Why Sex

Question 1

sex

Answer 1

• genetic recombination that involved meiosis with crossing over and mating
• creates variance, even within siblings

Question 2

the paradox of sex

- the costs of sex (4)

Answer 2

• cost of recombination
• cost of mating
• cost of meiosis
• cost of producing males

Question 3

cost of recombination

Answer 3

sex produces some bad allele combinations and may break coadapted gene complexes

Question 4

cost of recombination

- sickle cell anemia

Answer 4

healthy heterozygotes with high survival rate can produce unhealthy offspring

• 1/4 offspring are susceptible to malaria
• 1/4 offspring have sickle cell disease

Question 5

cost of recombination

- swallowtails (2)

Answer 5

to avoid predation, swallow tails replicate features of distasteful species
- fit combinations: tail/bright or no tail/dull
mating between these phenotypes produce a swallowtail that does not successfully mimic distasteful species
- produce unfit recombinant: no tail/bright

Question 5

cost of mating

Answer 5

• it takes more time and energy to find a mating partner than if an organism were to asexually reproduce

Question 6

cost of meiosis

Answer 6

only half of genome is passed on and is diluted through generations

• sexual: 50% genetic material transmitted
• asexual: 100% genetic material transmitted and not diluted over generations

Question 7

cost of producing males

Answer 7

1/2 population is not reproducing when 50% of offspring are males; two-fold cost of sex

• sexual: female population size remains the same
• asexual: female populations doubles in size after each generation

Question 8

what is evidence that sex can be eliminated (2)

Answer 8

• asexual species have arisen from sexual ones multiple times, even in vertebrates
• some species have sexual and asexual phases that alternate

Question 9

hypotheses for the maintenance of sex: long term advantage (2)

Answer 9

• Muller’s Ratchet

- Fisher’s rate of evolution

Question 10

hypotheses for the maintenance of sex: short term advantage (2)

Answer 10

• mutational: negative epistasis

- ecological: Red Queen hypothesis

Question 11

Muller’s Ratchet (3)

Answer 11

• in asexual species of finite size, deleterious mutations should accumulate and eventually lead to extinction
• sex would weed out deleterious mutations by recreating mutation-free variants
• assumes that mutations are independent of one another

Question 12

Fisher’s Rate of Evolution

Answer 12

sex brings together advantageous mutations, thus increasing evolutionary rate

• asexual: mutations have to be sequential and in same lineage
• sexual: recombination can quickly create lineage with good mutations

Question 13

Fisher’s Rate of Evolution

- problems (2)

Answer 13

• in small populations, beneficial mutations would not be frequent enough for sex to combine them any faster than those combinations arising in asexual populations
• sexual reproduction would not be better than asexual reproduction in small populations

Question 14

Muller’s Ratchet

- problems

Answer 14

• acting alone, it operates too slowly to provide a significant short-term advantage of sex

Question 15

Fisher and Muller hypotheses

• implication
• strength

Answer 15

• both imply long term effects and species-level selection

- not powerful enough to explain sex alone seeing that asexual species do appear more prone to extinction

Question 16

short term advantage: mutational hypothesis (3)

Answer 16

• negative epistasis: non-additive effect of deleterious mutations/additional mutations cause disproportionally greater decline in fitness
• sexual reproduction concentrates “bad” alleles in some offspring, thus requiring fewer selective deaths for sexual reproduction to win
• sexual reproduction combines bad mutations at same loci together

Question 17

short term advantage: ecological hypothesis; sibling (2)

Answer 17

• sex is advantageous in the face of an uncertain future; sex can create variety in phenotype so offspring have some fitness advantage in changing environments
• if environment changes slowly, process works only for very fecund organisms with high competition/low survival

Question 18

short term advantage: ecological hypothesis; Red Queen (3)

Answer 18

• pathogens and parasites are fast changing components of the environment
• sex and recombination create diverse offspring to which pathogens and parasites are not well matched
• implication: sex has evolved to confound our germs

Question 19

parasitic pressure and sexual mode (2)

Answer 19

• high parasitic pressure leads to greater number of sexual organisms
• lower parasitic pressure leads to greater number of asexual organisms

Question 20

short term advantage mutations hypothesis:

- requirements (3)

Answer 20

• high mutation rates (>1 per genome per generation)
• large population size
• negative epistasis: each deleterious mutations leads to greater decline in fitness than previous one

Question 21

short term advantage ecological hypothesis:

- requirements (2)

Answer 21

• sever effects of parasites on their hosts
  OR
• ‘hard’ rather than ‘soft’ selection: only healthiest hosts survive and reproduce

Question 22

advantages of combining hypotheses: (3)

Answer 22

• mutational and ecological explanations can complement each other
• we don’t discriminate among them, but rather estimate parameters
• different mechanisms may be relevant to different species or act simultaneously within the same population

Question 23

rate of environmental change

- examples (2)

Answer 23

• low rate: climate change

- high rate: pathogens or parasites

Question 24

evolutionary traction

Answer 24

• in asexual populations, deleterious alleles may fix by hitchhiking along favourable mutations

Question 25

when do asexuals prevail (4)

Answer 25

• graph: zone I
• when there is high rate of evolutionary change and low rate of genomic change
• when there is high rate of genome mutation rate and low environmental change
• when both factors are low

Question 26

when do sexual prevail (2)

Answer 26

• graph: zone II

- when both rate of environmental change and genome mutation rate are high

Question 27

when is Muller’s Ratchet sufficient to explain sexual reproduction (2)

Answer 27

• graph: zone III
• when genome mutation rate is high (rate of environmental change does not necessarily matter as long as mutation rate is high)