Evolution Of Sex

Question 1

What are the genetic effects of sex?

Answer 1

• Increases genetic and phenotypic variance

* Mixing of genes between individuals and segregation and crossing over during meiosis

Question 2

What is anisogamy?

Answer 2

• Females produce large costly gametes
• Males produce small uncostly gametes
• None, one or both can be motile

Question 3

What are the costs of sex

Answer 3

• Reduces a lineage’s growth rate
• Offspring number limited by females, males only give genetic contribution- the two-fold cost of sex
• Sex breaks up beneficial genotypes
- sickle cell anaemia- loss of genotype that gives best fitness (heterozygote)
- At equilibrium (in an adapted population) reshuffling of genes is disadvantageous

Question 4

What’s the paradox of sex?

Answer 4

Considerable costs yet very common
• Instances is asexuality are recent therefore asexual lineages are short lived
• Sex is advantageous when genetic mixing is favoured

Question 5

When is sex advantageous?

Answer 5

When genetic mixing is favoured:
• Optimal genotypes not common- non-equilibrium situation
• Components of optimal genotype dispersed- beneficial alleles in different individuals and LD
-> Both likely under directional selection

Positive selection- beneficial mutations very rare: mutation rate low and loss through drift
• Asexual adaptation limited by beneficial adaptations- all must occur in same lineage to fully adapt
• Clonal interference- two different adaptive mutations interfere with each other
• Sex breaks down LD and combines beneficial mutations

Negative selection- deleterious mutations can fix by drift and mutation-free genotypes can’t be restored
• ‘Muller’s ratchet’- deleterious mutations accumulate-> gradual decline in mean fitness
• Sex avoids this- restores genotypes with fewer deleterious mutations and generates very deleterious genotypes for selection

Question 6

Give experimental evidence for the benefit of sex in adaptation

Answer 6

Budding yeast:
• Adaptation to benign and hot environment by wild type (sex) strain and mutant with disrupted meiosis (no sex)
• Vegetative growth
• Sexual populations adapted faster to hot environment
(Goddard et al., 2005)

• Selection is less efficient in asexual populations- can’t separate linked mutations-> deleterious mutations hitchhike to fixation
(McDonald et al., 2016)

Question 7

Why can greater variance among offspring be beneficial?

Answer 7

Host-pathogen interactions: evolutionary arms race-> constant directional selection
• Red Queen Dynamics
• Host-parasite interactions are genetically determined- pathogenicity depends on genes
• Asexuals are competitive when rare, pathogen target when frequent
• Sexuals are genetically diverse so escape parasite specialisation

Question 8

Outline Red Queen dynamics experimental evidence

Answer 8

Testing predictions using aquatic snail:
- Trematode parasite sterilises host
1. Asexual genotypes show turn-over- high frequency clones later found at low/gone
2. Common genotypes are pathogen targets- pathogen susceptibility correlates with clone frequencies- local parasites adapt to common host genotypes
3. Sexuals escape pathogen specialisation- frequency locally stable
4. Frequency of sexual lineages increases with pathogen pressure
(Jokela et al., 2009)

-new asexual clones outcompete, but Sexuals have better long term resistance
—> parasitism Allows coexistence of asexuals and sexuals