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  #11  
Old 5th May 2012, 01:20 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Originally Posted by simonecuttlefish View Post
Sexual reproduction causes genetic variability, due to the way sex cells divide and jumble up the genetic compliment received from it's male and female parents. This allows for variations such as mutations to be passed on and enter more widely into the genetic variability of individuals of a species. Genetic variability is generally good for a species, as it is one of the mechanisms required for a species to adapt to changing environments, or to adapt over time into a species more suited to it's environment.

Sexual reproduction is here because it works, and distribution of variant genes within a population increases that populations chance of long term survival.

p.s. Sexual reproduction also increases the chance of different mutations ending up in the same individual. Genetic mutations often require the presence of other mutations before a major impact on the organism is expressed, and so mixing it all up increases the chance of these expressions occurring.
This is the old theory, but it's now (according to Matt Ridley's book at least) been refuted. The current theory for "why sex" - which is what this theory was dealing with, rather than why genders, is that most parasites have faster generations than their hosts, and so can evolve to be best adapted to their hosts faster than their hosts can respond. However by having sex you're ensuring that your offspring only have half of your genes, and so the parasites that are well adapted to you are less adapted to your offspring. This also explains why non-sexual reproduction is so common in parasites (especially prokaryotes) but much less common in hosts.
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Old 5th May 2012, 01:52 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Originally Posted by owheelj View Post
This is the old theory, but it's now (according to Matt Ridley's book at least) been refuted. The current theory for "why sex" - which is what this theory was dealing with, rather than why genders, is that most parasites have faster generations than their hosts, and so can evolve to be best adapted to their hosts faster than their hosts can respond. However by having sex you're ensuring that your offspring only have half of your genes, and so the parasites that are well adapted to you are less adapted to your offspring. This also explains why non-sexual reproduction is so common in parasites (especially prokaryotes) but much less common in hosts.
Only mutations cause new variation [in DNA itself], however, sex does mix combinations. As most genes contribute to multiple traits [pliotropy] you can have new variation that way, even if there were no mutations. [Of course there always are new mutations]. The other thing is regulation which effects gene expression, which can either be temporary [epigenetic] or a more permanent change in the regulatory network.
Epigenetics is an odd sort of animal as it can happen in real time [eg the insulin pathway affecting growth & sexual maturity] over a few generations, or become fixed in the popualtion.
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Old 5th May 2012, 02:51 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Only mutations cause new variation [in DNA itself], however, sex does mix combinations. As most genes contribute to multiple traits [pliotropy] you can have new variation that way, even if there were no mutations. [Of course there always are new mutations]. The other thing is regulation which effects gene expression, which can either be temporary [epigenetic] or a more permanent change in the regulatory network.
Epigenetics is an odd sort of animal as it can happen in real time [eg the insulin pathway affecting growth & sexual maturity] over a few generations, or become fixed in the popualtion.
I'm not really sure what your point is? While what you say is true, the so called "Vicar of Bray" theory, which is what I was criticising is a group evolution argument. To explain why something evolved, the explanation has to show why individual genes or combinations of genes would have an advantage, not why a species would have an advantage, which is what the diversity for evolving to environmental change argument is. Things don't happen in biology because they might have an advantage for future generations, depending on the future circumstances, they happen because they have (or had) an advantage for individuals. In the case of sex, it's an advantage of making sure immediate offspring have a stronger defence against parasites.

Last edited by owheelj; 5th May 2012 at 03:20 PM.
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Old 5th May 2012, 03:40 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Originally Posted by owheelj View Post
I'm not really sure what your point is? While what you say is true, The so called "Vicar of Bray" theory, which is what I was criticising is a group evolution argument. To explain why something evolved, the explanation has to show why individual genes or combinations of genes would have an advantage, not why a species would have an advantage, which is what the diversity for evolving to environmental change argument is. Things don't happen in biology because they might have an advantage for future generations, depending on the future circumstances, they happen because they have (or had) an advantage for individuals. In the case of sex, it's an advantage of making sure immediate offspring have a stronger defence against parasites.
The key to understanding evolution is to differentiate between selection on and selection for, and this depends on the definition of an individual. Vicar of bray was wrong, because obviously there is intra-species conflict eg between males, and inter-species conflict [ lions eat gazelle, so they are obviously not working for the betterment of mammals!

However, there is also cooperation. Is a beehive an individual or a group? Is a human an individual or a group of cells? Is Volvox? Lichen?

If there is very close cooperation, then you can conflate that as an individual for the purposes of selection. Selection acts on individual humans for certain traits. Traits [phenotype] is driven by genetics. So there is individual selection for genes [or more properly] gene teams. This group selection can break down in a human eg cancer, where individual cell lines go their own way.

So to answer any question on selection, we have to ask about the level of selection. There may be several levels of selection going on at the same time: the gene level, the cell line, the organism, and yes, the population. But population/species level selection is traditionally referred to as: " species replacement".

See:-
Keller, L. (1999). Levels of selection in evolution, Princeton Univ Pr.

Kin selection is a special case in group selection, where we have the Hamiltonian Relation:

Quote:
Formally, such genes should increase in frequency when
where
r = the genetic relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same locus is identical by descent.B = the additional reproductive benefit gained by the recipient of the altruistic act,C = the reproductive cost to the individual of performing the act.
Source:-
http://en.wikipedia.org/wiki/Kin_selection

Actually, the whole question of kin selection vs group selection is still quite open, mainly because of the question of what constitutes an individual. Dial over to Nature journal Volume 466 for a debate between scientists which I call the group selectors camp and the kin selectors camp!

http://www.nature.com/nature/journal...310/index.html
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Old 5th May 2012, 03:54 PM
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Default Re: Evolution: Why a male and female species of each organism?

Again, I'm not sure what you're point is. How does this relate to whether the "Vicar of Bray" explanation for sex that I was commenting on is true or old and outdated? It seems like you're just randomly offering technical details of evolutionary theory, whose only relation to the discussion is that they're about evolutionary theory (but not about the actual question being asked)?

Edit; I mean, are there group selectionists who still think that theory explains sex or genders?
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Old 5th May 2012, 04:15 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Originally Posted by owheelj View Post
Again, I'm not sure what you're point is. How does this relate to whether the "Vicar of Bray" explanation for sex that I was commenting on is true or old and outdated? It seems like you're just randomly offering technical details of evolutionary theory, whose only relation to the discussion is that they're about evolutionary theory (but not about the actual question being asked)?

Edit; I mean, are there group selectionists who still think that theory explains sex or genders?
Simply, yes, and you would know that if you had read my links. Part of the problem, as I tried to explain, is that if "the individual" is a murky concept, then selection will be murky too. In most situations, a human individual is just that, but it is also a group of cells which normally cooperate. BUT there are times when those cells don't cooperate, and I gave the example of cancer, where one group of cells go off and do their own thing.

With something as complex as biology, one is always going to get counter-factuals-examples where summat does not fit the model. It is best not to think in terms of truth or falsity, in group selection or in anything else.

To give another example, what is sex? Well, it depends on what organisms you are talking about because bacterial sex and sex in eusexuals like humans is like comparing oranges and apples.

And lastly, in science, there is no such thing as "explanation" in the sense that you seem to use it. It is simply about how well a model fits the data. So in that sense, evolutionary theory does fit the data, so long as you know what kind of organism you are talking about. Asexual and sexual organisms are both equally valid and sucessful.

How did sex come about? Noone knows. But we presume natural selection and drift. [NSD].
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Old 5th May 2012, 04:36 PM
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Default Re: Evolution: Why a male and female species of each organism?

Actually neither of your links provides an argument for the Vicar of Bray group selection theory as the reason for sex existing. If they do, please quote the relevant part.

Sex is typically defined as the process of reproduction that occurs through meiosis into gametes, followed by fusion producing a zygote. None of my biology text books define any of the other methods of reproduction as sex, and all of them are consistent on this. Binary fission, conjugation, transformation and transduction are not considered sex by any biology book I've read, or by any biology lecturer I've encountered. Sex definitely has a precise definition in Henderson's Dictionary of Biology ("any form of reproduction that involves fusion of gametes to form a zygote").

I'm not going to play word games - models are explanations. Certainly the term "explanation" is used frequently in many scientific papers, text books, and pop-science book in exactly the manner in which I've used it (do you really want me to start quoting these?)
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Old 5th May 2012, 04:50 PM
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Default Re: Evolution: Why a male and female species of each organism?

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Originally Posted by owheelj View Post
Actually neither of your links provides an argument for the Vicar of Bray group selection theory as the reason for sex existing. If they do, please quote the relevant part.

Sex is typically defined as the process of reproduction that occurs through meiosis into gametes, followed by fusion producing a zygote. None of my biology text books define any of the other methods of reproduction as sex, and all of them are consistent on this. Binary fission, conjugation, transformation and transduction are not considered sex by any biology book I've read, or by any biology lecturer I've encountered. Sex definitely has a precise definition in Henderson's Dictionary of Biology ("any form of reproduction that involves fusion of gametes to form a zygote").

I'm not going to play word games - models are explanations. Certainly the term "explanation" is used frequently in many scientific papers, text books, and pop-science book in exactly the manner in which I've used it (do you really want me to start quoting these?)
Bacterial sex and conjugation are synomyms in university level texts and in science journal articles.

Explanation in science is very specific. Does the data fit the model?, and does any predictions emergent from the model fit with the data of further experiments and observations? That is it! Nothing about truth, reality or anything else metaphysical. It is called methodological naturalism.
Whe Jeey Coyne wrote his book "Why evolution is true" he meant it in exactly the same sense as I have above. Natural phenomena confirm the fact of evolution, and evolutionary theory [plural sense] "explains it. I don't know how to make it any plainer for you.
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Old 5th May 2012, 04:54 PM
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Default Re: Evolution: Why a male and female species of each organism?

I don't understand what it is you're trying to make plain;

"Natural phenomena confirm the fact of evolution, and evolutionary theory [plural sense] "explains it.""

How does that contradict anything I've said?
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