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Sexual selection. A difference between the sexes is called a sexual dimorphism.

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Presentation on theme: "Sexual selection. A difference between the sexes is called a sexual dimorphism."— Presentation transcript:

1 Sexual selection

2 A difference between the sexes is called a sexual dimorphism

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4 Natural selection cannot always explain sexual dimorphism

5 Asexual reproductionSexual reproduction live long enough reproduce persuade partner to mate Asymmetries in sexual reproduction Sexual selection - differential reproductive success due to variation among individuals in success at getting mates

6 Sexual reproduction imposes different selection pressures on females versus males Asymmetries in sexual reproduction

7 Females Pregnant for 8 months Nurses for 3 years Protects until 7 or 8 yrs old Orangutan mating Males A few mL of semen

8 Sperm is cheap!

9 reproductive success potentialrealized femalessmallegg (preg) limited maleslargemate limited Asymmetries in sexual reproduction

10 male reproductive success limited by access to mates female reproductive success limited by capacity to produce offspring Asymmetries in sexual reproduction

11 Sexual selection theory predicts that males will compete with each other over access to mates and that females will be choosy Asymmetries in sexual reproduction

12 Intra-sexual selection - outcome determined within a sex male-male competition Inter-sexual selection - outcome determined by opposite sex female choice

13 Male-male competition combat over access to females sperm competition infanticide Intra-sexual selection

14 combat - elephant seals Male-male competition

15 sperm competition - damselflies Male-male competition

16 sperm competition - other examples Male-male competition large ejaculates mate guarding prolonged copulation copulatory plug repellant pheromones applied to female

17 sperm competition - other examples Male-male competition large ejaculates

18 infanticide - lions Male-male competition

19 infanticide - lions Male-male competition

20 Female choice better genes resource acquisition preexisting sensory bias ‘sexy sons’ Inter-sexual selection

21 Female choice barn swallows

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25 better genes - gray tree frogs Female choice long-call males had higher fitness offspring

26 resource acquisition - hanging flies Female choice nuptial gifts

27 pre-existing sensory biases - tungara frogs Female choice females prefer low frequency calls

28 pre-existing sensory biases - swordtails & grassfinches Female choice females prefer males with artificial enhancements

29 ‘sexy sons’ Female choice stalk-eyed flies peacocks

30 Runaway selection -females prefer some trait -males with that trait leave more offspring -male offspring will possess that trait and female offspring will prefer males that possess that trait -leads to a positive feedback loop, get exaggeration of the trait -if trait becomes a detriment to survival, sexual selection balanced by natural selection

31 Species in which males invest more in each offspring, and are thus a limiting resource for females, are the exceptions that can prove the rules of sexual selection Role reversal

32 male pipefish brood their young, and access to males is a limiting resource for females. females fight among themselves and males are choosy Role reversal

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