Why Sex?

Asexual reproduction produces offspring that are exact copies

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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

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Sperm egg(1n) (1n)

fertilization

One-celled Zygote--> 2n

Cell division (mitosis)And development

gametes

meiosis

2n

The problem with sex

Generation 1

Generation 2

Generation 3

Asexual reproduction Sexual reproduction

Why produce males?

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

Sex Generates Genetic Variation

• Sex produces offspring with new combinations of alleles

• i.e., sex reduces linkage disequilibrium

Linkage equilibrium Linkage disequilibrium

Gene A: f(A) = pf(a) = q

Gene B: f(B) = rf(b) = s

Then the frequency of AB gametes should be:

f(AB) = p*r= pr

If certain alleles are found together in higher than expected frequencies

i.e., f(AB) > pr

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

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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

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 Animal size, life span

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Burt & Bell:

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

fitness

freq

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sexuals

asexuals

The Red Queen Hypothesis

"Here, you see, it takes all the running you can do to keep in the same place."

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.

Prediction:

Sexual reproduction should be more common when the risk of parasitism is higher

Experimental evidence

Snails subject to parasitism by trematode worms

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Frequency of infection by parasites

Do highly parasitized populations have higher levels of sexuals?