The evolution of sex Flashcards
What is sexual reproduction?
Biparental mating includes the fusion of two gamates, which then separate through meisosis, giving rise to different combinations of genes because of segregation and recombination.
Sexual reproduction is successful
Sexual reproduction is common and evolutionarily successful.
Some sexual lineages can evolve into asexual lineages, but this is not evolutionarily successful and tend to be short-lived.
Costs of sexual reproduction
Twofold cost of sex:
- Sexual reproduction is half as effective for females.
- Males do not contribute to the increase in population, and females transfer half their gametes into their offspring.
The costs of mating
- Finding a mate
- Interacting with a mate
- The risk of an STI
- Competing for a mate
The cost of meiosis
- 50% reduction in relatedness between parent and offspring compared to asexual repoduciton
The cost of breaking down coadapted gene complexes due to recombination.
Linkage disequilibrium
Linkage disequilibrium is the nonrandom association of alleles of different loci due to their position on a chromosome.
Closely linked alleles experience less recombination.
Epistasis
The phenotypic expression of a mutation gene depends on the presence of other genes (genetic background)
Positive: the combined effect of the mutations is greater than the summed effect of each individual mutation.
Negative: the combined effect of the mutations is less than the summed effect of each individual mutation.
If there is positive disequilibrium and negative epistasis, then recombination is beneficial.
Origin vs maintenance of sex
Mechanisms leading to origin of sex cannot explain the maintenance of sex and vice versa.
- may have just evolved from asymmetric transfer of DNA (e.g. HGT) -> genes transferred can spread in the population
There are short-term and long-term effects of sex with differing benefits.
Example: Increased variance
- The introduction of sex will produce individuals with extreme genotypes that may have an average fitness lower then the intermediate phenotype. (If heterozygous individuals reproduce)
- In the long term, there can be selection on the homozygous dominant phenotype which has the highest fitness.
Increasing variance is only beneficial in the short term if the extreme genotypes have higher average fitness than the intermediate genotypes.
Increasing variance in the long term is beneficial if there is directional selection. -> more rapid adaptations
Hypothesis for the evolution of sex
Deterministic:
The Red Queen hypothesis
(Antagonist co-evolution)
Niche differentiation (tangled bank)
The space hypothesis
Mutational deterministic
Stochastic:
Fluctuating environment (lottery)
Fisher-Muller hypothesis
Ruby in the Rubbish
Muller’s rachit
The red queen hypothesis
Pathogens adapt to the most common genotype, leading to frequency-dependent selection.
Co-evolving parasites eliminate the fitness advantage of asexual reproduction by adapting to infect locally common clonal genotypes.
Sex is constantly producing new genotypes that are initially rare and can resist current pathogens
Evidence:
- The freshwater snails in Nz lakes can produce asexually or sexually.
- In shallow, parasite-full waters, sexual reproduction is favoured.
- In deep, parasiteless waters, asexual reproduction is favoured.
The space hypothesis
Positive disequilibrium is generated when some sites (loci) experience stronger selection than other sites or when alleles are favourable in some locations and not others.
Following migration, extreme genotypes may be selected for, however, due to linkage disequilibrium alleles that are not optimal to the new environment are also selected for.
Sex breaks up this disadvantageous genetic association.
Selection for sex only occurs if there is extreme negative epistasis which outweigh long term negative effect of breaking down extreme genotypes
Evidence:
- Rotifer populations in heterogenous environments have a greater propensity for sex than homogenous environments.
Fluctuating environment (lottery) hypothesis
Production of variable offspring is a hedge-betting strategy in fluctuating environments.
Evidence:
- There is little evidence to support this and asexual reproduction tends to be associated with unpredictable environments.
Niche differentiation Tangled bank hypothesis
Genetic polymorphisms enahnced by sex could be favoured in spatially heterogeneous environments if different genetically determined morphs were favoured in different habitats. Occurs when there is intense competition for food, space and resources.
Intraspecific competition would be reduced if different genotypes carve different niches.-> able to extract more resources from the environment
Hill Robertson interface
Selection becomes less efficient whenever linked loci are also under selection.
Advantageous alleles can be associated with disadvantageous alleles, reducing the efficiency of natural selection.
Sex and recombination alleviates this issues associated with Hill Robertson interface.
This idea links to:
- Ruby in the dust
- Muller’s rachit
- Fisher’s mullers fachit
Ruby in the dust
In asexual populations, background selection on deleterious mutations can effect the fate of advantageous mutations.
In sexual populations, the beneficial mutation can be separated from the deleterious mutation.
Muller’s rachit
Asexual populations can experience an accumulation of deleterious mutations that they are unable to purge from the population.
Sexual populations are able to purge these deleterious mutations and reduce the probability of fixation due to recombination.
Deterministic mutational model
The accumulation of deleterious mutations leads to a large decrease of fitness when there is negative epistasis (the combined effect of mutations is more deleterious than their individual effect summed).
The effect of the deleterious mutations is intensified.
Each additional deleterious mutation leads to large decrease in fitness
