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PHL243H1 (26)

The Origin of Sex Notes.docx

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Ronniede Sousa

- Individual starts from a single cell (zygote) formed by two sex cells coming together (gametes) - Gametes usually only have one set of chromosomes and the zygote contains two sets of chromosomes so the baby will contain genes from both parents - In reproduction, one cell divides into two, while in sex, two cells fuse to form one - Sex is unnecessary for reproduction since organism can reproduce asexually - Some species consist just of females reproducing daughters genetically identical to themselves, particularly plants but also some animals, although there are no mammals or birds that produce parthenogenetically in this way - It thus can’t be said that without sex reproduction is impossible - In animals and humans, we associate sex with sexual difference – males have sperm and females eggs, but in hermaphrodite species like some snails, the same animal produces eggs and sperm - There is a twofold advantage of parthenogenesis, and thus a twofold cost of sex - When a sexual lizard lays 100 eggs, only 2 will survive, one male and one female - When an asexual lizard lays 100 eggs identical to herself, 2 will survive, but they can reproduce twice as much because they reproduce asexually - However, in some species like birds, both parents care for the young, so two parents would be an advantage, even though this wouldn’t be an advantage in lizards - In a species where males provide nutrients to offspring, there would be no twofold cost to sex since a baby with two parents would have twice as many nutrients than a baby with one - Sex can act at any of three levels - It can benefit some populations at the expense of others, although there might not be any advantage of a sexual individual vs. an asexual one within the same population, ex. long term benefits like speeding up evolution - It can benefit individuals, ex. competition between genes and only the best offspring surviving - It may benefit certain genes at the expense of others - Sex can benefit populations because it can help the population evolve more rapidly to meet a changing environment, ex. there are two people with different advantageous mutations, they mate, their child has two advantageous mutations – this can’t happen with asexual reproduction - But this is only helpful if the environment is rapidly changing - Sex might also reduce the number of negative mutations in the population, ex. two individuals with harmful mutations mate and produce a child with no bad mutations - But the child might have both negative mutations rather than neither - But there are circumstances where a sexual population will carry fewer bad mutations than an asexual one, because in an asexu
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