If macro-evolution is to be considered the complete answer to our existence on earth please present some evidence or at least a scenario as to how life forms progressed from asexual reproduction to sexual reproduction for both plants and animals.
I don’t have a scenario offhand (other than “boy meets girl, boy loses girl, boy gets girl back” 
), but I do remember that the rationale behind progressing from asexual to sexual reproduction is that combining DNA from two different sources is supposed to give Mother Nature lots more options, more room to fiddle around with things. “Let’s try 10 legs this time and see what happens…” You get more chances for random, possibly useful mutations.
I think it was when they started Baywatch on TV. It just looked like more fun.
Seeing as how you want answers from a macroevolutionary viewpoint, here is one possible explanation.
I have no idea what was meant by the statement, “If macro-evolution is to be considered the complete answer to our existence on earth…” Macroevolution isn’t the complete answer, nor should it be considered as such.
Darwin’s Finch, your puzzlement with that statement is because creationists are using (and abusing) Macroevolution to say that
sexual reproduction could not have come about through evolution:
I can see the predictable “both sexes of each species would have to evolve simultaneously for macro evolution to be true” phrase coming up, but:
http://icarus.cc.uic.edu/~vuletic/cefec.html
“Even within humans, sex is not a case of being different from the moment of conception. The early human embryo (XY male or XX female) is sexually dimorphic (has both embryonic male and female parts). Normally a single gene (on the Y) sets in motion a cascade of events that leads to the emphasis of the male internal parts (Wolfian ducts) and degeneration of the female parts (Mullerian tract). Absent that gene, the male parts degenerate and a female develops. Most of what we call male and female traits are purely hormonally caused and depending on the hormonal environment you can get interesting events. I have a lovely picture of a busty young woman who is XY (she has a defect that makes her cells unable to respond to androgens). Internally, she is sterile because the genes that determine the degradation of the Mullerian tract are different. She also has no pubic hair (because that requires a cellular response to androgens). There are also XX individuals with various levels of penis and (empty) scrotum formation because of in utero exposure to androgens (the mothers had a tumor or took certain steroidal drugs). The external genitalia equivalents are very simple scrotum = labia and penis = clitoris. There are all kinds of intermediate situations since this is a question of different differentiation of organs rather than de novo creation of different organs. All this (and much more) can be found in most textbooks of development or genetics. But, amazingly, most creationists seem completely ignorant of these basic facts and somehow think that male and female were created separately. (Hershey 1996)”
** Darwin’s Finch **
Thankyou for the links. I must say though that they do not explain the difficulty of simultaneous transformation of two separate living entities mutually dependant in a very unique way to reproduce. Your second link clearly provides the answer as to how a new sexual species requiring similar mutation to occur in both male and female can result from viral infection in the parent population,and the advantages of sex are clear as well, but that does not address the problem how the first male and female differentiated and mutated from the parent population. What was the mechanism?
With regard to your puzzlement on the phrase" complete answer to existence", I believe there are basically only two camps, evolution or creation. However there are many like myself that believe that reality can encompass both. To disassociate my self entirely from a belief in creation and God, I most certainly would need a plausible mechanism initiating the first sexual encounter like viral infections for species mutation as discussed above to make the leap of faith and embrace evolution without God.
You seem to be asking two separate questions here grienspace. Firstly you want to know “how life forms progressed from asexual reproduction to sexual reproduction”. The answer to this is probably lost in the dim imists of the cambrian radiation. However speculate I can. Organisms became multicellular for various reasons. Organisms also became sedentary, as opposed to being free swimming. There are reasons for this I can go into but for now I’ll take it as a given. Sedentary organisms, being essentiallly immobile could only disperse by allowing some cells or cell clusters to break off and drift on the ocean currents. Eventually organisms developed cellular differentiation: stinging cells, nerve cells etc. Sedentary organisms developed specialised dispersal cells that could actually swim via flagella etc. For all intents and purposes zoospores. These zoospores when they reached a suitable colonisation site divided and became sedentary. Like all cells they developed various connections through the cell walls of adjacent cells. Now if two zoospores from separate organisms were to form such a connection the new colony would develop faster because there ar twice as many starting cells and potentially twice as many genes at work. It’s relatively easy in practice to force such a connection when grafting plants. From their it’s conceivably only a short step to complete fusion of the cell contents (plasmogamy), resulting in a binucletae organism where every cell can utilise all the genes of both nuclei. From their we need only get nuclear fusion (karyogamy) and we’ve got a diploid organism. While this would give an advantage for some time eventually the genomne would become huge, so meiosis would need to develop, at least for the zoospores. How plausible this is I don’t know. We then have sexual reproduction with alternation of generations, the presumed ancestral form. A process very similar to this, complete with plasmogamy followed by karyogamy is observed amongst some of the fungi. Of course at no stage do we have “simultaneous transformation of two separate living entities mutually dependant in a very unique way to reproduce”. We don’t need to because these earliest sexual lifeforms would have been equally able to reproduce sexaully and asexually, just as most microbes and plants can todya.
Then you ask “how the first male and female differentiated and mutated from the parent population”. That’s a completely separate issue. Most organisms aren’t differentiated male and female, they’re hermaphroditic. The number of dioecious plant species could probably by counted on your fingers and even amongst invertebrates hermaproditism is very common. Nonetheless I’ll give this a shot but you can’t really ask for a macroevolutionary answer because it’s not like all aniamls are either hermaphroditic or single gender. Many are geneticlly hermaphroditic and physically gendered and it’s from these that genetic sexual dimorphism will have sprung. Consider this:
We have a hypothetical ancestral animal, we’ll asume a chordate. It’s hermaphrodite. At spawning time it releases clusters of eggs and sperm simultaneously into the ocean currents. This happens amongst a lot of invertebrates. There is a problem with this. There is a good probability the gametes realeased will fertilise each other, meaning that no new genetic material is added to the offspring. This kind of defeats the purpose of wasting all that energy on external fertilisation. So if a mutant appears that only produces eggs at any given time it will have an advantage in that all its gametes will be fertilised by the sperm from other individuals. Similarly for a mutant appears that only produces sperm. So of the offspring of a solely sperm producing animal half will contain a gene that produces only one type of gamete, and half will produce both types. Now eventually, if outcrossing has a big enough survival advantage only those organisms producing one type of gamete at any given time will be found, so the only possible sperm donor for a solely egg producing animal is a solely sperm poducing aniaml and vice versa. Hey presto the offspring will either inherit a working gene for sperm, or a working gene for eggs, but never both. In other words we have gender.
Now in reality the mutations were apparently more commonly of a type that only allowed the production of one type of gamete at any given time, but the organisms are still potentially hermaphroditic. Not a great problem since all we need to do is to arrange for the hormone that causes sperm production to inhibit egg production and vice versa. There are numerous organisms that are genetically neither male nor female, yet have separate sexes. Many, if not most, fish have no genetic gender. In many cases they start out life male producing sperm etc, and then when conditions are right transform into female form and produce eggs. The species’ have two genders and yet this is only hormaonal, not genetic. No Y chromosomes.
Even closer to home many reptiles also have two genders but no genetic gender. Crocodilians are a classic example. All crocodile eggs are laid physically female in a mound of sand, mud and rotting vegetation. If the temperture gets too high the embryos transform to male. If it stays low they remain female. Once this has been set it can’t go back, unlike fish. A croc born male stays male.
So if you want to take it to the level of mammals where the sexes are permanently differentiated genetically all we have to do is get a mutant croc that can’t produce eggs at all, ever. This animal isn’t at a great disadvanatge so long as there are enough hermaphroditic females around. Any mutations that give this animal traits like increased aggression and territoriality can now have a survival advantage to its descendants, since there is no need for a totally male croc or its descendants to minimise agression and territoriality towarsd young. Such male crocs can carry genes that give them an advantage over genetic hermphrodites and will produce more young as a result. Very rapidly the gene makeup of the population will no longer be strictly hermaphrodite, but male/hermaphrodite. If other crocs, later or simultaneously, mutate into genetically female form they can also gain a survival advantage from various genes, such as genes for maternalism, that would be a disadvantage to half their offspring if they were hermaphroditic. The genetic makeup of the crocodile population will eventually switch from male/hermaphrodite to male/female because single gender animals have an advantage. Again we don’t need “simultaneous transformation of two separate living entities mutually dependant in a very unique way to reproduce” because some animals are genetically both male and female alongside the developing single gender animals.
Simple eh?
I hope that made some sort of sense.
Damn, Gaspode, you’re a tough act to follow.
Just wanted to add a piece about mammalian cloning technology. That a clone can develop from transplantaion of a diploid nucleus into an anucleate oocyte, is circumstantial evidence of our asexual past. Otherwise it’s hard to imagine why this would work. Think about it.
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So she’s beautiful, busty, naturally shaved, and you don’t have to worry that you might get her pregnant.
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Good explanation, Gaspode. Also remember that even asexual bacteria have forms of sex. Bacteria will exchange genetic material, so favorable mutations can be passed from one bacterium to another. As Gaspode pointed out, the thing to remember is that sex existed long before gender did. Organisms have been exchanging genetic material long before there were male and female organisms.
The simplest idea is to imagine an isogametic species. It doesn’t produce eggs or sperm, but simply undifferetiated gametes. Any two gametes can fuse to become a zygote. There are a few species even today that are like this (tries to remember specific examples but can’t…). But differentiation into sperm and eggs makes a lot of sense.
The thing to remember is that most organisms produce both eggs AND sperm. Although mammals have obligate sexuality, most creatures don’t.
Heeeeeeeeeeeeeere comes the Great Debate!!! Yeesh, there’s been a lot of evolutionism threads lately.
But suddenly, because of a cosmic ray, she starts responding the androgens, and her boobs shrivel and she sprouts a penis. During sex.
Actually, this thread is just fine for GQ. I expect that it will stay that way.