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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
+
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
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
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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
reco
mbi
natio
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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
uenc
y
xx
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.