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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
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Author | Topic: Descent of testicles. | |||||||||||||||||||||||
Modulous Member Posts: 7801 From: Manchester, UK Joined: |
An external scrotum presents more direct health cues (size, smell etc.) for females wishing to reproduce. Not entirely ruled out, but not thought to be the prime motivator. The pharyngula article gives several hypotheses and focuses on the last one:
A brief description as well as the authors reservations about them are given.
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nwr Member Posts: 6408 From: Geneva, Illinois Joined: Member Rating: 5.1 |
I. If such changes were possible in reproductive system of birds I see no reason why such changes in reproductive system would not have been possible in mammalian lineages too. It is only speculation that it was somehow impossible.
That seems to presuppose that evolution is purpose oriented, and will seek the best solution possible. I don't know of any support for such a view of evolution. Surely evolution is opportunistic, and takes a path that is good enough and that happens to be possible given the available variation within the genome (including variation provided by new mutations). And if it is opportunistic, there is no basis for expecting that it will find the best possible solution.
Now you consider sperms seeking lower temperature to be the source of their descent.
Sorry, but the idea of "sperms seeking lower temperature" is silly. Please don't ascribe that idea to me.
The opposite view - their lower temperature as adatation on environment is also possible.
I don't rule that out. Science answers the "how" questions, not the "why" questions. To me, the traditional explanation looks like a reasonable hypothesis. But it is only a hypothesis. I don't have a problem acknowledging that we don't really know.
We have two poles.
You have not been clear on what are these two poles. Let's end the political smears
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
Would you agree that if the animals that have internal testicles had either an alternative way of cooling their sperm, or had sperm that was not damaged by their body temperature then the cooling hypothesis still stands?
I consider all these "if" unreal examples as dialectical and off-topic.
The paper cited by PZ is quite interesting. The most parsimonious explanation for the pattern of scrotumless mammals is that the adaptation came no earlier than the common ancestor of golden moles and elephant shrews, with some independent adaptations in the monotremes and one group of tenrecs.
I cannot find there your deduction about golden moles and elephant shrews. Could you please quote it?
Thus, as you say: external scrota is not the best solution...unfortunately it was the best one mammalian ancestors had.
How are you so sure? Authors are more carefull- they call their hypothesis "untestable".
There are many examples of less than perfect solutions, and I don't see how a paper that gives evidence of less than perfect solutions being replaced by better solutions helps your theory that this is not neodarwinian in nature.
What I proposed is that neodarwinian explanation is wrong. If you insist that you posses valid explanation of bad solution so I respect it.
We also observed increasing structuring of mammalian bodies in areas that you have not designated 'poles'. How does observing an increase in structure contradict neodarwinian evolutionary predictions?
I've never heard about neodarwinian explanation of polarization head vs reproductive part of body. That is what I can really see. Of course I do not insist that everyone can see it same way. Maybe you do not see it and consider it as pure speculation.
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Modulous Member Posts: 7801 From: Manchester, UK Joined: |
I consider all these "if" unreal examples as dialectical and off-topic. You provided what you thought were counter-examples to the cooling hypothesis. You think that even considering ways in which their status as counter-examples might be challenged to be off topic? I'm not sure how that works.
I cannot find there your deduction about golden moles and elephant shrews. Could you please quote it? quote: I then looked at the phylogeny diagram for the red lineages. On the one side we have Chrysochlorida and on the other we have Macroscelidida. The descendants of their common ancestor are testicond strongly suggesting that testicondy originated with one of their common ancestors.
How are you so sure? Authors are more carefull- they call their hypothesis "untestable". Let's clarify a little here. They call their proposed evolutionary history of the scrota untestable at present. However, they do provide positive evidence for the proposal that testicles are a primitive adaptation that some mammal lineages have done away with for superior adaptations. The evolution of the scrota is more up for debate, but the evolution away from them is on more solid ground in view of this paper.
What I proposed is that neodarwinian explanation is wrong. Yes, and I suggested that the paper you cited does not support this position. How do you think repeating your position would advance the discussion?
I've never heard about neodarwinian explanation of polarization head vs reproductive part of body. I have never seen evidence of it, perhaps what you see is a private illusion? In order to convince me that this polarization exists I'll need to see actual evidence if you don't mind, not just appeals to subjectivity.
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
That seems to presuppose that evolution is purpose oriented, and will seek the best solution possible. I don't know of any support for such a view of evolution. Surely evolution is opportunistic, and takes a path that is good enough and that happens to be possible given the available variation within the genome (including variation provided by new mutations). And if it is opportunistic, there is no basis for expecting that it will find the best possible solution.
It's your neodarwinian view and I have nothing to add.
Sorry, but the idea of "sperms seeking lower temperature" is silly. Please don't ascribe that idea to me.
No it is not as silly as it sounds. Do you consider sperms to be dead matter or what? I suppose it not as impossible idea that they can differentiate between different temperature. But it is only thought. Silly is darwinian explanation of it. I don't know where originally testicles should have been placed in "common ancestor". Now moving by random mutation througout the body survived only those individuals whose testicles moved along some non-speciefied temperature gradient inside body or what?
To me, the traditional explanation looks like a reasonable hypothesis. But it is only a hypothesis. I don't have a problem acknowledging that we don't really know.
That's fine. I didn't expect such answer here.
You have not been clear on what are these two poles.
Head - centre of individuality. The opposite pole - centre of reproduction of species. Clearly visible in higher mammals.
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
I then looked at the phylogeny diagram for the red lineages. On the one side we have Chrysochlorida and on the other we have Macroscelidida. The descendants of their common ancestor are testicond strongly suggesting that testicondy originated with one of their common ancestors.
Look. We should be first clear what the cladogram or what it is means. I have before me Evolutionary chart of mammalia, the old morphological as well as the new molecular. In boths charts I see Lagomorpha and Rodentia having common ancestor and only their common ancestor having common ancestor with primates. The same here: http://whozoo.org/mammals/mammalianphylo.htm In the chart in Myers article primates and lagomorpha has the common point sooner than with rodentia. What this chart should represent? Is it evolution of mammals or what?
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Modulous Member Posts: 7801 From: Manchester, UK Joined: |
In the chart in Myers article primates and lagomorpha has the common point sooner than with rodentia. What this chart should represent? Is it evolution of mammals or what? The exact placing of lagomorphs is subject to some dispute. They were originally placed close to the rodents, then some evidence emerged that they should be placed where we currently see them. More recent evidence has the potential to shift them back under the glires clade. Quite fitting that the lagomorphs should do a bit of hopping These particular phylogenies do not have them under the glires clade but have them closer to the primates. However, this chart represents the relationships of the various mammals. The places where branches meet are common ancestors.
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
That chart do not agree with my charts not only in this point, but let it be preliminary.
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PaulK Member Posts: 17822 Joined: Member Rating: 2.2 |
Firstly, I strongly recommend that everyone who wants to discuss the this point reads the blog post referenced by the OP - carefully
Here it is again:
quote: Here is the first error. It is not a "cooling sperm" explanation - it is about cooling the testicles. It is a known fact that in some mammalian species raising the temperature of the testicles can cause infertility. This old paper is one example and it references more. This fact lends some plausibility to the idea that cooling is the reason - certainly it is needed in those species. Neither the example of birds nor those mammals that do have undescended testicles pose a serious problem for evolution. The blog post gives sufficient information to produce a plausible outline as follows: Evolution - as an undirected process, relying on random variation does not automatically jump to the best solution. More likely it will promote the first "good enough" solution that comes along - even if it brings other problems with it. We know that the problems associated with descended testicles are not sufficient to endanger the species that have them, through the obvious fact that those species survive. It is not implausible to suggest that increased fertility (a definite evolutionary advantage) more than compensates. So, the line which the mammals are descended from adopted the solution of descended testicles in a scrotum. Some mammals have evolved other solutions - and then the descended testicles are only a disadvantage. So natural selection would favour undescended testicles in those species. This explains not only why some mammals do not have descended testicles, but the pattern found in the phyologeny, as the blog post explains. Birds come from a different line altogether - and, given the undirected nature of mutation, it is not surprising that they ended up with a different solution to the problem.
quote: The pattern observed in the phyologeny indicates that mammals inherited the trait of descended testicles from pre-mammalian ancestors. In some species this condition has been reversed. How does this fit an "increasing structuring" ? Edited by PaulK, : No reason given.
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Granny Magda Member Posts: 2462 From: UK Joined: Member Rating: 4.0 |
MartinV writes: Granny writes: If the process was directed (and you neglect to mention who your director might be, but I think I get the hint) then why lower the ovaries at all? Why not descend the testicles and leave the ovaries where they were? Of course, if the whole business evolved, it might make sense... I don't see your point. The process of polarization involved both sexes. My point it simply that if the human body were designed, then there would be no need for the designer to cause both ovaries and testes to descend, when he only intended to affect testes.Both organs displaying similar properties suggests that they both evolved under the influence of the same basic adaptation. By the way, this "process of polarization" that you mention has been part of our basic phenotype since the evolution of the flatworm. This is why you don't see organisms with a head at each end. Incidentally, if you think that external testes are such a poor model, what makes you think that they were designed? Unless, of course, you think that they were designed by an idiot. Mutate and Survive
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AnswersInGenitals Member (Idle past 151 days) Posts: 673 Joined: |
Another crucial question, the answer to which I think would help inform this discussion, is whether bats and possums, which gestate and brood their offspring while hanging upside down, produce males with ascending testicles. Also, there is a pretty obvious explanation as to why birds would evolve a solution to gonadal temperature control other than descendable testicles. Can you imagine the problems involved in flying with all that baggage dangling in the wind? Not to mention the inconvenience of snagging those appendages on tree branches.
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
Evolution - as an undirected process, relying on random variation does not automatically jump to the best solution. More likely it will promote the first "good enough" solution that comes along - even if it brings other problems with it. We know that the problems associated with descended testicles are not sufficient to endanger the species that have them, through the obvious fact that those species survive. It is not implausible to suggest that increased fertility (a definite evolutionary advantage) more than compensates.
That discussed scrotal testicles has nothing to do with any kind of neodarwinian best or worst solution has been demonstrated by elephants. They testicles are inside their bodies near the kidneys. Having their penis 1 meter long evolution has somehow found the best solution and behold - cooling spermatozoa or testicles is no problem anymore. One should wonder what kind of force it is that has prevented other species like horses, deers, lions or apes and their ancestors to find the same solution for more than 50 million years.
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MartinV  Suspended Member (Idle past 5829 days) Posts: 502 From: Slovakia, Bratislava Joined: |
The evolution of the scrota is more up for debate, but the evolution away from them is on more solid ground in view of this paper.
Dialectical approach. Let us consider or reconsider the evolution of descent of testicles from as many points that no one knows anymore what we are discussing. The problem of descent of testicles is unresolved - on my opinion at least - but authors would like to solve ther ascent first. The same authors claim:
quote: The problem has been solved by elephants. The alternative solution to costly process in mammals exist. What's the problem?
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Modulous Member Posts: 7801 From: Manchester, UK Joined: |
Dialectical approach. Let us consider or reconsider the evolution of descent of testicles from as many points that no one knows anymore what we are discussing. You seem to be alone in not being able to keep up with what is being discussed in the paper. It is quite simple. The most parsimonious explanation for the phylogeny is that descended testicles represents a primitive adaptation and their ascent represents a more recent adaptation.
The problem has been solved by elephants. The alternative solution to costly process in mammals exist. What's the problem? There isn't a problem. The paper states that the scrota solution would be 'lost in mammal lineages as soon as an alternative solution' is found. The phylogeny shows that the lineage which includes elephants has indeed lost it. Why do you think there is a problem?
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PaulK Member Posts: 17822 Joined: Member Rating: 2.2 |
quote: It demonstrates nothing of the sort. Modern elephants have a better solution - as your yourself say. Therefore natural selection would favour that solution over descended testicles - when it arose. The evidence we have is entirely consistent with the idea that this solution arose in a species which had descended testicles.
quote: Why would there need to be a "force" preventing them ? A Darwinian view allows for a number of other possibilities. Either the necessary mutations have not arisen, or they would not constitute an immediate advantage to the ancestors of these species, or they have aquired other mutations which are in conflict with the changes needed for the elephant's solution.
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