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University of Toronto Scarborough
Biological Sciences
Maydianne Andrade

Lecture21 – Sexual Selection 1. Darwin’s Dilemma: Sexual Dimorphism 2. Differential parental investment 3. Types of Sexual selection a. Intrasexual (competition) b. Intersexual (mate choice) 4. Definitive Test: Sex Role Reversal Sexual Dimorphism: difference in the form and behaviour of males and females of one species - Eg: frigate Bird – difference in anatomy, behaviour, and color - Eg: American Toads – males sing and amourousness - Eg: Victoria Riflebird, Elephant Beetle, mammals, humans 4 types of sexual dimorphism - 1. Sexual dichromatism- difference in colour - 2. Structural dimorphism – eg: horns - 3. Sexual size dimorphism - 4. Behavioural dimorphism – difference in who chooses and who is persuading Darwins Dilemma: he proposed natural selection as mainly, differences in reproductive success that arise from adaptation to the environment. - So why are males and females so different in many species - And, why do many of the dimorphic traits reduce survival  very energetically expensive (eg the mule deer rack) and increase predation risk How do we explain sexual dimorphism? - Darwin: these differences are accounting for the differences in investment females invest more than males do - Conventional Sex Roles: male being bigger and more flamboyant and females being a lot more picky about who they mate with. Parental Investment: anything that increase the chance of success of your offspring and decrease the ability of the parent to produce future offspring  use of time and energy to make one offspring success leaving less time and energy to have other offspring -Differential Parental Investment: 1) Gamete dimorphism  Eggs: very expensive to produce so there are relatively few of them Sperm: is cheap to produce so there are very many of them 2)Parental Care: only in some species the maternal care is usually higher than paternal care so there is higher energy cost (more investment)in females than males In nature, in any given species, sperm are present in excess relative to eggs, and so anything that will increase the success of sperm in fertilizing an egg can be favored through selection. Differential parental investment has 3 consequences: 1. Female reproductive output is limited to access to resources Male reproduction is limited by access to female Eg: rough skinned newts – reproduce by external fertilization so you can collect the eggs and see who the father is and who the mother is and the reproductive success of specific individuals in the population -what you find is: for females who mate with 1 mate vs females who mate with 5, the number of offspring they produce is not significantly different  doesn’t matter really how many partners they have -For males however, the more they mate, significantly increases the number of offspring they produce -so MALES but not females significantly increase fitness by increasing mating frequency 2. Males have a higher MAXIMUM reproductive output than females  the difference for males that have vs have not (mate or don’t mate at all) is much greater than the difference between females with 1 or 5 partners.  the bigger the difference, the stronger the selection differential Eg: Elephant Seals: female elephant seals go on little islands to have their babies. Males try to defend females on the island from other males so that they copulate with all females (can be more than 100)- they are called beach masters. These fights can be very expensive  sometime they get killed but if they are successful in defending off other males then it can have a huge payoff in the number of offspring they produce -average female seal maximum is 25 pups but for males average maximum is 200. -For humans – the maximum a woman had is 69 and the maximum a man had is 867 - this means that traits that increase mating success can have a large effect on male fitness, but relatively small effect on female fitness - so strong selection for males to have traits that increase mating success. -EXAM QUESTION!!! - selection gradient – trait vs relative fitness and the slope of the relationship depends on how high the relative fitness of the individual is who are doing really well  if there is a big difference then you get a steep slope (zeros vs max reproducers) which results in a higher selection gradient which results in a higher selection differential -This does not mean that females have a lower fitness than male  average fitness of both is equal -some males have a lot and some have none but females all have someone in the middle  so average is the same -Newts  there are no females who have 0 mates and 0 offspring but there are a lot of males that have no mates and no offspring This leads to Sexual selection: differential reproduction due to phenotypic differences among individuals of one sex in their ability to obtain mates (and fertilizations) Lecture 22 – Sexual Selection Two types of Sexual Selection: 1. Intrasexual Selection: Agent of selection Competition – anything that they can do or they have that will allow them to beat other members of the same sex in competition for mates can be selected 2. Intersexual selection: Agent of Selection  Mate Choice- when one sex (usually females) is picky about who they mate with Intrasexual selection: - Members of the sex with more VARIABLE reproduction success compete for mating access to the other sex. - Usually males for females - Trait favoured (in competing sex): - A)large body size - B)weapons (eg: horns ) - C)Armour (eg: hard skulls in male big horn sheep) - D)ALTERNATIVE TACTICS – in individuals who are not as strong or likely to get mates, you see alternative tactics which also evolve (eg: hooknoses are large male fish who dig holes in the gravel and when a female chooses them then she comes and lays her eggs in his nest so that he can fertilize the. Jacks however, are smaller males and they hid in the vegetation around the hooknose nests and when a female comes and drops her eggs in, the jacks dart out, drop their sperm in the nest, and then run away.) (sometimes the male even looks like female which makes it easier to get close to the nest) - Another example is Redback Spiders – females like males who are large and who court for long periods of time. While the large males are courting on one side of the web, some smaller males come along and sneakily copulate with the female without courting - E) Adaptations for sperm competition- after mating when a female mates with several males in one fertilization period. (eg: sperm removal (males remove sperm of other males), prolonged copulation (the longer the male mates with the female, the less time she has to go mate with a different male), and copulatory plug (left in the female to prevent her from mating with someone else) - Eg: Damsel Flies – male has a penis that removes the sperm of any male that fertilized the female before him - Another adaptation is increased sperm production – eg: katydid  females mate about 23 times so the males have testes that account for 14% of their body mass which allows them to make a lot more sperm. Intersexual Selection: - Usually females who choose and males try to attract - Traits Favored (in competing sex): - A)Ornaments (eg: brightly colored feather, structures) - B) Displays (eg: dancing, singing) Why are females choosy? - From an evolutionary perspective, how does choosiness increase female fitness? - Although males fitness increase a lot with increasing the number of mates, this does not affect the fitness of females - How can females increase their fitness?  by increasing th
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