(Meant as a follow-up to The Other Dan's write-up on the two-fold cost of sex.)
Sex is a costly mode of reproduction. Evolutionary biologists have invested a great deal of time and effort into understanding why sex is ubiquitous and diverse. In this write-up, I would like to address the other costs of sex that were not discussed or only briefly touched upon in a previous writeup (on the two-fold cost of sex). This should present a persuasive case that the cost of sex is most probably greater than two, and places a greater onus on various hypothesized advantages of sex ( the deterministic mutation hypothesis, or the Red Queen hypothesis).
1. Sex destroys good combinations of alleles.
Sex breaks down linkage disequilibrium, so that alleles become more randomly associated. Consider a haploid organism with two genes in its genome:
–A1––B1–
'A1' and 'B1' are
alleles, or versions, of genes A and B. Suppose that putting 'A1' together with 'B1' in an organism is very good for survival and reproductive success –– it gives the organism a
fitness advantage. If this organism is a sexually-reproducing one, it will exchange genetic material with another member of its population. Unless it has some way of guessing what genes its mate has, there's little guarantee that it will end up with a combination that will give its offspring as much of an advantage. When compared to the asexual strategy of producing clones of your successful
genotype,
sexual reproduction seems reckless.
2. Why have sex when you can be eating food?
Sex is an energetically costly behavior. An organism has a finite amount of time and energy to allocate to all the tasks of living, which includes foraging, avoiding predation and parasitism, and locating or constructing shelter. In many species, sexual reproduction requires making a sacrifice so that the individual can search for a satisfactory mate, and subsequently compete against rivals directly or indirectly (e.g. by courtship display or holding good territory). Consider, for example, the satin bowerbird: the male of this species weaves reeds into a large and elaborate "bower" and decorates this construct with flowers and blue-colored litter. When the male leaves to forage, he risks the chance that another male will fly in, tear the bower apart, and steal the decorations. Moreover, if a female chooses his intact bower, he still has to perform a long and vigorous courtship "dance" or be rejected as a poor mate. All of these things represent an appreciable investment of resources.
3. A sexually-reproducing population is slower to colonize new space.
If a single obligately sexually-reproducing individual disperses into an unpopulated yet suitable habitat, it is forced to either wait to be fertilized by a second individual or to self-fertilize. On the other hand, an asexual individual can immediately begin to establish a population. Inbreeding depression will only be experienced by the establishing sexual population. If colonization is important for the persistence of a metapopulation, then sexual reproduction will be a costly strategy. One saving grace would be if each new habitat had slightly different environmental pressures, creating novel natural selection for alternate combinations of alleles (see above).
4. Parasites of a sexual population will tend to be more virulent.
If the probability that a parasite will be transmitted from one host to another is independent of its virulence, then parasites that tend to be transmitted vertically (i.e. pass from parent to offspring) will also tend to be less virulent. This intuitively makes sense: why harm your host if your long-term livelihood depends on the host having progeny? A sexually-reproducing population releases a parasite population from this constraint by providing a frequent corridor of transmission. Virulence tends to be associated with obtaining more resources from the host (i.e. increased parasitism). When this is true, a parasite population will be selected to increase its virulence.
On the other hand, sexual selection or discrimination between mates by the host population can reduce the rate of parasite transmission. One of the possible advantages of sex, moreover, is increased host adaptation against parasites.
Conclusion
What I have hoped to demonstrate in this write-up is that the cost of sex must be greater than two. In other words, the advantages of sex must allow a population to do more than twice as successfully than an asexual one. With the exception of the first example, these additional costs are not usually addressed in the literature. We have a hard enough time as it is, finding a two-fold advantage of sex in the deterministic mutation hypothesis! It will probably be important to be able to integrate several or all of the costs and benefits of sex together, to create a pluralistic hypothesis. Unfortunately, and to our probable loss, simple hypotheses are far more appealing.
In the composition of this write-up I have been motivated by and/or drawn material from the following sources:
- my lab mates (they know who they are)
- Otto SP and NH Barton (1997). The evolution of sex and recombination. Trends in Ecology and Evolution.
- Andersson and R May (19**). Parasitology.
- West SA, * and AF Read (2000). Genetical Research (?)