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EVOLUTION PART 2 NOTES Sex Primary sex ratio The sex ratio with which each generation begins Ideally it is the ratio among fertilized eggs o In many species (particularly those with parental care) ratio of fertilized eggs is difficult to measure Darwin-wide range of species, sex ratio=1:1 (male: female) o Individual leaving 10 surviving offspring had same RS whether these were ten males, all females, or 5 of each. o Sex ratio of one generation affects RS of these individuals in following generation Diff mortality by sex o During parental investment this does affect sex ratio o After period of PI, no effect o Expect to find that the sex that dies more quickly is initially produced in greater numbers----males Investment in male and female may change in a complex way with age of offspring o Mammals sex ratio slightly male-biased at birth o If males dying FISHER’S SEX RATIO THEORY Fisher’s says it’s usually 1:1 no matter social system (monogamy polygyny) because expected RS of male and female is equal when sex ratio is equal o Parents are selected to invest total amount of investment equal in males and females Imagine species where ratio is 1:3 (male to female)---males have 3 times more RS, inseminate 3 females to every females 1 partner o Same in reverse Mother makes a son instead of a daughter, she’ll have 3 times the amount of grandchildren o Pressure on mothers to produce males o As genes for investing more heavily on sons begin to spread, the ratio becomes less female biased, reducing advantage on males 1:1 sex ratio in equilibrium, no longer expected to change factors influencing selection on sex ratios o mortality of the sexes during the period of parental investment. o differences in the cost of producing offspring of the two sexes. o We talked about one particular type of example, that as Trivers-Willard which did involve female humans being able to marry men of higher socioeconomic status (but men are not able to do so). This results in females of high socioeconomic status and males of low socioeconomic status not finding mates.
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2 steps of Fisher’s theory o 1)sex ratios we expect to find in nature are those at which the return per unit work on a male = return per unit work on a female Ex: female investing in offspring of 2 sexes-she’ll be selected to produce sex ratio that maximizes her genetic return Genetic return measured as number of surviving grandchildren o 2) Fisher realized that expected RS of male relative to that of female is itself a function of primary sex ratio high ratios (many males per female) mean that each male has only a small chance of fertilizing a female and hence has low expected RS compared to female low ratios means each male has high expected RS individuals of underrepresented sex enjoy higher RS we expect ratio to reflect relative cost of producing a male compared to a female o Ex: species where it costs 2x as much to produce son as a daughter (half as
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This note was uploaded on 01/26/2012 for the course 70 204 taught by Professor Roberttrivers during the Spring '11 term at Rutgers.

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