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What is the primary advantage of sexual reproduction?
- Sex speeds up the pace of adaptation in a rapidly changing environment...
- combines genotypes in each generation into new combinations
- beneficial mutations are rare, and can be passed amongst sexually reproducing
- Bad mutations can be separated from good mutations over time (increased individual fitness)
- *note- red queen hypothesis- a co-evolutionary race in which each spp much continuously evolve new defenses at a rapid rate just to survive
What are two disadvantages of sexual reproduction?
- Sex is genetically expensive
- Asexual reproduction offers maximized genetic return (100% represented in offspring), sexual reproduction only results in 50% genetic return
- Recombination during meiosis can reduce linkage disequilibrium between genes & eliminate advantageous allelic combinations
- adaptive gene complex: only certain allelic combinations are adaptive
- EX- Primula Vulgaris (GA=short style, long stamen / ga=long style, short stamen)
- both produce flowers where style/stamen don't touch to prevent self-fertilization and are more fit than heterozygous genotype
What is the concept of Muller's Ratchet? How do inbred Drosophila lineages homozygous for the entire Chr #2 experience this effect despite sexual reproduction (Fig 8.10 in text)? How might sexual reproduction disable Muller's Ratchet and enable a species to persist?
- Muller's Ratchet: process by which genomes of asexual species accumulate deleterious mutations, despite occasional beneficial mutations (impossible to lose mutations)
- In inbred Drosophila lineages the # deleterious mutations per individual increased with time
- *note- no recombination when entire chr 2 is homozygous
- Sexual reproduction disables Muller's ratchet through genetic recombination. In a sexually reproducing species selection can act upon favorable traits and enable them to persist
How does sexual selection differ from natural selection?
- Sexual selection arises through variance in mating success
- *note- it is rare for sexually reproductive spp not to exhibit sex dimorphism
What are several categories of traits that seem to be the result of past sexual selection? w/ examples
- Size dimorphism: orangutang, sheep horns, lady butterflies larger than men to carry more eggs
- weaponry: hornlike structures have evolved independently in the males of many diverse animals
- ornaments: peacock, lizard throat patch (energetically expensive and increases predation risk)
What is the difference between intersexual and intrasexual selection? Can you provide explicit examples?
- Intrasexual: mating success determined by within-sex interactions (eg. male/male combat)
- Intersexual: mating success determined by between-sex interaction (eg. female choice of males)
Describe the mating systems of monogamy, polygamy, and promiscuity. What is the difference between polygyny and polyandry?
- Monogomy: one male mates exclusively with one female
- Polygamy: individuals mate with 1+ partner
- Polygyny: males mate w/ 1+ partner
- Polyandry: females mate w/ 1+ partner
- Promiscuity: males and females mate with 1+ partner
How does individual variance in reproductive success change when you consider monogamy, polygamy, and promiscuity in turn? In other words, how does sexual selection act in each mating system?
- Monogomy: all individuals have some mating success if the M:F is 1:1
- everyone gets a mate
- Polygamy: some may get 0 mates and some get upwards of 8 mates in extreme cases
- Promiscuity: more overall mating success than polygamy, but still not guaranteed as in monogomy.
- some will have more success than others
What sort of mating system do elephant seals have, and what evidence supports placing them in this category?
- Elephant seals practice harem polygyny (AKA mate-defence polygyny)
- Breeding females cluster on the beach (all beaches are created equal)
- Males defend a harem of many females at once in violent bloody fights where size bestows advantage (note males ~3x larger than females)
- *note- this system results in highly variable reproductive success (very few males do all the mating)
What sort of mating system do jacanas have, and what evidence supports placing them in this category?
- Harem polyandry ("sex role reversal")
- males maintain small territories and perform all parental care
- Females mate with multiple males, and leave eggs with the males in small floating nests
- These nests are highly vulnerable to crocodile attack
- # of males a female mates determines her reproductive success (since she doesn't care from the nest)
- *note- females 60% larger than males, aggressively fight other females and kill their chicks
What is a lekking polygyny? Provide an explicit example.
- lekking: males aggregate in particular areas (lekks) displayed for females
- Combination of male competition and female choice
- Males may fight for position in center of lekk
- Females choose mate (often the dominant male in the center)
- *note- huge energy investment in the display, dominant males must be very healthy to "perform"
- EX- black grouse thugging around
How might infantcide increase the reproductive success of a male lion who has recently taken over a pride of lionesses with cubs? Why do male chimpanzees refrain from killing infants in their own community?
- Male lions taking over harems observed killing infants to eliminate other males genes from the next generation, to redirect female energy to their own offspring, and to force the females back into heat
- Chimpanzee society is promiscuous, and it is often unknown if the child is genetically related to the chimp. Infanticide only usually occurs as a result of territorial conflict (different community)
How does male competition differ in a polygynous versus a promiscuous system (think elephant seals vs. Drosophila)
- Polygynous systems require intense intrasexual competition to determine which few males will get "mating rights"
- Promiscuous systems require intrasexual competition to increase "genetic return" (good chance that all will mate at least once if there is a 1:1 sex ratio)
How does territorial defense polygyny differ from harem defense polygyny?
- territory: an area that is defended for exclusive use of the defender against rivals (food, basking sites, dens, hiding places, etc)
- may be attractive to females
- In harem defence the physical area has nothing offer, the choice is made solely on male sexiness. In territorial defense the territory the male occupies is what attracts the female
How are alternative mating strategies implemented in side-blotched lizards? How is each phenotype under frequency-dependent selection?
- alternate mating strategies: types have distinct behaviors, sizes, coloration, and forms that increase reproductive success
- EX- side blotched lizards have 3 genetically determined mating strategies
- orange throat: most aggressive, try to control many females in large territory
- blue throat: less aggressive, guard mates closely in smaller territory
- yellow throat: mimic female behavior to sneak into territories and gain access to females
- The fitness of each morph is dependent on the frequency of the others (rock/paper/scissors)
- All three maintained, because no one morph is able to dominate in every situation
What is Bateman's principle? What usually limits female reproductive success? What usually limits male reproductive success?
- Bateman's principle: the sex that invests the most into offspring will become a limited resource over which the other sex competes (sexual selection)
- genetic investment- sperm are cheaper than eggs therefore...
- mating opportunity limits male reproductive success
- fecundity limits female reproductive success
In many fish species with external fertilization, females lay eggs in a nest and males fertilize them. The eggs are vulnerable to predation, so parental care is necessary. Which sex is normally the caretaker, and how did this pattern evolve?
- Males are normally the caretaker, because their entire genetic investment is in that clutch of eggs.
- Females increase their genetic investment with the amount of nests they can form (which will each have their own chance at suvival)
- Very similar to the jacanas
When is polyandry favored?
- When males become the limiting resource for reproduction
- Often occurs when decreased offspring survival requires male parental care, so that males have the greatest reproductive effort and lower reproductive rate
- Consequence - sexual selection is stronger on females than males
- *note- male paternity and offspring mortality are high
- *note- monogomy will occur when a single parent can't provide for children (Tamarin)
Why is mate choice (intersexual selection) so important, even in systems with intense intrasexual selection?
Mate choice is the strongest driving force of sexual selection. Mate choice literally determines which genes will make it to the next generation based on who receives matings. Even if intrasexual selection is high (say for size), intersexual selection may be acting on color.
Assuming that females are the choosy sex, what criteria might a female use in choosing a mate? What is the good genes hypothesis? What is a nuptual gift, and how might it influence female mate choice?
- Females choose a mate by requiring courtship or nuptual gifts, by being choosy they are more likely to choose a male who...
- provides parental care, territorial defense, resources, food, and less diseases
- good genes hypothesis: this male will invariably result in more fit offspring
- Nuptial gift: items transferred to females by males during courtship
- EX- food provided by hangingflies
- females allow males to mate only as long as they are eating. A larger nuptial gift results in increased duration of sex and increased deposition of sperm
What evidence is there that female preference for male traits can be genetically determined? (think stalk-eye flies and play and swordtail fishes). What is sensory bias?
- EX- stalk-eyed flies
- bigger flies have longer eyestalks. Most females prefer to mate with large eye stalk males
- Wilkinson and Reillo found a genetic correlation between male stalk length and female preference (both are heritable traits)
- Daughters had the same preference as mothers (correlated traits)
- EX- males have "swords" on tails in swordtail fishes, no platys (relatives of them) have swords
- Male platys were given artificial swords and females had preference for these males (even though it doesn't exist!)
- *note- this may indicate a preexisting sensory bias in the spp
- sensory bias: animals frequently show greater response to supernormal stimuli than normal stimuli
What is Fisher's Runaway Hypothesis for the evolution of male ornamentation? Why is female preference for such a trait under positive natural selection?
- Mate choice originally evolved to facilitate adaptive choice for traits conferring a survival advantage
- Once female preference evolved, any genes that conferred survival advantage but compromised attractiveness would not be passed on (surviving males would fail to mate)
What is Zahavi's Handicap Hypothesis for the evolution of male ornamentation? Why is female preference for such a trait under positive natural selection?
- Extravagant male traits are costly to develop/maintain
- Choosing mate w/ "good genes" requires an honest signal of genetic quality
- Only males in good condition (those with good genes) will be able to fully develop and maintain these traits (bright color, long tail, symmetry, etc)
In both Fisher's and Zahavi's hypothetical situations, how can the sexual selection imposed by female choice be opposed by natural selection on the male trait?
- Sexual selection overwhelms natural selection, because sexual selection is required to mate (and therefore to pass on genes into future generations)
- Even though ornamentation and behavioral modifications (lekking) require large amounts of energy and put the males at risk, THESE are the indicators chosen by a female for mating
- Only males that display these traits will even have a chance at mating, and thus they will necessarily be put into the next generation
How did the laboratory selection studies by John Endler demonstrate that sexual selection can favor traits that reduce survival? What evidence from natural populations supports Endler's experimental results?
- Guppies - pattern matches the substrate of their habitat
- Sexual selection can favor traits that reduce survival
- Allowed several generations with a predator in various substrates to "match" the guppy patterns with their substrate
- Transplanted some of the guppies into the opposite substrate (where they stood out to the predators) and they were more attractive to the females
- NS favors spot patterns that match the substrate, but sexual selection favors spot patterns that contrast the substrate
- Marine Iguanas (lekking reptile)
- Large males most successful, but during El Nino food was scarce and these large Iguanas starved to death
- Sexual and NS had conflicting effects on male body size
What is fecundity selection, and how might it influence female sexual dimporphism, even in a mating system that exhibits male combat?
- Fecundity selection: selection on female fecundity (# offspring produced)
- this is distinguished from mating success (# mates)
- In many spp where females are larger than males the dimorphism is a result of fecundity selection favoring large female size
- Sexual selection may still be acting on male body size, but the selective force acting on females is greater