The arcs of evolutionary genetics always cross back

If you have more than a marginal interest in evolutionary biology you will no doubt have stumbled upon the conundrum of sex & sexes. Matt Ridley's most prominent work, The Red Queen: Sex and the Evolution of Human Nature, covered both the theoretical framework and applied implications of the subject. Ridley leaned heavily upon William D. Hamilton's scientific work, which extended upon Leigh Van Valen's concept of the book's titular Red Queen. The complex interplay between pathogens & multicelluar organisms across the eons is a topic of such breadth and depth that a substantial proportion of the territory in evolutionary biology is still devoted to it, and how sex may relate this dance. Hamilton spent the second half of his career focusing on just this question, outlined in Narrow Roads of Gene Land, volume 2 of his collected papers.*

The question of sex begins with a simple self-evident curiosity: why not cloning? Assume that you are a creature who can be expected in any given generation to have two offspring reach maturity. Imagine two populations, one clonal and purely female and another sexual with females and males. In the later case let us assume a 50:50 sex ratio (this does not always occur, but often does for a peculiar evolutionary genetic reason). All things equal the former population should be able to produce twice as many offspring for subsequent generations. Males are a waste of a body, a "vehicle" as Richard Dawkins would say. Instead of producing offspring the bodies of males serve as halfway houses for genes. Consider the circumscribed lives of drones among the hymenoptera. An answer may be that if men are a plague upon women, they may also offer women a salvation from plagues. The evolutionary psychologist Geoffrey Miller constructs a speculative illustration of this principle in The Mating Mind, while the behavioral ecologist Bobbi S. Low reports on more concrete cases in Why Sex Matters. Why patriarchy? One must always engage the pathogens which inevitability threaten to turn into the bane of a multicellular organism's existence.

These are big questions, but fleshing out the details can only be done case by case. Though there are some ethnographic and biological anthropological research programs attempting to tease apart the nature of the relationship between disease load and sexual behavior, there are obvious limits to this sort of work. Humans are big slow breeding organisms constrained by ethical considerations. At the other extreme are those who play with parameters in silico (much of Hamilton's later theorizing was in simulation and not analysis). To split the difference one might look to fast reproducing model organisms, attempting to draw general inferences from specific results.

That is what a letter to Nature attempts to do in relation to sex, or more accurately sex with self or non-self, Mutation load and rapid adaptation favour outcrossing over self-fertilization:

The tendency of organisms to reproduce by cross-fertilization despite numerous disadvantages relative to self-fertilization is one of the oldest puzzles in evolutionary biology. For many species, the primary obstacle to the evolution of outcrossing is the cost of production of males, individuals that do not directly contribute offspring and thus diminish the long-term reproductive output of a lineage. Self-fertilizing ('selfing') organisms do not incur the cost of males and therefore should possess at least a twofold numerical advantage over most outcrossing organisms. Two competing explanations for the widespread prevalence of outcrossing in nature despite this inherent disadvantage are the avoidance of inbreeding depression generated by selfing and the ability of outcrossing populations to adapt more rapidly to environmental change. Here we show that outcrossing is favoured in populations of Caenorhabditis elegans subject to experimental evolution both under conditions of increased mutation rate and during adaptation to a novel environment. In general, fitness increased with increasing rates of outcrossing. Thus, each of the standard explanations for the maintenance of outcrossing are correct, and it is likely that outcrossing is the predominant mode of reproduction in most species because it is favoured under ecological conditions that are ubiquitous in natural environments.

Every evolutionary constraint a king! Now, the preamble above does not quite fit with what you see in the abstract here. "Selfing" is not asexual reproduction, as it literally means the self-fertilization of hermaphrodites. If you watch South Park you may note that there may be fictional analogs to C.elegans. In a typical "wild type" population ~5% of the individuals are male, who must "outcross" to reproduce. Outcrossing generally is not out of the question for any given individual, though it is not the modal behavior within the population. The analogy between outcrossing and conventional sexual reproduction is not difficult to grasp; hermaphrodites can go "both ways." But selfing is a bit stranger, as one is literally mixing & matching one's own genes as if one was engaging in sex. In other words, one isn't producing clones, but obviously the range of genetic variation is going to be relatively constrained vis-a-vis conventional sexual reproduction.

One can illustrate the consequences of selfing through a toy example. Imagine a gene which comes in two allelic forms, A and a. If you have an organism which is heterozygote, that is, they're Aa, and they self-fertilize then the expectation of outcomes in terms of genotype will be:

25% aa
25% AA
50% Aa

1/2 of the offspring will recapitulate the genotype of the heterozygote parent. And 50% will be homozygotes. Homozygotes only give rise to homozygotes, so down the generations of obligate selfing the frequency of homozygotes will increase vis-a-vis heterozygotes (barring overdominance). Like idealized asexual reproduction selfing is a study in contrast with conventional sexual reproduction which involves two individuals. Though you are only adding one more individual to the equation, summed together the process results in genetic material being passed around the population. With selfing as descendant lineages become more genetically uniform the operational outcome should converge upon cloning in individual cases (because there's no variance to reshuffle if you're a total homozygote).

Unlike humans C. elegans offer no ethical challenges in experimentation, and they come preloaded with an intriguing sexual wild type, mostly selfing, but with a minority of outcrossing through the existence of a small number of persistent males who must make recourse to this strategy.** As a descriptive matter the ratio of males to hermaphrodites might seem a curiosity, but as an evolutionary question one has to wonder why it exists at all. If 95% selfing, while not 100% selfing?

It may be a historical contingency, some sort of left-over from a bygone era of sexual promiscuity in the C. elegans lineage. Perhaps there are peculiar kluges which generate a small number of males every generation, and selection has not worked up the enthusiasm to slough them out of the gene pool. Or, perhaps there is an aspect of irreversibility on some deep genetic level. The dial of sexuality can be turned mostly back, but the last step is obstructed.

I don't mean to sound speciesist, but I'm skeptical that a worm whose every cell has been mapped out is so fundamentally complex that the male morph could not have been abolished by now (this assumption does not exist in a vacuum, there are plenty of organisms with purely clonal and purely sexual lineages). The paper itself offers evidence that this mix of morphs is not a random act of evolutionary history, but is adaptive.

How so?

First, they tweaked C. elegans mating behavior by introducing mutations which resulted in two exotic lineages:

- obligate (always) outcrossing
- obligate selfing

These two mutant phenotypes were then set against the wild type, which is predominantly but not obligately selfing. More specifically the mutations mentioned above were introduced into two different genetic backgrounds, N2 and CB4856, which varied in outcrossing frequency.

Once they had the genetic and phenetic combinations that they wanted to test, they modulated the evolutionary genetic parameters. Specifically, they:

- increased basal mutation rate via a mutagenic environment
- increased selection pressures which should drive adaption (or extinction) through introduction of pathogens

The experiments ran on the order of 50 generations, with 500 individuals per generation. Remember that the power of random genetic drift is inversely proportional to population size (think sample variance). They presumably selected this N because it is large enough so that stochastic deviations won't swamp out the selection effects which they're looking for as signs of adaptation.

Additionally, there is one theoretical issue that they nod to in the paper which must be mentioned, and that is expunging of genetic load (genetic load being the deleterious mutations within a population). Selfing lineages presumably should offload rare deleterious alleles of large effect, by exposing said alleles to the full force of selection in their homozygote state. Recall that selfing populations should drive themselves toward homozyogosity rapidly over the generations, so masking of such alleles will be temporary in these populations. In contrast sexual populations may maintain enough heterozyogosity that deleterious alleles remain extant at low frequencies and so shielded from selection because so few homozygotes are produced. Because of selfing's effect on long term population size presumably deleterious alleles which are closer to selective neutrality, but not quite, could be fixed more easily.

What they found when they elevated mutation rates experimentally was that the selfing populations could not purge the higher rate of introduced deleterious alleles (new mutations are invariably neutral or deleterious, very rarely beneficial). Consequently the population mean fitness decreased rapidly under conditions of high mutability. Also, wild type lineages which were not subject to obligate behaviors (selfing vs. outcrossing) exhibited increases in outcrossing under conditions of high mutation, suggesting an adaptive response and a shifting in phenotype (presumably genetically controlled because the response was not immediate and had to be selected for). Interestingly, obligate selfers seemed to lose fitness even under the natural mutation rate, indicating that the less than 5% of elegans which are male hang around for a reason.

Next they focused on adaptation to pathogens, as opposed to purging genetic load introduced by mutation. When exposed to pathogens it was the outcrossers who began to show increases in fitness, that is adaptation, quickly in relation to the selfers. This fits with our intuition, outcrossers constantly reshuffle genetic information across each other, rapidly moving beneficial mutations and generating novel genetic combinations. Among selfers presumably novel beneficial mutations would spread through natural increase of the lineage which possessed the mutation, but among outcrossers the mutation could rapidly spread across the web of lineages. Increasingly in the wild type populations which were not fixed for either strategy the frequency of outcrossers increased before declining again. This may be a sign that outcrossing becomes a dominant strategy as the population sweeps across an evolutionary transient toward a new adaptive peak, but once at that peak selfing comes to the fore as the more efficient strategy for self-replication.

Let me allow the authors to speak with their last paragraph:

The fact that obligate outcrossing yielded a much larger response than natural outcrossing rates is something of a surprise, because it is thought that moderate amounts of outcrossing are sufficient to escape the problems associated with obligate selfing. One additional feature of this system that has not been previously considered, however, is that an increase in the frequency of males within a population also increases the opportunity for sexual selection, which has been shown to reduce the overall genetic load within a population. Males therefore play multiple roles within these populations, both for enhancing genetic exchange across generations and increasing the efficacy of natural selection within generations. Mutation, changing environmental conditions, and pathogens are nearly ubiquitous selective pressures for many organisms, which probably explains outcrossing's relative prevalence in nature.

As I said, the the plague of males is simply the consequence of the plague of pathogens.

Figure 1 illustrates their results concisely. I have reformatted as usual to 500 pixels.


The above experiment uses the outcrossing:selfing dichotomy to serve as an analogy to sexual:asexual reproductive strategies. More abstractly one could consider both forms of the relation long term vs. short term. All things equal organisms which lack males and have all individuals dedicated to producing offspring (as opposed to an extant population of fertilizers) has sexual species outgunned. Similarly, selfers have all the means of reproduction at hand, and don't need to worry about mixing up their own genetic material with that of a stranger if they simply rearrange what they have on hand. What sexual species which have males bring to the table is flexibility under duress. If you are a multicelluar organism you're likely handicapped against pathogens who are out to get you because they breed fast and can respond to your defenses. How to level the playing field? Sex may be one way; the constant reshuffling of defenses by mixing & matching genes between individuals keeps the pathogens guessing, who are after all asexual (though horizontal gene transfer thickens the plot considerably these days). Males may incur the cost of decreased short term productivity, but a species with males is made to last, at least if that species reproduces rather slow in relation to microorganisms with whom it is presumably in a biological arms race. To use an example with contemporary relevance, clonal lineages are undercapitalized when market conditions shift and overleveraged and stuck with only a few viable strategies. Sex may not offer up the same short term yields, but it is a diversified portfolio designed to weather, and even benefit from, downturns and market volatility. Until a great selective moderation, males are here to stay.

Citation: Mutation load and rapid adaptation favour outcrossing over self-fertilization, Nature, doi:10.1038/nature08496

* Of William D. Hamilton's the three volumes of collections of papers the second is the most interesting because he died before his biographical introductions could be reedited. They are therefore voluminous, and quite often shockingly frank and unguarded. Volume 3 has biographical introductions written by friends and collaborators. Volume 1 is slimmer, and evinces scientific conventionality in both substance & style.

** If these worms intrigue you, I recommend Andrew Brown's In the Beginning Was the Worm: Finding the Secrets of Life in a Tiny Hermaphrodite .

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I just skimmed this paper but haven't read it's great. A couple notes: C. elegans are essentially clonal because even in the wild individual strains tend to be homozygous at almost all loci. Outcrossing estimates in the wild are also very low, but there aren't a lot of data.

However, perhaps counter to this report, many elegans strains show outbreeding depression under stable conditions, and the population structure shows evidence for selection against recombinants (see Rockman and Kruglyak). This paper is slightly different because the variation comes from novel mutations rather than existing variation, and they manipulate the enviroment -- very cool.

There is evidence that certain stressors induce a higher frequency of males--you heat shock hermaphrodites when you want some male offspring--it would be interesting if C. elegans produced males "as needed" in response to environmental fluctuation, but off the top of my head it's hard to imagine mechanisms without invoking some kind of group selection.

Also of note: if you look at nematode phylogeny, species with hermaphrodites are always on the tips (with maybe one exception with speciation within a herm lineage)... given that you might expect selfers to speciate more readily, all of these might be dead ends. I think males are here to stay.

just a note, right, these are operationally clonal, so we're talking mostly de novo mutations here. but i believe in the sequence of experiments with pathogens they introduced the mutations ahead of time, so that it was actually selection on standing variation by the time the selection began.

Awesome post.

By Paul Jones (not verified) on 22 Oct 2009 #permalink

The commonality, if not ubiquity, of sexual reproduction has always seemed to me one of the most interesting of evolutionary questions.

It poses the problem of "lineage" interest vs. short-term gene interest. We all know the Selfish Gene -- but here, almost every explanation is one that goes way past kin selection as a short term pressure, to long-term selection between lineages. That makes it a much nastier problem -- we have two levels at work, not the one of simpler (and obviously preferred for analysis) problems.

Sometimes we apparently go uphill instead of downhill to find the next valley (using a minimization metaphor). Something appears (from the perspective of the Selfish Gene) to have foresight or memory. Nasty, nasty, nasty.

Because of selfing's effect on long term population size presumably deleterious alleles which are closer to selective neutrality, but not quite, could be fixed more easily.

What they found when they elevated mutation rates experimentally was that the selfing populations could not purge the higher rate of introduced deleterious alleles
The first statement makes the second a surprise. Why couldn't they purge the mutations? Were there just too many of them to purge and so outcrossers muddled on with heterozygotes?

Why couldn't they purge the mutations? Were there just too many of them to purge and so outcrossers muddled on with heterozygotes?

when effec pop size drops, the power of drift v-a-v selection increases. so presumably a lot of crappy deleterious alleles were being fixed randomly.

Nice explanation, but one nitpick; I think you should have written "new mutations are almost invariably neutral or deleterious, very rarely beneficial". Without the "almost", it's too easy for creationists to quote mine. All they have to do is lop off the last three words...