W9L1 Evolution of Sex

Question 1

Two general reproductive modes

Answer 1

1. Asexual reproduction = creation of progeny without contribution from another individual
2. Sexual reproduction = requires gametes produced by two different individuals

Question 2

The hydra (animal) example

Answer 2

Their individual is reproducing both sexually and asexually
The crown of tentacles at the upper left surround the hydra’s mouth
Along the body below the mouth are rows of testes. Below the testes are two asexual buds

Question 3

Fitness advantage of asexuality

Answer 3

-Asexuals can breed without a partner
-No need for meiosis (which takes 5-100x longer than mitosis)
-No STIs,no time wasted on sex/courtship, no ‘sexual conflict’
-avoids the need for male, the sex that cannot “make” offspring
-preservation of high fitness alleles

Question 4

The two fold cost of male

Answer 4

-asexual population leave 2x more descendant than sexual population with 1;1 ratio
Asexuality
The two individuals each produce one offspring per time step
Sex+ isogamy (al and. A yeast)
Because each individual contributes to the zygote, they produce two offspring per time step
Sex+ anisogamy
Because sperm contribute basically nothing, the two individuals only produce one offspring per time step

Question 5

Problem with sex

Answer 5

-Sex makes some loci homozygous, which can be fatal
-Sexual reproduction exposes recessive alleles to selection, lowering average fitness of individuals in sexual populations
- Recombination breaks up co-adapted alleles (favorable epistatic effect)

Question 6

Overview on sex (recombination)

Answer 6

• Recombination increases the variability in offspring fitness
• Often, it reduces the average offspring fitness

Question 7

The prevalence of sexual reproduction is a paradox

Answer 7

• Despite the apparent advantages of asexual reproduction, the vast majority of multicellular species reproduce sexually
• This suggests that sexual reproduction must in general confer higher fitness than asexual reproduction; that is, one or both of the assumptions of all else equal are incorrect
• Assumption #1 - equal numbers of offspring
• Assumption #2 - equal survival

Question 8

Why do we recombinate (sex)

Answer 8

• Recombination allows increased variability in subsequent generations
• Variability allows capacity to change = adaptation to changing environments
• Recombination and crossing over = mechanism to edit mutations

Question 9

Sex: Linkage Disequilibrium & Selection

Answer 9

• Recombination (sex) brings favorable alleles together faster than mutation
• Recombination (sex) allows chromatids to lose disadvantageous alleles

Question 10

Sex advantage in adaptation vs Asexual

Answer 10

Question 11

Asexual vs sexual: muller ratchet

Answer 11

-Asexual populations are stuck with whatever mutations they pick up
-Sexual populations can recover the mutation-free genotypes by recombination

Question 12

Basic introduction on Group selelction

Answer 12

-Some traits help the group/species survive, so they are favoured by competition between groups or species”

Question 13

Why does sexual reproduction enhance adaptation

Answer 13

• Crossing-over during meiosis
• Mixing of genes from 2 parents
• Sex “reshuffles” genes to create new multilocus genotypes in every generation

Question 14

Co-evolution arm race

Answer 14

Question 15

The RedQueen and sex

Answer 15

-Cycles like this might make sex advantageous
-An asexual individual might lose all its descendants to a well-adapted parasite
-Sex creates and preserves genetic diversity, giving large eukaryotes a fighting chance at out-evolving fast-growing things like bacteria

Question 16

Sex and parasitism in a freshwater snail

Answer 16

• In snails, evidence that when heavily infected with parasites switch to sexual reproduction
• Snail populations consist of:
• males
• obligately sexual females (whichproduce male and female offspring)
• obligately asexual females
  Frequency of of sexual individuals correlate with infection frequency