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This 6 page Class Notes was uploaded by Hiba Kouser on Wednesday February 17, 2016. The Class Notes belongs to BIOL 4700 at Clemson University taught by Michael J Childress in Fall 2016. Since its upload, it has received 38 views. For similar materials see Behavioral Ecology in Biology at Clemson University.
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Date Created: 02/17/16
L05 Notes: Sexual Selection I. Sexual Selection A. Sexual selection: selection for traits that increase mating success 1. Traits that can increase a male’s mating success is likely to be very different than a trait that increases a female’s mating success and this can lead to sexual divergence 2. Written by Darwin in Descent of Man and Selection In Relation to Sex (1871) B. Cost and Benefits of Sex: 1. What are the advantages of asexual reproduction? a. Is faster b/c you don’t need to find a mate b. Leads to increase in the number of genetic copies you contribute to future generations c. Keeps superior gene combinations together 2. Why don’t more animals reproduce asexually? a. Is very rare in the animal kingdom partially because of the costs b. Genetic material gets diluted from one generation to the next c. Can end up with an allfemale species (no allmale species are known) d. Lack of genetic variation (can be very bad if there is a change in the environment) e. Can be harmful if you don’t have superior gene combinations 3. Benefits of Sexual Reproduction a. Increase the disease resistance of offspring b. Increase the relative viability of offspring across a greater range of environmental conditions c. “more fun” 4. Why only two sexes? a. Larger gametes have greater energy and have higher success b. Small gametes have greater mobility and thus have higher success c. Gametes in the middle don’t make it typically and so sexual selection depends on the size of the gametes where the largest and smallest gametes tend to be the most successful thus only two sexes favored d. Disruptive selection should favor the largest and smallest gametes and eliminate any intermediate size gametes C. Conflict between the sexes 1. Can the differences between egg and sperm explain why males and females behave differently? a. Trivors(?) hypothesized that the differences as to why males and female behave differently may be caused by the difference between egg and sperm Male Female Sperm is Cheap Eggs are expensive Fitness is limited by number of mates Fitness is limited by the quality of one’s mate Mate with as many females possible Extract the most resources from the male as possible (energy, protection, parental care, etc.) 2. Evidence for this from Bateman and his studies for fruit flies a. Mated male fruit flies consecutively with 03 females and measured the fitness (positive relationship between the number of mates and the amount of fitness) b. Mated female flies with 03 males and measured fitness (saw a negative relationship between the number of mates and fitness) i. Sperm is toxic to the females and can reduce lifespan; so an increase in mates led to a reduction in the lifespan of females 3. Reason that there seems to be more male success in mating over female success in mating? a. More females reproduce actually than males b. A lot of males die before reproductive age c. Variance is the cause of the perception because some males reproduce a lot while some do not reproduce at all) 4. Why do females invest more in offspring care than do males? a. Males are less certain about paternity than females b. Males have more to gain by mating with multiples mates than females D. Intrasexual v. Intersexual 1. Darwin noted that there is two sexual struggles: intrasexual (malemale competition) and intersexual (intersexual) 2. Intrasexual malemale competition which was competition for access to females that can be violent and even deadly 3. Intersexual selection female choice; ultimately it is the female’s choice which male gets to mate II. Intrasexual Selection A. Why is male v. male competition more common than female v. female competition? 1. What resources do females need that males can defend? a. Defensive cavities b. Nesting sites i. Male sticklebacks compete for the best of nesting locations c. Foraging patches d. Territories B. Competition for females 1. How do males monopolize access to females a. Ex. Elk bugling i. Sign of competitive dominance (for other males) Males will bugle back and forth and can occasionally engage ii. Honest indicator of size and strength (is hard to fake) Size/strength of bugle can determine if males will engage and usually do so by parallel walking (stride length can determine strength), will occasionally escalate into a fight iii. Stimulates the reproductive system to come into estrus b. Ex. Elephant seal fighting i. Suitable mating locations have gone down due to urbanization of beaches so competition for mates has gotten higher and more violent ii. The larger the male, the larger the harem size C. Competition for matings 1. What is the only mating strategy if you can’t monopolize female access? a. Create more sperm and attempt to fertilize anyway possible b. Every copulation counts and so males in lesser conditions that aren’t favorable to mate with females will try to sneak in copulations to achieve a few offspring c. Ex. Bullfrogs i. Male bullfrogs compete for good egglaying territories in the pondwinners advertise their success and females come to mate with them alternative male strategies exist so losers can slide in and try to fertilize some eggs too ii. Element for female choice also exists 2. Alternative Mating Strategies a. What male strategies reduce the reproductive skew of dominant males? i. Ex. Disguise as female, be fast, etc. b. Frequencydependent selection strategies i. In certain species: larger males will build an defend nests, while satellite ales adopt female color patterns and sneak in copulations ii. Depends on what others individuals’ roles are in the population Is allowed by natural selection because larger males and satellite males will have similar finesses iii. Ex. Isopod crustaceans Alpha males: are the largest, will defend territory on sponges, and mates with all of the females in its harem Beta males: resemble females, gain access to alpha harem b/c of similarity to the females and mates with females in harem Gamma males: are the smallest, gain access to harem by entering sponge cavity by side canals All have similar finesses III. Intersexual Selection A. Direct Benefits 1. Bird song learning is costly 2. But males that sing more songs/ more complicated songs have better territories a. Also tend to have parental ability which correlates with heavier chick weights and an increase chance of offspring survival 3. Thus, females prefer males with more songs 4. Ex.: hanging flies a. Male hanging flies bring females gifts b. These gifts influence the mating success of males b/c larger gifts allow males to mate for longer and mating for longer increases the number of eggs fertilized 5. Females can change things by: a. Sexual suicide which will decrease the chance of female remating b. Increased egg number/quality c. Increases copulation time B. Good Genes: females choose males based on superior genetic quality or superior genetic compatibility 1. High quality males a. Resistance to parasites b. Better foragers c. Higher survival 2. Genetically compatible males a. Decreased inbreeding costs b. Decreased autoimmune costs 3. Ex. “redness” and female choice in stickleback a. Good honest signal for a good mate i. Males get color from diet and energy ii. Parasites will reduce the pigmentation Intensity of red coloration in male stickleback is an indication of parasite load More red=less parasites b. Females will prefer males with the most intense red coloration c. Females will also use other things for judge for good mate i. Prefer males with more MHC alleles (up to the optimum #8) ii. Can tell by pheromones (smell MHC alleles( iii. Females with fewer MHC alleles respond stronger to MHC supplementation 4. Females will also prefer the male with symmetrical features a. Assymtery is usually the restul of developmental gene incompatibilities i. Is correlation to the underlying genes that the individual has 5. Handicap Principle: extreme traits that act as an honest indicator of male health a. Unhealthy males could never survive such a large handicap b. Ex. Bowerbirds: why do they spend so much time making bowers? i. Indicates to females a male’s vigor ii. Iridescent blue feathers is a trait that indicates male health iii. Is more of structural pigment (unlike red stickleback color) and males can decorate their bowers with blue to trick females and enhance their own “blueness” C. Attractive sons 1. Fisher’s runaway sexual selection: females mate with the most attractive males and then that is passed down to son 2. Genetic correlation between male trait and female preference a. Genetic linkage (males receive trait and daughters will receive preference) 3. Sons inherited preferred trait and daughters inherit preference a. Leads to exaggerated male traits b. Is checked by natural selection 4. Ex. Stalkeyed flies: a. Males will develop longer eye stalks and daughters will prefer longer eye stalks b. A genetic correlation exists between trait and preference D. Sensory exploitation 1. Females initially prefer male traits that elicit the greatest amount of stimulation from their sensory systems 2. Ex. Bower birds: why decorate with blue objects? Blue typically indicates that blue males are healthier so more blue indicates more vigor 3. Ex. Frogs: some females cells prefer “chuck calls” even when their own males don’t use them thus the preference evolved before the trait 4. Ex. Swordtail in fishes: swordtail is preferred even in species where the sword is absent IV.Sexual Conflict A. Sex role reversal (reversal of Bateman’s principle) 1. Ex. Male parental care in pipefish a. Females compete for access to males b. Males are choosey about female mates c. Here the male is the limiting resource and female competition exists in pipefish because male brood pouches are limited 2. Ex. Brush crickets a. Males produce rich spermatophores b. Females are choosy when resources are abundant c. Females compete for best male when resources are scarce 3. When males cannot mate with many females, females will compete for the best male a. Leads to the appearance of female exaggerated traits that will indicate her quality to the male 4. Can lead to sexual diphormism B. Extra pair copulations (EPCs) 1. Why is social monogamy not a good predictor of genetic monogamy a. Extra pair copulations are only favored if the costs are low b. DNA fingerprinting has revealed a lot of hidden EPCs 2. EPCs occur when females have the highest chance of fertilization 3. Extra pair mates tend to have better gene indicators such as longer, brighter, or larger traits 4. Social mate usually has better parenting skills 5. Ex. In certain bird species a. Malefemale plumage dimorphism was not predicted by social mating system b. Malefemale plumage dimorphism was predicted by extrapair paternity C. Antagonistic Coevolution 1. Evolutionary “arms race” between male traits and female traits (b/w reproductive organs) 2. Traits that increase fitness in one sex will decrease fitness in the other sex 3. Ex. Water striders a. Male water striders force copulations b. Males evolved grasping genetilia c. Females evolved longer defensive spines d. A species comparison suggests coevolution of these traits. 4. For each male trait that has evolved to assure fertilization success 5. A female counter trait has evolved to reduce fertilization success 6. Ex. Fruit fly (drosophila) the seminal fluids are bad for females a. Females that are exposed more continuously to male sperm and mating, had a decreased lifespan b. Toxicity of seminal fluid is due to malemale competition and making more sperm that is more deadly and can move faster is necessary for reproductive success D. MaleMale sperm competition 1. Males compete to fertilize eggs 2. Multiple mechanisms: high sperm production, sperm removal a. Males show variation in genetilia for sperm removal b. Males show variation in sperm morphology for effective fertilization 3. Coolidge Effect a. Males show strategic allocation of sperm b. More sperm are allocated to novel females (new females) i. Novel females make sperm production go way up E. Cryptic Female Choice 1. Female (cryptic) choice: a. After copulation with multiple males and before fertilization, females can use a variety of methods to “choose” which sperm fertilize the eggs including: i. Sperm expulsion Females can remove unwanted sperm by cloaca contractions ii. Sperm choice Females can utilize sperm selectively by sperm storage tubes iii. Reproductive tract complexity Female reproductive tract is convoluted to reduce male fertilization success iv.
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