and murids respectively (perhaps not an advantage but a requirement). The LTRs of several retroelements are critical to the proper function of their neighboring genes. It is also not clear if in general, reverse transcriptase activity is required by the cell and the benefit of RT activity outweighs the burden of dealing with a genome full of transposons. So while I agree that not every trait need be advantageous i.e. finding beneficial mutations is difficult, it would be equally wrong to claim that such a large and diverse part of the genome "junk DNA" has no benefit or consequence for the fitness of the bearer. I agree with the last part of your statement
quote:
We should be very wary of asking questions like "what is the evolutionary advantage of trait X". How can we tell if a trait is adaptive? The usual approach is comparative analysis. We find a similar trait that has evolved repeatedly in different species or pupulations, and test to see if it could have become fixed in these populations or species in response to equivalent environmental pressures.
this needs to be applied to "junk DNA" as well where for example, Syncytin 1 and 2 in humans and A and B in mice are an example of convergent evolution with the trait becoming fixed in the respective lineages.
Dupressoir A, Marceau G, Vernochet C, Benit L, Kanellopoulos C, Sapin V, Heidmann T. Syncytin-A and syncytin-B, two fusogenic placenta-specific murine envelope genes of retroviral origin conserved in Muridae.
Proc Natl Acad Sci U S A. 2005 Jan 18;102(3):725-30. Epub 2005 Jan 11.

This message is a reply to:
 Message 1 by mick, posted 02-18-2005 3:21 PM mick has replied

Replies to this message:
 Message 8 by mick, posted 02-24-2005 7:47 PM Mammuthus has replied

  
macaroniandcheese 
Suspended Member (Idle past 3928 days)
Posts: 4258
Joined: 05-24-2004


Message 7 of 12 (188173)
02-24-2005 3:09 PM
Reply to: Message 1 by mick
02-18-2005 3:21 PM


i would argue that a main source of many of these issues is the idea that somehow humans have completed their evolution and are the pinacle of such. a very arrogant stance and one shared by both those who support evolution and those who don't.

This message is a reply to:
 Message 1 by mick, posted 02-18-2005 3:21 PM mick has replied

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mick
Member (Idle past 4986 days)
Posts: 913
Joined: 02-17-2005


Message 8 of 12 (188265)
02-24-2005 7:47 PM
Reply to: Message 6 by Mammuthus
02-24-2005 5:30 AM


good point
Hi Mammathus,
Thanks for your very useful and up-to-date comments. It's nice to think that eutherian viviparity might be viral. On a side-note... I recall that the syncytin genes are only present in haplorhine and not strespirhine primates. I didn't know they had been found in rodents. But syncytial cell layers are common to all eutherian mammals (and I think, to some extent, marsupials). Did the virus strike repeatedly?
Just wondering, this is a bit off-topic but there you go!
Thanks,
Mick

This message is a reply to:
 Message 6 by Mammuthus, posted 02-24-2005 5:30 AM Mammuthus has replied

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mick
Member (Idle past 4986 days)
Posts: 913
Joined: 02-17-2005


Message 9 of 12 (188266)
02-24-2005 7:49 PM
Reply to: Message 7 by macaroniandcheese
02-24-2005 3:09 PM


hi brennakimi,
You are dead right. But evolutionists have no excuse!
Cheers
Mick

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Mammuthus
Member (Idle past 6475 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 10 of 12 (188342)
02-25-2005 2:49 AM
Reply to: Message 8 by mick
02-24-2005 7:47 PM


Re: good point
Hi mick,
I don't think the same virus struck. In humans Syncytin 1 is coded by a HERV-W env gene and Syncytin 2 is a HERV-FRD env gene (not particularly close relatives though both are gammaretrovirus-like). In mice the ERVs are not related to human ERVs since most mammalian groups have distinct ERVs. So in both groups, env proteins from different viruses took over the same function i.e. convergence. What I have not seen reported is what genes strespirrhines (or marsupials)use to form syncytia. It may be that more surprises of ERV convergence will be found.

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Brad McFall
Member (Idle past 5033 days)
Posts: 3428
From: Ithaca,NY, USA
Joined: 12-20-2001


Message 11 of 12 (189439)
03-01-2005 11:08 AM
Reply to: Message 1 by mick
02-18-2005 3:21 PM


You said,"Cancer/other diseases"
3. Cancer (and other genetic diseases)
Cancer is caused by uncontrolled cell growth and division. It's a pretty negative trait, causing many deaths worldwide. Nevertheless recent physiological work suggests that genes causing rapid cell growth are very useful to have when you're a fetus. It's only when you get middle aged that these genes start to cause problems. This is an example of a trait that is adaptive and beneficial at one life stage but negative at another. Genes don't have a "value" independent of the context in which those genes are expressed. Cancer genes are good for you when expressed in a placenta, bad for you when you expressed in a lung.
My idea on cancer seperates these two
things somewhat categorically but if the notion is more than a mere idea what makes the c-word good or bad might depend on the differential benefits taxagenically of different tissue NONrejection patterns of different subdiploid DNA content TOTAL lengths. These accpetable values might differ by organs. There are at least two different molecular means being proposed to destroy even ostensibly naturally cell chromatin. Perhaps it was more like this idea of mine that you noticed and rejected as nonsense? Yes indeed there is no "sense" as in the "five" senses! but I dont write about other"" genetic diseases and cancer in the same 'line.'

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 Message 1 by mick, posted 02-18-2005 3:21 PM mick has replied

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mick
Member (Idle past 4986 days)
Posts: 913
Joined: 02-17-2005


Message 12 of 12 (189643)
03-02-2005 12:23 PM
Reply to: Message 11 by Brad McFall
03-01-2005 11:08 AM


pleiotropy
Hi Brad,
Thanks for your comments, I apologise once again for the time I used the word nonsense. I was probably in a bad mood that day and wrote undiplomatically. I'm sorry.
I think it is reasonable to consider cancer and other genetic diseases together, to some extent. The thing that unites cancer with diseases such as diabetes, hypertension, diseases of old age - I mean diseases with a "genetic component" even if they are not genetically determined - is that the genes underlying these disorders are evolving under antagonistic pleotropy. They confer benefits on young individuals but costs on old individuals. Many examples are given in the work of Randolph Nesse (index). But of course you are dead right to say that the genetic mechanisms, the architecture of these diseases, are likely very different from each other. These diseases are only linked conceptually by the idea of antagonistic pleiotropy.
Thanks for your reply.
Mick

This message is a reply to:
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