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bild 3 lecture 9

bild 3 lecture 9 - A B 57 73 51 85 52 61 94 66 96 55 One...

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Sexual reproduction B) Asexual reproduction A) 96 52 61 66 94 55 85 51 57 73 One mutational difference In this simulation, I have followed two populations made up of organisms with chromosomes carrying 100 genes, each of which has a low mutation rate. In the asexual population (left) only mutational differences between entire chromosomes accumulated and the result was a set of chromosomes with very few differences between them. In the sexual population (right) mutational differences that accumulated at the same rate were allowed to recombine, producing a wide variety of recombinant chromosomes. There were no more mutant alleles in the second population than in the first, but the number of diffferent chromosomes and the differences between them were much greater because the mutant alleles could be brought together on different chromosomes in a wider variety of combinations. Under some circumstances, especially when the environment is changing rapidly and many members of the population are dying, this ability to produce a range of different genotypes may be advantageous. This is because high-fitness sexual reproducers are more likely to appear. But recombination could be disadvantageous if most of the new genotypes confer a lower fitness than the original genotype. This might be the case when environmental conditions are constant, and most of the organisms in the population are well-adapted. Another disadvantage of sexual reproduction is that it permits the spread of sexually transmitted diseases. And a third disadvantage is that asexual
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organisms can outreproduce sexual ones — all asexual organisms can have babies, but only half of sexual ones can! (See below) Generation One Two Three Asexual Sexual The figure shows the so-called twofold cost of sex, which gives a strong advantage to asexual organisms provided that there is an advantage to rapid population expansion. In spite of almost eighty years of theory and experiments, the disadvantages of sexual reproduction remain obvious, while the advantages continue to be elusive! One likely advantage, for which there is some evidence, is that we may be in an evolutionary race with our pathogens, so that we must evolve as quickly as possible to keep up. Genetic recombination will speed up this process of adaptation. Red Queen Races In 1976, paleontologist Leigh van Valen suggested that many species are running Red Queen races against changes in their environment. Van Valen began by recalling Lewis Carroll’s famous children’s book Through the Looking- Glass , the sequel to Alice in Wonderland , in which Alice ventures into a looking- glass world where everything is backwards. Here chessmen are alive rather than inanimate, and here she encounters the forbidding figure of the Red Queen (Figure below).
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John Tenniel’s illustration of Alice running with the Red Queen. This famous scene has become a metaphor for how species can coevolve with their rapidly changing environment.
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