1. Origins and maintenance of sex Sexual conflict Sex ratios
Dr. Sally Otto, UBC

2. Asexuality vs. self-fertilization
Give examples of the mechanism – pseudo copulation footage in snakes
Video footage?

3. Costs of sex

4. Advantages of asexuality: which sex limits population growth?
Sexual lineage
Asexual lineage
Note: no figure, but attempted to convey same idea with letters.
What would happen if an asexual mutant occurred in a population? Would it increase or decrease?

5. Advantages of asexuality: fitness
of individuals
Frequency

6. 99.9% of species can’t be wrong
So, is sex good?
99.9% of species can’t be wrong
Cnemidophorus
Sexual
Asexual
Dandelions
Daphnia

7. (but a few asexual lineages persist)
Bdelloid rotifers
freshwater filter feeders
This implies that asexuality should have some major problems, or sex some major advantages.
What exactly?

8. Hypotheses to explain the maintenance of sex
Harmful mutations: Mller’s ratchet
Speed of adaptation and the Red Queen

9. Disadvantages of asexuality: Müller’s ratchet
Mutations happen
and accumulate
mutation-free chromosome

10. # of deleterious mutations # of deleterious mutations
Müller’s ratchet
LLC: “least-loaded class”
“CLICK”
frequency
frequency
# of deleterious mutations
# of deleterious mutations

11. Müller’s ratchet in sexuals? No.
“CLICK”
frequency
frequency
# of deleterious mutations
# of deleterious mutations
frequency
# of deleterious mutations

12. Müller’s ratchet : an experiment
Set up 444 cultures of Salmonella
Transfer one individual every 24 hours
1700 generations
Prediction: lower fitness
Test: generation time:
parental:
Experimental populations:

13. Evidence for Müller’s ratchet : the human Y chromosome
In XX females, recombination
Y chromosome does not pair with X

14. Müller’s ratchet overall

15. Reason’s for sex: adaptationB1 A2 B2
Low fitness
High fitness A1 B1
parental A1 B2 A1 B1
recombinant
meiosis A2 B1 A2 B2
recombinant A2 B2
parental

16. Does recombination increase genetic variance for fitness?

17. The results of sex: cost of recombination
If parents have higher than average fitness, what effect of recombination?
The results of sex: cost of recombination A1 B1
meiosis A2 B2
parental
recombinant

18. Experimental test of recombination and adaptation
Adaptation may require new combinations of alleles
Asexuality does not allow this
Experiment: flour beetles (Tribolium)
Have stock population
Allow one population to evolve
The other is restocked from original population, as if asexual.
Asexual has 3x reproductive advantage
Asexuals start 0.5 of population
Selection: pesticide Malathion
What proportion are sexual?

19. Advantage of sex: adaptation
Malathion concentration
Proportion sexual 30
Generations
figure 7.18

20. Do organisms need to adapt? Red queen hypothesis
Red Queen to Alice:
“Now, here, you see, it takes all the running you can do, to keep in the same place.”
--Lewis Carrol,
Through the Looking Glass (1872)
Concept: constant adaptation needed. Why?

21. Parasites and hosts
Imagine four parasite genotypes, four host defense genotypes
Parasite
Host

22. Parasites and host: Red queen
start
after selection on host

23. Parasites and host: Red queen
after selection on host
after selection on parasite

24. Evolution over time: cycling genotypes

25. Red queen, evidence: topminnows of Mexico
Interspecific hybrids from asexual triploids
or: sexual diploid fish
Infected by parasites that cause spots

26. Data I: sexuals vs. asexuals: which has more parasites?
Asexuals have more parasites
Parasites
Fish length (mm)

27. What if there is no variation in sexual population?
Heart pool: dried up in 1976
Recolonized by just a few sexual minnows and a few asexual minnows.
Which will have the higher fitness?

28. Why sex, summary

29. Why two sexes? Most species have two sexes
Some have multiple sexes (mating types): mating type 1 can mate with anyone except mating type 1, etc.
Advantage: higher proportion of population available for mating
Why only two mating types??

30. Implication of two sexes
Sexual selection
Sexual conflict

31. Intrasexual competition: sperm competition
When females mate with multiple males, sperm can compete
Example: yellow dung fly Scathophaga stercoraria

32. Yellow dung flies Females mate with multiple males
Last male to mate fertilizes most ovules
Selection experiment: compare wild to 10 generations monogamous, 10 generations polyandrous
Expected effect on sperm competition?
Monogamous
♀ + ♂
Polygamous
♀ + 3♂
10 gen.
10 gen.

33. Yellow dung flies: polyandrous males vs. monogamous males
Mate each female with two males: one from polyandrous line, one from monogamous line
Hosken & Ward % offspring sired by second male to mate

34. Sexual selection and intersexual conflict evolution
Traits that favour male success may harm female success
Example: sperm competition
faster sperm win: higher fitness for male
too many sperm: polyspermy – egg is lost
human spontaneous abortions: 3% of conceptions end due to triploid embryos (1 egg, 2 sperm)

35. Female evolutionary response: slow the sperm
Yellow dung fly experiment

36. Evolution of sex ratios
Which sex should be more common?
Why aren’t there many females per male?
imagine monogamous species
females become more common
which would be better to have has offspring: male or female?

37. Sex bias? If females could choose:
What if polygamous species?
some males have many mates
most males do not mate
If in excellent condition, should a female produce male or female offspring?
If in poor condition, should a female produce male or female offspring?

38. Sex bias: data Case 1: Kakapo (NZ parrot)
Captive breeding, plenty of food.
When well fed: 70% sons
Adequately fed: 50% sons

39. Sex bias in humans: data
Mormon second (or higher) wives
Sons Daughters Ratio
One co-wife 4,217 3,
More than one

40. References
Hoskins, Garner, and Ward Sexual conflict selects for male and female reproductive characters. Current Biology 11:
Hoskins and Ward Experimental evidence for testes size evolution via sperm competition. Ecology Letters 4:10-13.
Lively, Craddock, & Vrijonhoek Red queen hypothesis supported by parasitism in sexual and clonal fish. Nature 344:
Ridley, M The red queen: sex and the evolution of human nature. Harper. Nice job reviewing Muller’s ratchet and other hypotheses for the maintenance of sex. Goes boldly into explaining human nature, far beyond the evidence.
Robertson et al Sex allocation theory aids species conservation. Biology Letters 2:

41. Readings and questions
1. In the beetle evolution experiment (figure 8.18) Dunbrack et al did not actually asexual beetles, as there aren't any. Instead they used two different lines of beetles that differed in color, treating one line as if it were asexual by replacing individuals with individuals from a stock population. The researcher's simulated asexual population was not allowed to evolve at all in response to competition and the presence of the insecticide. Is this realistic?
2. In general, would you expect asexual lineages to persist longer with small population sizes or large population sizes? Why?
3. If the offspring of sexual and asexual individuals have equivalent fitness, why would asexuals take over a population?
4. Explain how Muller's ratchet affects sexual populations differently from asexual populations.
Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease?
In most species, the sex ratio is 50 / 50 male: female. Why wouldn’t evolution favor a higher proportion of females, since one male could mate with many females? Discuss using the idea of frequency dependent selection.