1. Why Sex?

2. Asexual reproduction produces offspring that are exact copies
Amoeba porn flicks

3. Sexual reproduction produces offspring with genetic variation
Specialized cells called gametes fuse together to form a one-celled zygote, which develops into the offspring of the next generation 2n
Cell division (mitosis)
And development
fertilization +
Sperm egg
(1n) (1n)
One-celled
Zygote--> 2n
gametes
meiosis

4. The problem with sex Asexual reproduction Sexual reproduction
Generation 1
Generation 2
Generation 3

5. Why produce males?

6. Evolution is an arms race
Asexuals will outcompete sexuals in terms of numbers
Sexual reproduction must provide an advantage in terms of the quality of offspring

7. Sex Generates Genetic Variation
Sex produces offspring with new combinations of alleles
i.e., sex reduces linkage disequilibrium
Linkage equilibrium
Linkage disequilibrium
If certain alleles are found together in higher than expected frequencies
i.e., f(AB) > pr
Gene A: f(A) = p
f(a) = q
Gene B: f(B) = r
f(b) = s
Then the frequency of AB gametes should be:
f(AB) = p*r= pr

8. Why is genetic variation advantageous?
Muller’s Ratchet
Sex reduces genetic load
Tangled Bank Hypothesis
Genetic variation reduces competition for resources
Lottery Hypothesis
Sex can produce offspring with higher fitness in a temporally changing environment
Red Queen Hypothesis
Genetic diversity provides an advantage in escaping biological enemies

9. Muller’s Ratchet
In an asexual population, clones will accumulate deleterious mutations
Less common clones will be lost through genetic drift--> distribution “ratchets” to the right
Genetic load causes asexuals to go extinct
Sex can regenerate individuals with fewer mutations
Problem:
Asexuals will outcompete sexuals before genetic load becomes large enough to take effect

10. Tangled Bank
"It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us…growth and reproduction…inheritance…variability…a ratio of increase so high as to lead to a struggle for life." -Darwin 1859
In a spatially variable environment, parents that produce offspring that can use a variety of resources will be favored those that produce genetically identical offspring
Problem:
Prediction: Large animals have smaller broods, less sib competition--> less selection for sex
Burt & Bell:
recombination
Animal size, life span

11. Lottery Model
Sexuals are favored when the environment changes over time
Asexuals: buy 100 lottery tickets all with the same number
Sexuals: buy 100 tickets, each with a different number
Asexuals have a higher mean fitness, but a few sexual will be “winners”
Problem:
Bell (1982): sexual reproduction is associated with complex but stable environments x x
asexuals
sexuals
frequency
fitness

12. The Red Queen Hypothesis
"Here, you see, it takes all the running you can do to keep in the same place."

13. Red Queen dynamics:
Results from a computer simulation for host-parasite coevolution. Note that both genotypes oscillate over time, as if they were "running" in circles.

14. Prediction: Sexual reproduction should be more common
when the risk of parasitism is higher

15. Experimental evidence
Snails subject to parasitism by trematode worms

16. Frequency of infection by parasites
Do highly parasitized populations have higher levels of sexuals?
0.40
0.30
0.20
0.15
Male frequency
0.10
0.05
0.01
0.00
0.00
0.05
0.15
0.30
0.50
Frequency of infection by parasites