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the evolution of SEX
Transcript of the evolution of SEX
Why do species reproduce sexually?
"Birds do it, bees do it, people do it...but sexual reproduction often seems more trouble than it's worth." (Culotta & Hines, 1998,
Advantage of asexual reproduction
sex v. asex
Examples of organisms that utilize asexual reproduction?
Parasite Coevolution Theory
Side Effect of Cell Survivor Theory
Deleterious Mutations Theory (Mueller's Ratchet)
Hypothesis (RED QUEEN)
"there are few groups of higher eukaryotes that have persisted by asexual reproduction for a long period of time, and prokaryotic reproduction in nature involves occasional parasexual events that lead to genetic exchange between different individuals" (Barton & Charlesworth, 1998)
seems like asex should win...
SEXUAL reproduction involves a reduction of the somatic chromosome number by meiosis followed by a restoration of the somatic chromosome number by fertilization
In many species, haploid (n) gametes are descended from germ cells that are originally diploid (2n) (via meiosis)
haploid – they contain ½ the genetic content
Gametes then combine in fertilization to reconstitute the diploid complement found in parental cells
Sex results in more genetically varied offspring, thus sexual reproduction can increase fitness by producing an offspring that will survive in a never-ending cycle of predator/prey and host/pathogen interactions.
Red Queen Hypothesis
“For an evolutionary system, continuing development is needed just in order to maintain its fitness relative to the systems it is co-evolving with.” --Leigh Van Valen (1973)
The Red Queen: “It takes all the running you can do, to keep in the same place.”--Lewis Carroll, “Through the Looking Glass”
In a strain of asexual species the number of deleterious mutations accumulate with time. The only way to purge the mutation is by death.
Mueller described it as a "ratchet": When the genotype with the smallest number of mutations nx dies , the genotype with the smallest number of mutations in now nx+1 and the "ratchet" has clicked up one. With each mutation event clicking the genome closer to it's demise...
Sex breaks the ratchet, Sex allows advantageous alleles in different individuals to be combined together into the same individual.
Parasites invent new keys; hosts change the locks
Parasites invent new keys; hosts change the locks; sex stores temporarily bad alleles that have promise for reuse, continually shuffling them in combination…
RECOMBINATION has been strongly selected for
With sexual reproduction, favorable mutations at different loci can be combined rapidly. Mutation alone can take very long...
Individual asxexually reproducing organisms fused together to form “mega-cells” under stressful conditions, resulting in two or more sets of genetic info. Fused cells increase survival rates than individual cells under stress conditions. Cells that fused w/others during stressful periods but could reduce their amount of genetic material during good times would be the most successful.
"if sexual reproduction increases the additive genetic variance in fitness, it will increase the rate of adaptation of the population" Barton & Charlesworth, 1998
why do we have sex?
(The Lottery Model)
Competition Theory (Tangled Bank)
reuse of genetic information through recombination without "direction"
Van Valen, 1973
Hedging bets – most plants are asexual and sexual: e.g. Aspen’s strategy
Even though sex is more costly (time, risk, energy)