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L16 – Why Sex

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Biology 1001A
Tom Haffie

L16 – Why Sex? Cnemidophorus uniparens – every organism is born female Sex Determinism - Which traits determine gender? Recombination leads to genetic diversity (generate new combinations of alleles) - via crossing over - independent assortment - offspring distinct from either parent/other offspring - exception: Armadillos always produce 4 genetically identical offspring (quadruplets) - polyembryonic wasps produce a single zygote which develops and fragments into thousands of embryos - results in the production of a wide variety of offspring genotypes - sex = recombination - reproduction is not sex, you can reproduce without sex in a lot of cases (ex. Binary fission) - “facultatively sexual” – option of reproducing sexually through flowers or asexually “obligately asexual” – do not have the option of sexual reproduction ex amazon molly recently descended by sexually reproducing species dioecious – “two households” male and female functions are housed in two different individuals. Each individual is male or female, NOT both ex. Ash trees, humans, many mammals monoecious – hermaphroditic, every individual has both male and female function ex. Very common in a lot of plant species Sequential monoecy – “sex change” ex. Bluestreak wrasse can change from female to male at a given time - phenomenally common in coral reef fish - once individuals reach a certain threshold body size they will change sex within a matter of hours sometimes ex. Clownfish change from male to female - when female mate disappears they change into a female - finding nemo: if it were realistic, nemos dad would have become female when his mom died Size-advantage model of sex change - size vs. reproductive success - linear positive relationship in males, bigger males have greater success than small males - steeper linear positive relation with females - larger females can lay a lot more eggs - males are not necessarily constrained by amount of sperm they can produce - protandry: individuals should be born small and change to females so the big ones can turn into females. “fitness payoff for a given body size” Distribution of sex - first life f
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