Lecture 21

Question 1

What is sex in the broad sense definition?

Answer 1

genetic material from different ancestors brought together in a single descendent

Question 2

what is sex definiton in the strict sense

Answer 2

eukaryotic haploid-diploid cycle, meiosis and syngamy (the whole point of meiosis is recombination)

Question 3

What are the costs of sex (say all of them here generally)?

Answer 3

1. the cost of producing males
2. the genetic cost of meiosis
3. the cost of finding mates
4. the cost of mating

Question 4

What is the cost of producting male as a cost of sex?

Answer 4

• also referred to as the twofold cost of sex
• an asexual producing female is able to have their reproductive output be 4, but if it is a sexual reproducing female the reproductive output is 2
• so asexual female has twice the fitness and asexuality should be selected and increase in frequency over time

Question 5

Explain this cost to sex: the genetic cost of meiosis

Answer 5

• “reduced relatedness”
• only half the alleles are possed on to offspring and this halves the relatedness between parents and offspring
• so asexual females have twice the relatedness between parent and offspring

Question 6

Explain this cost of sex: the cost of finding mates

Answer 6

• this cost is also axacerbated by low population density
• time and energy spent on searching for mates could be spent foraging

Question 7

Explain this cost to sex: the costs of mating

Answer 7

• mating provides effective means of transmitting pathogens and infections
• exposure to predators (ex. snails that part of mating pairs are more likely to be attacked and captured by predators)

Question 8

What are the four benefits of sex (just listed generally here)?

Answer 8

1. combining beneficial mutations in a lineage
2. sex generates novel genotypes through recombination
3. sex allows for faster evolution
4. sex can remove deleterious mutations (Muller’s ratchet)

Question 9

Explain this benefit of sex: Combining beneficial mutations in a lineage

Answer 9

• in asexual species, advantageous mutations must occur in the same lineage
• in sexual populations, advantageous mutations can be combined across lineages through meiosis and syngamy

Question 10

Explain this benefit of sex: sex generates novel genotypes through recombination

Answer 10

• through recombination, meiosis provides an opportunity for paired chromsomes to cross over, creating gametes with unique combinations of alleles

Question 11

Explain this benefit of sex: sex allows for faster evolution (red queen effect)

Answer 11

• species must be able to continuously run/evolve to track changing environment
• if species fail to adapt, they may go extinct

Question 12

Explain this benefit of sex: sex can remove deleterious mutations, “Muller’s Ratchet”

Answer 12

• we can purge bad mutations
• sex produces genotypes with fewer mutations than parents

Question 13

What is muller’s ratchet?

Answer 13

a process which, in the absence of recombination ( in an asexual population), results in an accumulation of irreversible deleterious mutations

Question 14

does muller’s ratchet occur in sexual populations?

Answer 14

NO. sex breaks the ratchet

Question 15

how does sex break the ratchet?

Answer 15

by constituting the least mutated classes (by recombination, assuming the deleterious mutations are different among lineages)

Question 16

what is sex?

Answer 16

sex is recombination

Question 17

how do deleterious alleles act on a graph in asexual populations vs sexual?

Answer 17

• deleterious alleles will become fixed in an asexual population
• in a sexual population they are purged

Question 18

Is it common for eukaryotes to be asexual?

Answer 18

no

Question 19

How are predominantly asexual eukaryotes able to live?

Answer 19

most predominantly asexual species have sex under some conditions

Question 20

where are almost all obligatory asexual species derived from?

Answer 20

are derived recently from sexual ancestors

Question 21

What is the trend for when asexual eukaryotic species arise?

Answer 21

asexual species arise, proliferate for a while, but eventually go extinct

Question 22

Why do asexual species evolve?

Answer 22

becuase of the huge short term benefits of asexual reproduction (two fold cost, relatedness, search, STI)

Question 23

why do asexual species go extinct?

Answer 23

because the short term benefit of asexual reproduction is countered by the long term advantages of sex

Question 24

For species that can do both asexual and sexual reproduction, what happens?

Answer 24

• there is a trade-off
• depending on the environment and other conditions, either sex or asexual will be favored