Evolutionary Biology 9

Question 1

what is Asexual reproduction

Answer 1

• reproduction without sex
• it is easy, error-free and simple
• offspring genetically identical

Question 2

Sexual reproduction

Answer 2

two haploid gametes fuse to form diploid zygote

• involves meiosis (thus recombination+segregation)
• results in the formation of gametes containing a combination of alleles from both parents

Question 3

What is the paradox of sex

Answer 3

-inefficient, complicated and 
  time-consuming
-difficult + dangerous as 
  have to find a mate
-Costly, often involves sexual 
 selection + twofold cost of 
 sex  (AKA cost of males)

Question 4

assumptions of two fold cost of sex

Answer 4

1. every female produces an equal number of offspring
2. no differential survival
   - for sex to evolve at least one of these must be violated

Question 5

Reasons why sex is good/may have evolved

Answer 5

• Male parental care may increase reproductive out of females (this cant explain all sex)
• Natural selection can lead to differential survival if there is variation, lack of variation in asex = no selection
• Sexual populations have more genetic variation. (recombination + segregation etc.)

Question 6

Recombination

Answer 6

process by which pieces of DNA are broken and recombined to produce new combinations of alleles

• consequences of recombination depend on populations composition and mating method (i.e. random or non-random) - recombination can bring together two beneficial alleles that occurred in different organisms
• in asexual populations a mutation would have to occur twice in a genotype to have a similar effect (very unlikely)

Question 7

How can the short term cost of sex (affecting the next generation) be overcome to generate long term benefits?

Answer 7

1. Genetic drift (Mullers ratchet)

2. Selection in a changing environment (red queen)

Question 8

Muller ratchet

Answer 8

• Mutation and genetic create problems that sex can solves
• the number of deleterious mutations in asexual lineage can only increase over time
• recombination stops passing on of deleterious alleles
• an optimal genotype lost by drift can be reconstructed by recombination.

Question 9

evidence for Mullers ratchet

Answer 9

E.g. Chao et all (1990) RNA viruses

- started with a single 
 bacterium and then 
 propogated it to 20 lines of 
 RNA virus 
- took one line and then 
 created a new culture 
-continued for 40 
 generations 

• Fitness (growth rate) declined over subsequent generations

Question 10

Assumptions of Red queen model ( selection in a changing environment)

Answer 10

1) -Recombination increases the rate of adaptation (directed reading)
2) -Some novel genotypes have high fitness

E.g. (2) Lively (1992)
- found positive correlation in number of males and level of trematode infection in population of snails as expected if high level of parasitism favour sexual reproduction

• both sexual and asexual snails occur