chapter11 ecology

chapter11 ecology - Chapter 11 - Sex and Evolution: The...

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Chapter 11 - Sex and Evolution: The ecology of reproduction Thursday, November 08, 2007 1:23 AM Shorebirds: so many ways to mate and to achieve the same: off-spring for future generations Background Among the most fascinating attributes of organisms are those related to sexual function, such as: o Gender differences o Sex ratios o Physical characteristics and behaviors that ensure the success of an individual's gametes Sexual reproduction mixes genetic material of individuals In most plants and animals reproduction is accomplished by production of male and female haploid gametes (sperm and eggs): o Gametes are formed in the gonads by meiosis Gametes join in the act of fertilization to produce a diploid zygote , which develops into a new individual No sex! Asexual Reproduction Progeny produced by asexual reproduction are usually identical to one another and to their single parent: o Asexual reproduction is common in plants (individuals so produced are clones ) o Many simple animals (hydras, corals, etc.) can produce asexual buds, which: May remain attached to a form colony May separate to form new individuals Cost of Meiosis Sex has a hidden cost for organisms in which sexes are separate: o Only half of the genetic material in each offspring comes from each parent o Each sexually reproduced offspring contributes only 50% as much to the fitness of either parent, compared to asexually produced offspring This 50% fitness reduction is called the cost of meiosis For females, asexually produced offspring carry twice as many copies of her genes as sexually produced offspring: Thus, mating is undesirable The cost of meiosis does not apply: o When individuals have both male and female function (are hermaphroditic ) o When males contribute (through parental care ) as much as females to the number of offspring produced: If male parental investment doubles the number of offspring a female can produce, this offsets the cost of meiosis Advantages of Sex One advantage to sexual reproduction is the production of genetically varied offspring:
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o This may be advantageous when environments also vary in time and space Is this advantage sufficient to offset the cost of meiosis? Who's asexual? If asexual reproduction is advantageous, then it should be common and widely distributed among many lineages: o Most asexual species (fishes, salamanders, plants) belong to genera that are also sexual o Many asexual species do not have a long evolutionary history: Suggests that long-term evolutionary potential of asexual reproduction is low: Because of reduced genetic variability, asexual lines simply die out over time An example: Sex and Pathogens Rapid evolution of virulence by parasites that cause disease ( pathogens ): o Populations of pathogens are large o Their generation times are short The Red Queen Hypothesis
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chapter11 ecology - Chapter 11 - Sex and Evolution: The...

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