Dear Mr. Darwin: How is babby formed?
April 20, 2011 8:25 AM   Subscribe

How did sexual reproduction become evolutionarily advantageous?

I'm unsure about how early life first reached the point of being reproduced through sexual couplings that required more than a single partner. It would seem to me that asexual reproduction is more "efficient," and therefore "preferable" to the process of natural selection. Wouldn't evolution favor a system of reproduction that doesn't require the cooperation (or at least, the simple presence) of another organism?

Does scientific consensus support the hypothesis (see The Red Queen, cited in this very close earlier question) that it boils down to engineering immunity to disease and parasites? Logically that makes sense to me, but only as an explanation for why sexual reproduction endured, not why it originated.

Does that make sense? Obviously, I'm not a biologist. Feel free to explain this to me using small words.
posted by AngerBoy to Science & Nature (23 answers total) 10 users marked this as a favorite
Well... it endured (immunity to disease/parasites, genetic diversity, etc), which is how it became evolutionarily advantageous.

The way it originated is the same as all other things, I'd imagine, random mutations.
posted by Grither at 8:37 AM on April 20, 2011

The key seems to be that by mixing genes from two parents something wholly new can come to life; and that may have better chances for survival than its parents, through some fluke.

Whereas asexual reproduction only produces clones, unable to adapt to new circumstances, should the circumstances have changed. So, in fact natural selection is harsh when an ecosystem changes for some reason or other.

How, or why it originated? It just did. Isn't it that biologists can explain about life, except how it ever came to be?
posted by ijsbrand at 8:38 AM on April 20, 2011 [1 favorite]

Wouldn't evolution favor a system of reproduction that doesn't require the cooperation (or at least, the simple presence) of another organism?

Not necessarily. Sexual reproduction mixes DNA between organisms which, generally, helps spread out risk of disadvantageous traits across the population.
posted by The Michael The at 8:39 AM on April 20, 2011

You've just asked one of the most difficult and contentious questions in biology. Richard Dawkins goes into this in detail in several of his books (the selfish gene particularly), so that would be a good starting point for you perhaps - One of the problems is odd examples like Bdelloidea which apparently gets away with asexual reproduction and has done for millions of years with lots of species. This kind of thing puts the red queen hypothesis in doubt. This page may have more information too
posted by BigCalm at 8:39 AM on April 20, 2011 [1 favorite]

A big benefit from an evolutionary perspective to sexual vs asexual reproduction would be the amount of genetic diversity available. More diversity equals faster responses to environmental pressures. I am not a biologist either and may slip up here, but I think what I recall is that asexual reproduction, while efficient, is also pretty stagnant compared to sexual reproduction. In long term species survivability diversity is preferred over stability
posted by edgeways at 8:40 AM on April 20, 2011 [1 favorite]

Sexual vs. asexual reproduction: scientists find sex wins. This article covers most of the argument. Obviously, being an asexual reproducer has some advantages. You just clone yourself, all your genes get passed, etc. Long story short, beneficial genes rise to the top quickly in sexual reproduction but with asexual they stagnate.
In the experiments, conducted over a year and a half, the researchers were able to track individual mutations over time. They found that with sexual reproduction, the gene for red eye color rose rapidly toward "fixation" or establishment in the population. The proportion of the red eye color gets higher and higher over time until it is fixed at 100 percent. With the asexual populations the mutation began to accumulate, but eventually stopped in most cases.
posted by damn dirty ape at 8:42 AM on April 20, 2011

This is my favorite book on the subject. I do not seem to be able to keep a copy, since people borrow it and "forget" to return it.
posted by francesca too at 8:44 AM on April 20, 2011

You also mustn't forget that there are organisms, for instance yeasts and even some animals (where it's called parthenogenesis), that can choose to have sexual reproduction (though yeast of course, without genders) or asexual reproduction. In different circumstances, they do one or the other. As you mention, asexual reproduction has an element of efficiency, but it has downsides in terms of accumulation of harmful mutations and maintenance of genetic (and phenotypic) diversity. Sexual reproduction is better at handling those, but not as fast. There is a tradeoff, and certain conditions (and organisms) make one strategy more fit than the other based on the balance of negatives and positives.
posted by Schismatic at 8:55 AM on April 20, 2011 [1 favorite]

If you have a species that reproduces solely through parthogenesis, then improvements must occur sequentially in a single line for the species to evolve. If two favorable traits appear in two distinct lines, there's no way for those traits to become incorporated into a single individual.

The advantage of gene mixing is that it permits those traits to come together. That results in much, much faster evolutionary change.

Bacteria engage in gene mixing. Occasionally two bacteria will run into one another, and temporarily open holes in their cell walls and exchange protoplasm. The protoplasm may contain small rings of DNA (known as "plasmids"), which might contain useful traits.
Interestingly, this can even happen between bacteria of different species.

It's one of the ways that antibiotic resistance has been spreading.

Sex, as we know it, takes this gene mixing to the logical extreme, and the reason it's so successful is that creatures which create their offspring through sex are especially well suited to playing the evolution game. Simply put, they evolve at a much faster rate per thousand generations.
posted by Chocolate Pickle at 8:56 AM on April 20, 2011 [1 favorite]

You've actually asked a very astute question that is a source of debate in biology.

Here is a Cal Tech press release about a paper - it's a decade old and involves computer modeling of evolution, but it does go directly to the heart of your question - why from some perspectives it seems that the evolution of sexual reproduction wouldn't be favored by selection, why it is difficult to study directly, and a possible explanation (in a nutshell, the model they proposed suggested that in circumstances where there were unusually high rates of mutation, sexually reproducing organisms were favored).
posted by nanojath at 9:55 AM on April 20, 2011

Both sexual and asexual reproduction have been favored by evolution -- just in different ecological niches. It's not correct to say that sexual reproduction is evolutionarily advantageous -- and in fact is essentially meaningless. In the particular ecological niche that humans occupy, sexual reproduction has stuck around. But in the particular ecological niche that many microorganisms occupy, asexual reproduction has been found to be better. Don't forget that every organism on this planet has evolved for exactly the same amount of time. The better question to ask is: why is sexual reproduction favored in some niches and asexual reproduction in others?
posted by peacheater at 10:32 AM on April 20, 2011

It might help to ask the question the other way around: "Does evolutionary advantage require require sexual reproduction?" If you haven't read The Red Queen, IIRC Ridley does go into detail about sex/asex.
posted by rhizome at 11:13 AM on April 20, 2011

Long story short, beneficial genes rise to the top quickly in sexual reproduction but with asexual they stagnate

There is lateral gene transfer in many (almost every? we keep discovering them) asexually reproducing life form. Plasmids, ...

So I would say that sexual reproduction made sense for those of us that evolved without that particular bit of history.
posted by rr at 11:22 AM on April 20, 2011

Hi. I'm an evolutionary biologist. I am, in fact, an evolutionary biologist specializing in the questions surrounding the maintenance of sex in populations.

There is no consensus on the mechanisms involved in the origin or the maintenance of sex.

I can’t speak fluently to the origins of sex – partly because it’s not one topic; it’s the origin of meiosis and the origin of anisogamy and probably other things I’m not thinking of. But the advantages pertaining to the origin of sex are likely to be similar to the advantages pertaining to the maintenance of sex. Features which will help a sexual population resist invasion by an asexual will also help sexuality invade an asexual population, though many of the details are different.

Sex is, as you’ve observed, costly, and requires explanation. A popular explanation is that sex begets variation, and variation is good because it gives selection more to act upon (though I hasten to correct the misapprehension that natural selection somehow fails to operate on asexual organisms!) The problem with this is that it requires individuals to sacrifice for the benefit of populations. Biologists have known that this basically doesn’t work since shortly after Van Valen proposed it in the 60’s. Also, frankly, variation is often bad. If you’re well-adapted to your environment, why on earth would you want your offspring to be any different from you?

Well, one observation is that environments change. Therefore, it might be good to have variable offspring just in case the environment is very different from yours. This is called the “Lottery” model of sexual reproduction. Like a good theory, it makes very specific predictions – particularly that sexual reproduction should be found in temporally heterogenous environments, such as high latitudes and high altitudes. However, those are precisely the regions where asex is observed to predominate. An important researcher in the field, Graham Bell, wrote a book on the subject, (The Masterpiece of Nature) where he gathered all the available comparative ecological data. It’s a lovely coherent sensible theory which has been rejected by the data, and is not considered important by researchers in the field. It also has weakness on a mathematical-theoretical level; the kinds, amounts, and timescales of change required seem implausible in real biological situations.

Another theory is the “Tangled Bank” theory. This basically observes that environments vary, not in time, but in space. Therefore, it might be advantageous to have your offspring escape competition with each other by occupying slightly different ecological niches. While it’s not rejected on the ecological data in the way that the Lottery is, it has its own problems – e.g., the Tangled Bank works better the more offspring an individual has, while sex increasingly predominates in organisms with few, not many, offspring.

The theories that have really strong currency are not the above, but “Muller’s Ratchet” (or ”Kondrashov’s Hatchet”, which is a refinement of the Ratchet), and the “Red Queen”. Muller’s Ratchet basically treats sex as a mutation clearing mechanism. As genomes accumulate mutations, the only way to generate an organism with fewer mutations is to recombine two genomes. You’ll create one worse genome, and one better genome. Depending on a bunch of conditions surrounding rate of mutation, severity of mutation and the combinatorial effects of mutations, this can create an advantage to sex (especially if there’s a threshold level of mutations that are unacceptable.) A twist on this hypothesis is that recombination also allows good mutations to be brought together quickly. This provides an advantage if the mutations are more beneficial in combination than separately. There is laboratory data in micro-organisms to support this hypothesis.

The Red Queen basically takes the Lottery one step forward: it’s not just that environments are changing, they are changing capriciously: the biotic environment includes co-evolving parasites which preferentially infect the most common genotype. There’s a strong advantage to being rare, and, therefore, to generate rare genotypes by sex. There is a substantial amount of field data (much of it in freshwater snails) and laboratory data (in micro-organisms, roundworms, and beetles) to support the Red Queen.

The Red Queen can be taken another step forward: under the Red Queen, each genotype in a population is driven rare periodically by parasite pressure. While rare, the effects of Muller’s Ratchet are magnified (because the Ratchet is stronger in small populations). So the Ratchet and the Red Queen may act in synergy. Certain researchers regard “different mechanisms working in tandem or synergy” as the only possible explanation. Certain researchers regard synergistic explanations as utterly intellectually repugnant. It’s a dynamic field.

I think The Red Queen, the book, sets out issues of the maintenance of sex in layman's terms fairly well. If you're really curious, I recommend it. The Selfish Gene is a great (and very widely misunderstood and maligned book), but it is primarily in my mind about the evolution of cooperation, rather than sex.

One of the problems is odd examples like Bdelloidea which apparently gets away with asexual reproduction and has done for millions of years with lots of species. This kind of thing puts the red queen hypothesis in doubt

Actually, recent work on the biology of the bdelloid rotifers supports the Red Queen. In a nutshell, the Red Queen Hypothesis argues that sex is advantageous because shuffled genomes are beneficial under certain kinds of pressure from co-evolving parasites. The Bdelloids, however, are putative ancient asexuals (there are some problems with that assertion, but the genetic work indicates they've been asexual for a heck of a long time). The question, then, is, "why don't the bdelloids, of all creatures, need sex?" Turns out, they escape their parasites efficiently through periodic dessication, which their common fungal parasites can't tolerate.
posted by endless_forms at 12:37 PM on April 20, 2011 [190 favorites]

I suppose I should have said, "I am not your evolutionary biologist, and the following does not constitute evolutionary advice."
posted by endless_forms at 12:48 PM on April 20, 2011 [42 favorites]

endless_forms did a really nice job laying out the concepts and research involved here.
I would like to add Muller's Ratchet to the pile (Don't we get really cool names for things in evolutionary biology?).

Mutations are always carried forward in an asexual lineage. The 'Ratchet' refers to a gradual decline because of mutation accrual; there's no way back to zero. In a sexually reproducing population, recombination repair can fix deleterious mutations.

posted by vortex genie 2 at 12:57 PM on April 20, 2011

Whoops, you totally mentioned Muller's Ratchet. Reading comprehension fail, my apologies.
posted by vortex genie 2 at 12:58 PM on April 20, 2011 [1 favorite]

Oversimplified: Why sexual reproduction?

Because if we only cloned, we would still just be amoeba after all these years.
posted by ovvl at 6:38 PM on April 20, 2011

I didn't realize Van Halen had also made important contributions to biology.
posted by neuromodulator at 8:03 PM on April 20, 2011 [3 favorites]

As the person who asked that earlier question, I do have to heartily recommend The Red Queen-- it answered all sorts of questions I was never clever enough to come up on my own.
posted by shakespeherian at 8:08 PM on April 20, 2011

That's an outstandingly lucid answer, endless_forms; thanks for posting it.
posted by LobsterMitten at 1:23 PM on April 21, 2011

"Sex is, as you’ve observed, costly, and requires explanation."

I think we can all agree on that.
posted by webhund at 6:11 PM on April 21, 2011 [7 favorites]

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