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Lecture 8

BIOL 359 Lecture 8: Sexual Selection

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BIOL 359
Kristen Muller

Sexual Selection Sexual Dimorphism: Phenotypical differences between male/females Sexual Selection: Differential reproductive success resulting from differential abilities to find a mate Example: Females that unconsciously choose mates that smell differently Asymmetric Reproductive Potentials Parental Investment:Energy, time and resource devoted to mating, gestation and caring for offspring. Females typically have higher investment than males • Eggs are expensive, sperm are cheap • Daily female egg production needs 3x the energy, whereas male sperm needs about 4/1000 of energy needed for metabolism • Females typically invest more than males — reproductive success limited by number eggs they can produce and rear • Males reproductive success is limited by number of mates they can obtain Example in Newts: • Males that encounter more mates results in more offspring, no mates means no offspring (Statistically Significant) • No females present didn’t get to mate or have offspring (No Significance) • Access determines reproductive success Example in Pipefish: • Females competing for males, large number of females had no offspring • Access to mates for females is important, in comparison to males • Limited by ability to find mates Asymmetry in reproductive potential predicts that differences in mating behaviour will exist between the two sexes. • Males: will usually be competitive - strong sexual selection o Infanticide: young from previous male dominating are killed • Females: usually be selective or choosy - weak sexual selection • Intrasexual: interactions between members of the same sex • Intersexual: interactions between members of opposite sex (not Mutually exclusive) Male-Male Competition: Combat • Intrasexual selection in male-male combat can favour morphological traits o Body size, armour, antlers etc • Males tend to be larger than females, sexual dimorphism exists in populations. • Two lucky Males o Females not seen copulating more than once, and provided considerable amount of parental care o Two males with more females than other male, these males were larger than other males in the colony o Intrasexual selection influencing directional selection on body size o Increase in size of males in population, but natural selection stabilizing on population (antagonistic Selection: two selective forces acting against each other) • Severe Male Competition in Bees o Bees are dying to fight for females (Extreme) Male-Male Competition: Sperm Competition • Female mates with more than one male, sperm from different males are in race to fertilize egg • Prolonging copulation (Males attach to females to ensure fertilization), copulatory plug (males insert plug into females so no other males can mate with them), sperm removal (spike that can remove sperm from other males, so they can fertilize egg), and larger ejaculates Male-Male Competition: Infanticide • Male lions move from pride to pride, and nursing females do not breed • Males will kill other male lion’s cubs and gain more opportunities to mate • Male infanticide created conflict between new males who take over, and it’s resident female • females will try to defend their cub, and can be killed in process • Geladas and Bruce Effect o Gelada Baboons ▪ Males often fight to take over groups of females ▪ Infanticide of previous males offspring occurs ▪ Pregnant females spontaneously abort pregnancy from scent of new males ▪ No point in birthing offspring that will result in being killed anyway Female Choice: • Males advertising themselves for mates, and females choose “best” individual • Leads to elaborate courtship displays or morphological features in males • Red Collared Widow Birds: o Experiment to test the mating success of males by altering tail feathers o A lot of energy put into maintain tail feathers o Tail feathers cut off: condition of birds significantly different from experimental group o Longer tail feather — more active n
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