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1. You need sex if you have a nucleus.
No you don't. I realize this makes for a simply dichotomy that LabRat uses to contrast in the next sentence, but it is not correct. First, this ignores the basic problem, which is why is there sex. If nucleus makes sex essential (need), then there should not be asexual eukaryotes, but there are. Many of them in fact and many that are sister species to eukaryotes that still have sexual reproduction. BTW don't forget whiptail lizards (I will beat on them like a dead horse).
2. If you don't, gaining genetic diversity is very, *very* easy you can just pick up random genes from anywhere and try them out.
There are two assumptions in this statement that I want to call into question. The first assumption is that the nucleus is the barrier to picking up random DNA from the environment (which is the idea I think LabRat was making). Is that true? If it is, that means eukaryotic cells still take up DNA from the environment just fine (it is a great source of nutrients) but cannot get the DNA into the nucleus. My experience in eukaryotic genetics suggests this isn't true. Simply making cells more permeable is sufficient to get DNA into the nucleus, which means the nucleus isn't the issue.
The second assumption is that bacteria can easily take up DNA from the environment. In the case of Vibrio cholerae that is true, if they're growing on a crab shell (freely available article). I think LabRat is highlighting the importance and sexiness of Horizontal Gene Transfer (HGT). HGT clearly occurs between bacteria, not just different cells of the same species, but also different species, hell even different phyla! It is a phenomenon built of WIN! and really HGT was a game changer for how we (scientists) think about biology. HGT is important and its discovery revolutionized bacterial genetics. But let's not overstate it. HGT has occurred and is important. But is it a potential force of evolution? Yes. Is it the driving force? No, well at least not necessarily. In fact, bacterial species have a way to generate genetic diversity without taking up DNA from the environment, which is based on DNA polymerase IV. DNA polymerase IV is an error prone DNA polymerase, which means when replicating the DNA of a bacterium it makes more mistakes than other DNA polymerases. This results in a bunch of additional mutations. This is not generally a good thing, but DNA pol IV is only expressed when things are bad for the bacterium. If you're in the process of dying anyway, generating a bunch of mutations will help you on your way or may actually result in something happening that lets you survive.
3. Once you package all the DNA away though you need to have all sorts of random twisted 'crossing over' and specialised sharing-DNA events in order to get any kind of diversity in your genome.
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4. And the risks of no genetic diversity at all are far too high.
Maybe, but this is a hypothesis. An almost certainly absolutely correct hypothesis in most circumstances, but still a hypothesis. However, I want to stress that genetic diversity is meaningless except as a proxy for phenotypic diversity and as I noted above there are many ways to generate phenotypic diversity independent of genetic diversity. Further, genetic diversity can be generated in the absence of sex.
Finally I will close with my last rejoinder to why I do not think the current explanations for the evolution of sexual reproduction are sufficient.