5. Sex, Sexes, Hermaphroditism
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What is the essence of sex?
sex without reproduction
when bacteria cells join to transfer genetic material via plasmid
The fusion of gametes resulting in the formation of a zygote, which develops into a new organism with new combination of alleles
cost of recombination
breaks down a demonstrably fit genotype for a completely untested combination of alleles
cost of meiosis
sexual reproducers only contribute half of genes. Sex producers have to produce twice as much as asexual
Cost of producing sons
because males in sexual reproducers do not produce offspring. Therefore sons, genetically, are a waste.
cost of mating
time and energy, exposure to predators, disease
The Adaptability Hypothesis
- Fisher (1958) - sex increases diversity, and therefore increases the rate at which population can evolve and avoid extinction
- does not explain how sex became popular. How sex became a thing when it is so costly.
- mutation is usually harmful
- Asexuals - any one mutation will be copied and copied and copied.
- sexuals - those with serious mutations are less likely to reproduce and therefore edit out the mutation.
- the advantage of sex is fleeting. sex is costly.
The Lottery Principle
- G.C. Williams 1975
- If offspring are likely to face an environment different from parents, it makes sense for parents to produce many diverse children.
- applies to r strategists who produce a lot but who do not invest in offspring.
- does not apply to k strategists who produce few offspring and invest and have low mortality rate.
Hamiltons Red Queen Hypothesis
- the environment for k strategists is an equally moving target (all predators are adapting with time too)
- best explains parasite resistance
- sex changes game by producing a new unique genotype that parasites have never seen before
Definition of male and female
- male = small gametes
- female = large gametes
- an = not
- iso = same
- gamy = gamete
- differing gamete size
when selection favors the extreme of a trait, not the mean (ex. gamete size)
why doe sexes exist?
Parker, Baker, and Smith
- multicellular organisms can't conjugate
- large gametes = quality
- small gametes = quantity
- selection favors both, not a compromise. Therefore, sexes evolved.
why do sexes exist?
Cosmides and Tooby
- sexes evolved to prevent the organelles of one parent from competing with the organelles of the other parent.
- intercellular competition would hurt zygote
- therefore small gametes destroy their organelles, large gametes monopolize organelles, and zygote is safe.
Offspring sex ratio
- usually 50/50 across species
- male fitness = generation size (G)/number of males (M)
- female fitness = G/ number of females (F)
- happy at G/M = G/F
- selection will favor whichever sex is rare, except when one sex is too costly to produce.
when does sequential hermaphrodites evolve?
when size provides some kind of advantage
when does protogyny evolve?
when males need to be bigger to protect females or to compete for males
when does protandry evolve?
when it is more favorable that females produce large eggs
when does simultaneous hermaphroditism evolve?
when species is spread out and meeting another one of your species is rare. Low density
why don't more species change sexes?
the main reason why traits are obligate is that it would be too costly to change, even if there is an advantage to do so.
what is r strategy
lots of kids, not a lot of time
few offspring, time and care into each
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