Descent of testicles.

You mention two categories of counter-example.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?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. Thus, as you say: external scrota is not the best solution...unfortunately it was the best one mammalian ancestors had. Evolutionary pressures are decreasing the numbers of mammal species with this less than perfect solution.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. 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?

That's not right, really. The critter and its descendants have to reproduce at a rate at least the same as the other critters in its deme, otherwise the proportion of critters with the mutation will decrease over time.The only exception to this is if the deme has not reached its maximum size with respect to its environment. If the deme has little competition for resources and can grow, then minor disadvantages are less likely to be penalized as much. However, once competition kicks up again, when there are limited numbers who get to successfully reproduce...any disadvantage relative to your deme-mates will tend to lead to the extinction of that disadvantage.A descent of testicles may well increase fecundity, an advantage in a competitive world and so it could spread as long as the accompanying survival disadvantage was not too severe. There may be better ways of increasing fecundity but scrota got there first, and once we had it we'd have to wait until a better solution came along. That seems to have happened to some mammals, but not all of us.

Not entirely ruled out, but not thought to be the prime motivator. The pharyngula article gives several hypotheses and focuses on the last one:

1. The display hypothesis.
2. The cold storage hypothesis (epididymis rather than testes)
3. The training hypothesis.
4. The temperature hypothesis. (testes rather than epididymis)
   

A brief description as well as the authors reservations about them are given.

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.

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The black bars are us, the species that let it all hang out. The most parsimonious explanation for their distribution, the one that requires the smallest number of novel adapations, is that they represent the ancestral condition...the rare red lineages, elephants and manatees most prominently, have dispensed with the whole silly arrangement and keep their testicles safe and deep in the body cavity.

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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. 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. 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 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.

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.

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. 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?

OK, you are catching up. Yes, this is a hypothesis that is based on the evidence presented in the paper. Whose taking anything for granted? All I said was that "The most parsimonious explanation for the phylogeny is that descended testicles represents a primitive adaptation and their ascent represents a more recent adaptation."Do you disagree that it is the most parsimonious explanation? Please provide a better one if you have it. Yes, those are two divisions: scrotal and ascrotal. There is also descended and not descended. Ignoring this latter classification would surely render your topic absurd wouldn't it? You are being now-centric. The phylogeny seems to show an increase in the frequency of ascrotism over time. Whose to say that this pattern won't continue until all mammalian families are ascrotal?What would you expect from an inefficient search process that neodarwinism proposes? Would you expect that disparate lineages would simultaneously find a better solution?

If you prefer you can reword my original phrase to reflect this. Change it to "Yes, this is a hypothesis that is based on the observed facts presented in the paper." I focussed on all of the data, and the patterning I observed. Can you give a more parsimonious explanation for the data in its totality? It's the same explanation. The most parsimonious explanation is that black represents the primitive condition and that grey represents a later type of adaptation. I see that grey universally comes out of black or white lines. Indicating that the black solution is probably ancestral to the grey solutions. What do you think the best explanation for the general pattern of black to grey to red is?It looks quite clear that the number of lineages which are grey increases as we go from bottom to top. I'm not sure how you are reading the chart that would give any other impression. Do you contend that there is a better way to colour the lines that makes it more parsimonious? Please, share your more parsimonious explanation.Edited by Modulous, : No reason given.Edited by Modulous, : No reason given.

I thought you would understand that my insistence on 'most parsimonious' would imply that it was not gospel truth and that you understand the tentativity of science by now. Correct. The paper presents one piece of evidence for the hypothesis that one common ancestor with scrota had some kind of advantage, and the trait became common, but over time even better solutions were came upon.From this one paper alone, that is the most parsimonious explanation. Maybe other papers would give different conclusions.Still, I remain unconvinced that the pressure to develop scrota came about as a result of the cooling problem. I still think that the cooling problem came after the scrota, as a response to a different optimum operating temperature that the scrotum demanded.Once we have to come at least a basic agreement of the paper's contents you have presented I am happy to discuss some alternative hypotheses. And I pointed out that all the paper really says is that scrota are a primitive adaptation to a problem that better solutions came along to. The chart shows that it has taken millions upon millions of years for even a fraction of mammal lineages to solve the problem. Whether or not it is 'difficult' - it certainly doesn't seem to be trivial. I thought the neodarwinian explanation doesn't give an explicit time within which it has to have happened. Could you explain this?The evidence shows that 40 million years isn't enough time for all lineages to arrive at a solution.

Would you be shocked to learn that I agree? That isn't a supposition, it is their conclusion. You'd be on better grounds if you stated that their erroneous supposition is that the phylogeny tree is correct.As Conrad notes:

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What a difference a few years makes!

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And how! Isn't the advance of science great? Clearly the central hypothesis of Werdelin and Nilsonne was testable: since it failed!I had read the interesting (to this discussion) part of the wiki article on 'testicle' earlier and it says:

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The basal condition for mammals is to have internal testicles. Only the Boreoeutherian land mammals, the large group of mammals that includes humans, have externalized testicles. Indeed their testicles function best at temperatures lower than their core body temperature. Their testes are located outside of the body, suspended by the spermatic cord within the scrotum. The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen.[3] There are also some Boreoeutherian mammals with internal testes, such as the rhinoceros.

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It then goes on to give the hypotheses about why we have scrota:

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More efficient. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis...However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes.[4] It was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.[4]

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2) Irreversible adaptation to sperm competition. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster) for sperm competition and thus had to place its testes outside the body.[5] This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they were stuck with enzymes that operated best at cooler temperatures and had to keep their testicles outside the body.

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3) Protection from abdominal cavity pressure changes. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.[6]

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As I stated in a previous post, I don't think the cooling hypothesis is the best one. I prefer the second hypothesis, irreversible adaptation to sperm competition.

I'm still interested in teasing out the details of your alternative hypothesis if you are interested in doing so. Hopefully your arguments against the irreversible adaptation to sperm competition hypothesis would be different than your arguments against the testes cooling hypothesis? If not, then you're probably right - there's no point repeating them.

Ah - it's just a basic observation of body plans. As far as I am aware, this is well documented and discussed, perhaps you can dig up a paper on it for discussion sometime?I suppose, since you have nothing to add I will close with a barb - intended to spur the discussion forward if it is at all possible (that is to say, it isn't personal):

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I think that Modulous knows very well that I will defend my opinion in open discussion.

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And yet Remains undefended as far as I can tell. All you have done is established that there is no definitive evolutionary hypothesis for the natural history of the scrota that we can have any degree of confidence in. I don't see evidence for evolutionary forces standing above those in the consensus view of evolutionary theory (though obviously there is more to it than just mutation and natural selection) and I don't see how this relates to the observation you describe.If you do indeed have nothing to add, the previous comment can be seen as my conclusionary statement in this thread. If it motivates you into further defence of your position, all the better

So your conclusion is that 50 years ago some people were wrong about spermatogenesis in aves? It seems you have simply reverted us back to where we were in Message 43 where we had agreed the cooling hypothesis wasn't the best one given contemporary evidence and data.

Yes, fascinating. This is a debate, remember?Edited by Modulous, : No reason given.