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Lecture 16

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Biology 1001A
Tom Haffie

Biology 1001 Why Sex? November 5, 2012 Sex as recombination  Recombination -> genetic diversity  Crossing over, independent assortment  Offspring distinct from either parent and (usually) each other  Armadillos always have 4 offspring at a time that are genetically identical to one another  Poly-embryonic wasps provide a single zygote that fragments into thousands of embryos Sex does not mean reproduction, it means recombination  Reproduction can occur without sex: in bacteria and other things that reproduce through binary fission  Sex can occur without reproduction: genetic combination without reproduction  Some plants have the option to reproduce sexually or asexually  By vegetative propagation (asexual): creates genetically identical offspring  Can also reproduce sexually  “Facultatively sexual”  The Amazon molly is obligately asexual: cannot reproduce sexually at all. Sexually reproducing organisms may be dioecious or monoecious  Dioecious: male and female function are housed in different individuals  Every individual is either male or female  Monoecious: hermaphroditic  Every individual has both male and female functions Sequential Monoecy (sex change)  Change sex at some point in their lives  Bluestreak Wrasse: female to male  Once they reach a relative body size, they change their sex  Every individual is born female  Clownfish: male to female Size advantage model of sex change  In males, there is a weak positive success  In females, there is a steeper fitness function with body size (larger females can lay more eggs?)  They cross at a certain point  If they start of being smaller, it would be better to start of being male until a certain threshold, where it starts to get bigger, it would pay off to be female (protandry) Distribution of Sex  First life forms almost certainly reproduced asexually  Most things, except plant and animals, reproduce mostly asexually Biology 1001 Why Sex? November 5, 2012  But among animals… there are
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