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Biology tells us that sex evolved as a way to defend against viruses, bacteria, and parasites. Homogenous populations (where everybody is a near-clone) are extremely vulnerable to parasites. Once they find a trick that works against one host, it works everywhere. Sexual reproduction shuffles genes, so a sexual population will have many different genes. Some individuals will survive even the worst plague if they're diverse.

Humans are very diverse today. If we suddenly switched to cloning, we would start from high diversity. How long would it take for parasites to destroy the civilization and most humans?

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  • $\begingroup$ We'd maintain the diversity we have. So, I imagine we'd last a very long time. $\endgroup$ – Samuel Feb 27 '15 at 23:45
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    $\begingroup$ What would be benefit to switch to cloning? Sexual reproduction is fun and it works, so what is the reason to abandon it? $\endgroup$ – Peter M. Feb 28 '15 at 0:12
  • $\begingroup$ It's a bad idea, so {insert plausible sounding reason here} $\endgroup$ – Sophit Feb 28 '15 at 0:15
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    $\begingroup$ Sex doesn't just shuffle genes, it allows them to be exchanged/combined between separate bloodlines, making a species's heritage web-shaped instead of a simple branching tree. Philosophically therefore one could argue that "the" species died the second you started cloning, because there's no longer any element mechanically linking the "branches" of the human family to one another; every human individual is their own separate species now. $\endgroup$ – Leushenko Feb 28 '15 at 1:09
  • $\begingroup$ What would be the incentive to clone vs sex? My answer is, it won't last very long unless the species has a different motivation for cloning... $\endgroup$ – Glowie Feb 28 '15 at 1:32
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There would be (to start with) 7 billion human-descended species (since species are defined by cross-reproduction, and we're excluding that).

Some lines would die out while others might thrive (and have lots of clones). Depending on the technology you use to generate and grow your clones, there would still be genetic drift. If chromosome recombination can occur, a fraction of the clones would suffer from lethal or debilitating genetic maladies. If your genetic-copy quality control function were about as efficient as our current system (1 error per million to 1 error per hundred million base pairs), and chromosome recombination did occur as part of whatever mitotic process replaces gametes meiosis, your clones would slowly descend into inbred imbeciles, probably too slowly for anyone to notice.

PS: I just noticed Leushenko already covered some/most of this in his comment. If you wish to turn your comment into an answer, I'll delete mine.

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You are disregarding other advances in genetic engineering

When you say:

Biology tells us that sex evolved as a way to defend against viruses, bacteria, and parasites.

...you are in effect postulating that evolution through random mutations and natural selection would cease and that evolution through planned and engineered mutations, along with artificial selection would not take place. I challenge that premise. I claim that this postulate is flat out wrong. And here is why...

We are about to do not only do human cloning but also to start editing/improving human genes. So for you to assume that we will only do carbon-copy clones of ourselves instead of also improving the genome of our cloned offspring, is — I must say — kind of short-sighted.

Also, the time-span you are postulating here, where so much time has passed that we supposedly have achieved a genetically near homogeneous population, means that centuries of advances in genetic engineering will happen in the mean time. This in turn makes your assumption, that we will only do carbon copies of ourselves and never improve the genome to meet threats of the sort you are talking about, comes off as really silly.

Hence your question "How long would it take for parasites to destroy the civilization and most humans?" is made on a false premise and is entirely moot (at least for the reason stated).

When imagining scenarios like this, always remember that technology marches on.

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So in this switch to cloning we are assuming that we only have clones of present day humans, exactly as they are. My children will be clones of me and their "mother" and so on for ever and ever.

With our present technological level, I would wager that we would be able to stay ahead of the viral/bacteria/parasitic evolutionary curve for quite some time. If everyone knows who they were cloned from, in a few generations we would know exactly what to expect from each clone and how to better meet their medical needs. This would extend the lifespan of each successive cloned generation.

To answer your question I believe it would take a very, very long time, and that we would have much bigger things to worry about in the interim.

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