IB31Lecture+14+Outline+Evolution+of+Sex+I+Sp08 - IB31...

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IB31, Lecture 15 March 12, 2008 SEX: EVOLUTIONARY MECHANISMS I. Sex is an inefficient, risky way to reproduce. A. It takes time and energy to get the two gametes together. B. Recombination associated with sexual reproduction breaks down good gene combinations that have evolved to be effective given local conditions. C. Often, sex cells go unfertilized and die (spawning in open ocean and concentration effect – mention corals and abalone). In an animal that reproduces asexually, every sex cell could become a member of the next generation. D. Another cost of sex is that it defeats the basic tenet of life - get as many of your genes as possible into the next generation. In an asexual species, 100% of the offspring’s genes come from the parent. In a sexual species, only 50% of a parent's genes are represented. II. Lets look at the problem in a slightly different perspective. In a sexual species at population equilibrium, each female produces on average two offspring, one male and one female. A mutant parthenogenetic (development of ovum without fertilization) female that entered the population and reproduced would have an immediate 2-fold advantage because she produces two daughters, both of which can reproduce, for every one daughter that a sexual female produces. All else being equal (with respect to competitive ability), the parthenogenetic allele would rapidly spread through the population and eventually eliminate the sexual allele. III. Given these disadvantages, WHY SEX? Many evolutionary biologists consider this to be one of the greatest remaining biological questions. Over the past 50 years, a series of hypotheses have been proposed to explain the advantages of sex. I am going to go through a few of the major ones in chronological order and use the names that their originators gave them. They are not mutually exclusive. A. Weissman in late 19 th century and later R.A. Fisher: Sexual Recombination leads to greater genetic variation and therefore a faster evolutionary response. Assume two mutations (A and B) arise and that the combination AB is a superior genotype. The chances of both A and B
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arising in the same asexual individual (clone) are very poor. Therefore, asexual reproduction is unlikely to come up with this combination. In a sexual species A and B could develop in different individuals and through sexual recombination come together in the same individual giving it a fitness advantage. This will happen much faster in sexual species and therefore sets the stage for a faster evolutionary response. This model has been criticized on the basis that beneficial mutations are so rare and such recombination would occur so infrequently that the benefit could not out weight the advantages of asexual reproduction. Also, there is a cost to recombination. It breaks up beneficial combinations of genes.
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IB31Lecture+14+Outline+Evolution+of+Sex+I+Sp08 - IB31...

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