EVOLUTION OF SEX

Why is there sex? We assume, anthropocentrically, that reproduction requires two individuals. But in many organisms that is not true. Life originated without sex (as best we can tell) so sexual reproduction is something that had to evolve.

First what is sexual reproduction? 1) production of haploid gametes by meiosis, a reduction division, 2) fusion of these gametes produces a zygote and restores the full diploid complement of chromosomes.

There are thus two parts to the evolution of sex: 1) the origin of sexual reproduction (cellular evolution) and 2) the evolution and maintenance of sexual reproduction and recombination (recombination is like sex in that it reassorts genetic material)

Recombination probably evolved ~ 3 billion years ago as a mechanism of DNA repair; sex evolved ~ 1-2 billion years ago in the early eukaryotes; the reason is unclear but it its likely that it is maintained in the current day by selection.

One "story" about the origin of sex is as follows (from J. Maynard-Smith, The Evolution of Sex, Cambridge University Press, 1978):

1. Binary cell fusion (advantage ~ hybrid vigor; masking deleterious mutations)

2. Evolve the use of one spindle apparatus (advantage = maintaining both sets of chromosomes and hence any hybrid vigor effects

3. Homologous pairing and chiasma (next step but advantage unclear; generates variation but also creates regions of genetic homozygosity)

4. Reduction division + syngamy (favored to restore heterozygosity)

This may be one plausible scenario but why higher eukaryotes spend so much of the haploid-diplod life cycle in the diploid stage is unclear

Subsequent evolution and maintenance of sex and recombination. Observation: one can select (both up and down) for rates of recombination between two loci without affecting other rates of recombination (see figure 8.6, pg.213). This means that there is genetic variation affecting recombination, which in turn means that selection can alter the rates of recombination.

The central problems with sex: 1) sex and recombination mix up any "coadaptation" that a genotype might have to a particular environment; why disturb this if its is adaptive; 2) there is a cost of meiosis associated with putting genes into males that cannot produce eggs. Consider the following model: k = number of eggs, S = probability of survival, Parthenogenetic = unfertilized eggs develop into females.

If n is small, the parthenogenetic strain has effectively doubled in the next generation. This assumes that the egg production by a sexual and a parthenogenetic female are Å same and that the eggs of sexual females are equally divided between males and females. Why be sexual?

Advantage of sex in terms of genetic variation and rate of evolution. Consider two loci, A and B and new mutations to alleles a and b which can interact to produce a genotype of high fitness in a novel environment.. Since mutations are rare originally the only new chromosomes in the population will be Ab (from a B -> b mutation ) and aB (from an A -> a mutation). An asexually reproducing stain would have to wait for the second mutation because it has no way of reassembling the existing alleles into new combinations. The sexual strain could produce the high fitness genotype much faster by recombination (see figure 11.1 page 286).

This advantageous effect of recombination would be reduced in small populations because the number of mutations occurring is the product of mutation rate and population size, so the advantage of sex and recombination would be reduced in small populations.

These two observations suggest that sex might evolve by group selection: we have 1) genetically isolated "groups" (sexual and parthenogenetic strains), 2) disadvantage to the individual and 3) advantageous to the long-term survival of the group.

But G. C. Williams notes that the cases of facultative parthenogens (species that can reproduce either parthenogenetically or sexually) strongly suggest that there must be some short-term advantage to sex (otherwise it would be selected out of population as a strategy; note that we are dealing with the same group here so the advantage of sex at the group level does not apply). Some possible advantages:

Unpredictable environment theory: sex and recombination produces more genetically varied offspring, individual using this strategy will have higher fitness since they have a high chance of leaving an offspring that will survive in an unpredictable and changing environment.

Problems/qualifications: 1) if selection were sufficiently strong in the unpredictable environment then there is a chance that the genetic variation produced by sex/recombination might not be sufficient to "cover" the range of genotype needed. Considers the hard selection/soft selection continuum

2) if there were continuous selection such that genetic variation was continually removed from the population, there might be little variation remaining for sex to have an advantage.

3) theoretical models show that one needs to have the unpredictability be in the form of a switch from hot-dry vs. cool-wet environments in one generation to hot-wet and cool-dry in the next generation for recombination to really have a significant advantage.

Sib competition theory states that offspring in the next generation will be competing for the same resources and thee genetically identical offspring will experience more severe competition that sexual siblings because the latter will be genetically different and will not utilize resources in identical manners. Supporting evidence from plants where offspring were shown to have higher fitness when grown in competition with different genotype than in competition with its own genotype.

Muller's ratchet and recombination. In a strain of asexual species the number of deleterious mutations accumulate with time. The only way to get rid of them is by death. When the genotype with the smallest number of mutations n_x dies , the genotype with the smallest number of mutations in now n_x+1 and thee "ratchet" has clicked up one notch. With recombination one can mate two genotypes and their offspring might have fewer mutation (and more as well), so variation is generated that can be a higher fitness than any of the existing genotypes.

The lower chromosome now has fewer deleterious mutations than either parental chromosome.

Hitchhiking effect Deleterious alleles can become associated with advantageous alleles by virtue of finite population size which will impose disequilibrium on loci (or association could be due to lack of recombination and a deleterious mutation at an adjacent locus). Any other locus which increases the rate of recombination will be favored because it will tend to break up the association between the advantageous allele and the linked disadvantageous allele.

Converse of this is that any increase in fitness that occurs as a result of the interaction between two loci, selection will act to reduce recombination because recombination would break up favorable associations of alleles.

Parasites may also have played a role in the evolution of sex. Various data suggest that more genetically diverse (heterozygous) organisms can mount a more effective defense against parasites than homozygous hosts. Sexual reproduction might again have a short term advantage in terms of sexually produced organisms having a greater defense system against parasites and other infectious agents.