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Linkage disequilibrium and the evolution of sex
Q: What distinguishes sexual from asexual reproduction?
Linkage disequilibrium and the evolution of sex
Q: What distinguishes sexual from asexual reproduction?
A: Meiosis and Syngamy
Linkage disequilibrium and the evolution of sex
Q: What distinguishes sexual from asexual reproduction?
A: Meiosis and Syngamy
Male Female
2N 2N
Linkage disequilibrium and the evolution of sex
Q: What distinguishes sexual from asexual reproduction?
A: Meiosis and Syngamy
Male Female
2N 2N
Meiosis N N
Linkage disequilibrium and the evolution of sex
Q: What distinguishes sexual from asexual reproduction?
A: Meiosis and Syngamy
Male Female
2N 2N
Meiosis N N
Syngamy
2N
A model for the evolution of two sexes
• in many species, sexual reproduction has entailed a shift from isogamy to anisogamy.
A model for the evolution of two sexes
• in many species, sexual reproduction has entailed a shift from isogamy to anisogamy.
Isogamy
+ -
A model for the evolution of two sexes
• in many species, sexual reproduction has entailed a shift from isogamy to anisogamy.
Isogamy
+ -Anisogamy
♂
♀
A model for the evolution of two sexes
Mating Gamete type (M) size (G)
+/- Small (S)/Large (L)
Linkage disequilibrium
MG
A model for the evolution of two sexes
Mating Gamete type (M) size (G)
+/- Small (S)/Large (L)
Linkage disequilibrium
MG
MG
+S
♂
A model for the evolution of two sexes
Mating Gamete type (M) size (G)
+/- Small (S)/Large (L)
Linkage disequilibrium
MG
MG MG
- L+ S
♀♂
MG MG
- L+ S
♀♂
MG
+ L
MG
- S
Recombinants:
fitness due to low sperm number
fitness due to inviable eggs
What is linkage disequilibrium?
Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus.
What is linkage disequilibrium?
Linkage equilibrium occurs when the genotypes present at one locus are independent of the genotypes present at a second locus.
Linkage disequilibrium occurs when genotypes at the two loci are not independent of each other.
Q: What causes linkage disequilibrium?
1. Natural selection
• can be produced by epistatic selection
Q: What causes linkage disequilibrium?
1. Natural selection
• can be produced by epistatic selection
• epistasis occurs when the fitness of a genotype at one locus depends on its genotype at another locus
Q: What causes linkage disequilibrium?
1. Natural selection
• can be produced by epistatic selection
• epistasis occurs when the fitness of a genotype a one locus depends on its genotype at another locus
2. Random genetic drift
• much weaker than selection in creating disequilibrium.
Q: What causes linkage disequilibrium?
3. Population admixture
• can be as important as selection in creating disequilibrium.
Q: What causes linkage disequilibrium?
3. Population admixture
• can be as important as selection in creating disequilibrium.
Q: What eliminates linkage disequilibrium?
Q: What causes linkage disequilibrium?
3. Population admixture
• can be as important as selection in creating disequilibrium.
Q: What eliminates linkage disequilibrium?
A: Recombination!
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
Some alternatives:
1. Parthenogenesis (both mitotic and sexual forms)
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
Some alternatives:
1. Parthenogenesis (both mitotic and sexual forms)
• organisms develop from unfertilized eggs.
Examples: lizards, aphids, many plants
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
Some alternatives:
1. Parthenogenesis (both mitotic and sexual forms)
• organisms develop from unfertilized eggs.
Examples: aphids, many plants
2. Hermaphroditism (obligate or sequential)
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
Some alternatives:
1. Parthenogenesis (both mitotic and sexual forms)
• organisms develop from unfertilized eggs.
Examples: aphids, many plants
2. Hermaphroditism (obligate or sequential)
• organisms possess both male and female reproductive organs, or change sex at some point in their lives.
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
Some alternatives:
1. Parthenogenesis (both mitotic and sexual forms)
• organisms develop from unfertilized eggs.
Examples: aphids, many plants
2. Hermaphroditism (obligate or sequential)
• organisms possess both male and female reproductive organs, or change sex at some point in their lives.
Examples: many fishes, snails, worms
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
3. Haplodiploidy
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
3. Haplodiploidy
• haploid males develop from unfertilized eggs, diploid females from fertilized eggs.
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
3. Haplodiploidy
• haploid males develop from unfertilized eggs, diploid females from fertilized eggs.
Examples: ants, bees, wasps
How and why did sex evolve?
or… how is sexual reproduction maintained in the face of so many alternative strategies?
3. Haplodiploidy
• haploid males develop from unfertilized eggs, diploid females from fertilized eggs.
Examples: ants, bees, wasps
4. Pseudogamy
• contact with sperm stimulates development from unfertilized eggs.
Example: some nematodes and freshwater fishes
But… of the world’s 2 million named species less than 2,000 are totally asexual
… and they don’t appear to persist very long
Asexual species are typically found at the tips of phylogenetic trees
S = sexual speciesA = asexual species
S A S S S A S A S S
The “costs” of sex
2. The cost of finding mates
• exacerbated by low population density.
3. The costs of mating
The “costs” of sex
2. The cost of finding mates
• exacerbated by low population density.
3. The costs of mating
• mating is a risky business!
The “costs” of sex
2. The cost of finding mates
• exacerbated by low population density.
3. The costs of mating
• mating is a risky business!• also vulnerable to sexually transmitted diseases.
The “costs” of sex
2. The cost of finding mates
• exacerbated by low population density.
3. The costs of mating
• mating is a risky business!• also vulnerable to sexually transmitted diseases.
4. The cost of recombination
The “costs” of sex
2. The cost of finding mates
• exacerbated by low population density.
3. The costs of mating
• mating is a risky business!• also vulnerable to sexually transmitted diseases.
4. The cost of recombination
• recombination creates superb combinations of genes then quickly breaks them apart.
Why then does sexual reproduction persist?
1. Adaptive evolution is enhanced
• in asexual species, advantageous mutations must occur in the same lineage:
Why then does sexual reproduction persist?
1. Adaptive evolution is enhanced
• in asexual species, advantageous mutations must occur in the same lineage:
advantageous mutation
Abcd Abcd’
Ab’cd’ advantageous mutation
Why then does sexual reproduction persist?
1. Adaptive evolution is enhanced
• in asexual species, advantageous mutations must occur in the same lineage.
• in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy).
Why then does sexual reproduction persist?
1. Adaptive evolution is enhanced
• in sexual populations, advantageous mutations can be combined across lineages (through meiosis and syngamy):
advantageous mutation
Abcd Abc’d
x Abc’d’
Abcd’ Abcd’ advantageous mutation
Why then does sexual reproduction persist?
2. The Red Queen hypothesis
• originally proposed by Leigh Van Valen in 1973.
Why then does sexual reproduction persist?
2. The Red Queen hypothesis
• originally proposed by Van Valen in 1973.
• species must continuously “run” (evolve) to track changing environments.
Why then does sexual reproduction persist?
2. The Red Queen hypothesis
• originally proposed by Van Valen in 1973.
• species must continuously “run” (evolve) to track changing environments.
• if species fail to adapt, they may go extinct.
Why then does sexual reproduction persist?
2. The Red Queen hypothesis
• originally proposed by Van Valen in 1973.
• species must continuously “run” (evolve) to track changing environments.
• if species fail to adapt, they may go extinct
• sexual reproduction facilitates this process.
The Red Queen process is an evolutionary arms race
Target species
“Enemies”(parasites, predators, competitors)
The Red Queen process is an evolutionary arms race
Target species
“Enemies”(parasites, predators, competitors)
Adaptation
The Red Queen process is an evolutionary arms race
Target species
“Enemies”(parasites, predators, competitors)
AdaptationCounter-adaptation
Muller’s ratchet
• deleterious mutations occur in asexual lineages…
• … causing the least mutated class to dwindle…
Mutation
Muller’s ratchet
• deleterious mutations occur in asexual lineages…
• … causing the least mutated class to dwindle…
• … and be lost by random drift
Mutation
Mutation
?
Muller’s ratchet
• deleterious mutations occur in asexual lineages…
• … causing the least mutated class to dwindle…
• … and be lost by random drift
• now the ratchet has “clicked” forward once.
Mutation
Mutation
?
Muller’s ratchet
• deleterious mutations occur in asexual lineages…
• … causing the least mutated class to dwindle…
• … and be lost by random drift
• now the ratchet has “clicked” forward once.
• now the ratchet has “clicked” forward again.
Mutation
Mutation
Mutation
? ?
?
Muller’s ratchet
Asexual populations can only evolve towards ever greater loads of deleterious mutations!
Muller’s ratchet
Asexual populations can only evolve towards ever greater loads of deleterious mutations!
Does Muller’s ratchet occur in sexual populations?
Muller’s ratchet
Asexual populations can only evolve towards ever greater loads of deleterious mutations!
Does Muller’s ratchet occur in sexual populations?
NO! Sex breaks the ratchet.
Muller’s ratchet
Asexual populations can only evolve towards ever greater loads of deleterious mutations!
Does Muller’s ratchet occur in sexual populations?
NO! Sex breaks the ratchet.
How? By reconstituting the least mutated classes (by recombination).
Muller’s ratchet
Asexual populations can only evolve towards ever greater loads of deleterious mutations!
Does Muller’s ratchet occur in sexual populations?
NO! Sex breaks the ratchet.
How? By reconstituting the least mutated classes (by recombination).
SEX IS RECOMBINATION!
Q: So why are asexual species at the tips of phylogenetic trees?
S = sexual speciesA = asexual species
S A S S S A S A S S