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.
|# adults||# eggs||adults next generation|
|Ratio Partheno/total||n/(2N + n)||(Skn) / (SkN + Skn) =
n / (N + n)
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 nx dies , the genotype with the smallest number of mutations in now nx+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
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.