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Questions and Answers
Anisogamy, the production of different-sized gametes, is thought to be favored by natural selection primarily because it:
Anisogamy, the production of different-sized gametes, is thought to be favored by natural selection primarily because it:
- increases the probability of fertilization due to the mobility of larger gametes.
- leads to zygotes with higher survival rates due to larger initial resource provision. (correct)
- ensures genetic uniformity within a population through specialized gametes.
- reduces the energy expenditure on gamete production for both sexes.
According to the model explaining the evolution of anisogamy, what is the initial selective pressure that leads to the divergence of gamete sizes?
According to the model explaining the evolution of anisogamy, what is the initial selective pressure that leads to the divergence of gamete sizes?
- Competition for resources among gametes of the same size.
- Predation pressure on intermediate-sized gametes, favoring extremes.
- The trade-off between gamete size and gamete mobility.
- The advantage of larger zygote size for survival combined with the trade-off between gamete size and number. (correct)
Disruptive selection in the context of gamete size favors:
Disruptive selection in the context of gamete size favors:
- larger gametes exclusively, as zygote size is the primary determinant of survival.
- a unimodal distribution of gamete sizes centered around a moderate size for optimal fusion.
- a bimodal distribution of gamete sizes, with very small and very large gametes being most successful. (correct)
- gametes of intermediate size to maximize both mobility and resource provision.
Fisher's model for sex ratio evolution posits that a 1:1 sex ratio is an evolutionary stable strategy because:
Fisher's model for sex ratio evolution posits that a 1:1 sex ratio is an evolutionary stable strategy because:
Hamilton's verbal model explaining Fisher's principle suggests that if male births are less common than female births, then:
Hamilton's verbal model explaining Fisher's principle suggests that if male births are less common than female births, then:
In the blue moon butterfly example, the initial skewed sex ratio (99% female) was caused by:
In the blue moon butterfly example, the initial skewed sex ratio (99% female) was caused by:
The rapid return to a 1:1 sex ratio in the blue moon butterfly population after 2001 demonstrates:
The rapid return to a 1:1 sex ratio in the blue moon butterfly population after 2001 demonstrates:
In the mathematical model of sex ratio evolution, the term 'k' represents:
In the mathematical model of sex ratio evolution, the term 'k' represents:
According to the sex ratio model, when there are more females than males in a population (f > m), natural selection favors parents who:
According to the sex ratio model, when there are more females than males in a population (f > m), natural selection favors parents who:
The experiment with blue moon butterflies from Moorea infected with Samoan Wolbachia was designed to:
The experiment with blue moon butterflies from Moorea infected with Samoan Wolbachia was designed to:
The finding that Moorean butterflies infected with Samoan Wolbachia produced only female offspring indicated that:
The finding that Moorean butterflies infected with Samoan Wolbachia produced only female offspring indicated that:
The rapid evolutionary shift in blue moon butterfly sex ratio highlights the importance of which factor for natural selection to act?
The rapid evolutionary shift in blue moon butterfly sex ratio highlights the importance of which factor for natural selection to act?
The fundamental difference between males and females that drives many aspects of sexual selection is:
The fundamental difference between males and females that drives many aspects of sexual selection is:
According to Bateman's principle, male reproductive success is typically limited by:
According to Bateman's principle, male reproductive success is typically limited by:
The Bateman gradient often shows a steeper slope for males compared to females. This indicates that:
The Bateman gradient often shows a steeper slope for males compared to females. This indicates that:
Why are females generally considered to be the 'choosier' sex in mate selection?
Why are females generally considered to be the 'choosier' sex in mate selection?
Sexual selection, as defined by Darwin, depends on:
Sexual selection, as defined by Darwin, depends on:
Sexual dimorphism, differences in traits between males and females of a species, often arises due to:
Sexual dimorphism, differences in traits between males and females of a species, often arises due to:
Intersexual selection typically involves:
Intersexual selection typically involves:
Intrasexual selection primarily involves:
Intrasexual selection primarily involves:
Mutual mate choice, where both sexes are selective, is most likely to occur when:
Mutual mate choice, where both sexes are selective, is most likely to occur when:
In the firefly Photuris versicolor study, artificial light pollution disrupted courtship behavior by:
In the firefly Photuris versicolor study, artificial light pollution disrupted courtship behavior by:
The firefly study demonstrated that artificially lit plots acted as 'sinks' for fireflies. This means that:
The firefly study demonstrated that artificially lit plots acted as 'sinks' for fireflies. This means that:
Initially, research on sexual selection heavily focused on male-male competition. What is suggested as a potential reason for this historical bias?
Initially, research on sexual selection heavily focused on male-male competition. What is suggested as a potential reason for this historical bias?
Current research in sexual selection is now more focused on:
Current research in sexual selection is now more focused on:
The hypothesis that sexual selection might increase speciation rates suggests that sexual selection can lead to speciation by:
The hypothesis that sexual selection might increase speciation rates suggests that sexual selection can lead to speciation by:
The analysis of 34 families showed a positive association between species richness and the strength of sexual selection, suggesting that:
The analysis of 34 families showed a positive association between species richness and the strength of sexual selection, suggesting that:
In Mercurialis annua, pollen dispersal distance is identified as a key factor for male reproductive success. This is primarily because:
In Mercurialis annua, pollen dispersal distance is identified as a key factor for male reproductive success. This is primarily because:
The Bateman gradient in Mercurialis annua was found to be steeper in males than females. This implies that in this plant:
The Bateman gradient in Mercurialis annua was found to be steeper in males than females. This implies that in this plant:
Figure 16.24 shows the distribution of reproductive success in bitterling fish. Based on this figure, males have a greater variance in reproductive success compared to females. This is mainly because:
Figure 16.24 shows the distribution of reproductive success in bitterling fish. Based on this figure, males have a greater variance in reproductive success compared to females. This is mainly because:
Based on Figure 16.24, which sex of bitterling fish likely has a higher median reproductive success?
Based on Figure 16.24, which sex of bitterling fish likely has a higher median reproductive success?
Which of the following is an example of sexual dimorphism?
Which of the following is an example of sexual dimorphism?
The large horns of male stag beetles are primarily a result of:
The large horns of male stag beetles are primarily a result of:
The bright coloration and elaborate songs of many male birds have likely evolved due to:
The bright coloration and elaborate songs of many male birds have likely evolved due to:
The larger size of male Northern elephant seals compared to females is primarily driven by:
The larger size of male Northern elephant seals compared to females is primarily driven by:
Fiddler crab males have one greatly enlarged claw, while females have two smaller claws. This is an example of sexual dimorphism likely driven by:
Fiddler crab males have one greatly enlarged claw, while females have two smaller claws. This is an example of sexual dimorphism likely driven by:
In species with internal gestation, female reproductive success is often limited by:
In species with internal gestation, female reproductive success is often limited by:
The concept of 'sensory pollution', exemplified by light pollution's effect on fireflies, refers to:
The concept of 'sensory pollution', exemplified by light pollution's effect on fireflies, refers to:
In the context of gamete evolution, disruptive selection favors which gamete size strategy?
In the context of gamete evolution, disruptive selection favors which gamete size strategy?
According to Fisher's model on sex ratios, if a population has a skewed sex ratio with fewer males than females, what is the predicted evolutionary outcome?
According to Fisher's model on sex ratios, if a population has a skewed sex ratio with fewer males than females, what is the predicted evolutionary outcome?
Consider a population where there are 10 males and 30 females, and they produce a total of 200 offspring. According to the sex ratio model, which sex has a higher average reproductive success, and which sex would parents be favored to produce more of?
Consider a population where there are 10 males and 30 females, and they produce a total of 200 offspring. According to the sex ratio model, which sex has a higher average reproductive success, and which sex would parents be favored to produce more of?
In the blue moon butterfly example, the rapid evolutionary change in sex ratio from female-biased to 1:1 after the introduction of a suppressor gene highlights the importance of:
In the blue moon butterfly example, the rapid evolutionary change in sex ratio from female-biased to 1:1 after the introduction of a suppressor gene highlights the importance of:
Bateman's principle suggests that the primary limiting factor on reproductive success differs between males and females. What is generally considered the limiting factor for female reproductive success?
Bateman's principle suggests that the primary limiting factor on reproductive success differs between males and females. What is generally considered the limiting factor for female reproductive success?
A steeper Bateman gradient in males compared to females indicates that:
A steeper Bateman gradient in males compared to females indicates that:
Why are females typically considered the 'choosier' sex in mate selection, based on evolutionary theory?
Why are females typically considered the 'choosier' sex in mate selection, based on evolutionary theory?
Sexual selection, in contrast to natural selection in its broader sense, specifically emphasizes:
Sexual selection, in contrast to natural selection in its broader sense, specifically emphasizes:
Sexual dimorphism, the difference in traits between males and females, is often a result of:
Sexual dimorphism, the difference in traits between males and females, is often a result of:
Intersexual selection is primarily driven by:
Intersexual selection is primarily driven by:
Mutual mate choice, where both sexes are selective in choosing partners, is most likely to evolve when:
Mutual mate choice, where both sexes are selective in choosing partners, is most likely to evolve when:
In the firefly Photuris versicolor study, artificial light pollution acted as a 'sink' for fireflies. What does this imply about the firefly population in artificially lit areas?
In the firefly Photuris versicolor study, artificial light pollution acted as a 'sink' for fireflies. What does this imply about the firefly population in artificially lit areas?
The historical bias in sexual selection research towards focusing on male-male competition might be attributed to:
The historical bias in sexual selection research towards focusing on male-male competition might be attributed to:
The hypothesis that sexual selection can increase speciation rates suggests that sexual selection can lead to reproductive isolation by:
The hypothesis that sexual selection can increase speciation rates suggests that sexual selection can lead to reproductive isolation by:
The study of 34 families showing a positive association between sexual selection strength and species richness suggests that:
The study of 34 families showing a positive association between sexual selection strength and species richness suggests that:
If the Bateman gradient in Mercurialis annua is steeper in males than in females, what does this imply about the relationship between mating success and reproductive success in this plant?
If the Bateman gradient in Mercurialis annua is steeper in males than in females, what does this imply about the relationship between mating success and reproductive success in this plant?
Based on Figure 16.24 showing reproductive success in bitterling fish, the greater variance in male reproductive success compared to females is primarily due to:
Based on Figure 16.24 showing reproductive success in bitterling fish, the greater variance in male reproductive success compared to females is primarily due to:
Based on Figure 16.24, and considering that median reproductive success represents the midpoint value of reproductive success, which sex of bitterling fish likely has a higher median reproductive success?
Based on Figure 16.24, and considering that median reproductive success represents the midpoint value of reproductive success, which sex of bitterling fish likely has a higher median reproductive success?
Which of the following traits is LEAST likely to be a result of sexual selection?
Which of the following traits is LEAST likely to be a result of sexual selection?
The experiment with Moorean butterflies infected with Samoan Wolbachia was designed to:
The experiment with Moorean butterflies infected with Samoan Wolbachia was designed to:
Flashcards
Anisogamy
Anisogamy
The production of different-sized gametes, specifically small sperm and large eggs.
Sex Ratio
Sex Ratio
In sexual reproduction, the proportion of males to females in a population.
Fisher's Sex Ratio Model
Fisher's Sex Ratio Model
A model that suggests natural selection favors an even (1:1) sex ratio in most species.
Prediction of Sex Ratio Model
Prediction of Sex Ratio Model
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Sexual Selection
Sexual Selection
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Difference in Gamete Production
Difference in Gamete Production
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Bateman Gradient
Bateman Gradient
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Sexual Selection
Sexual Selection
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Sexual Dimorphism
Sexual Dimorphism
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Intersexual Selection
Intersexual Selection
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Intrasexual Selection
Intrasexual Selection
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Mutual Mate Choice
Mutual Mate Choice
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Study Notes
Sexual Reproduction and Anisogamy
- Sexual reproduction sets the stage for sexual selection.
- Anisogamy, the production of different-sized gametes (small sperm and large eggs), is favored by natural selection once sexual reproduction is established.
- Producing very small proto-sperm allows for the production of many gametes, however zygotes formed from these have low survival rates.
- Zygotes formed from the fusion of larger gametes (proto-eggs) have higher survival rates.
- Natural selection favors proto-sperm fusing with proto-eggs due to the rarity of proto-eggs; intermediate-sized gametes decrease in frequency.
- Disruptive selection for proto-eggs and proto-sperm leads to the prevalence of these two types of gametes.
The Evolution of Anisogamy (Box 16.1)
- A trade-off exists between gamete size and number: larger gametes are fewer, smaller gametes are more numerous.
- Larger gametes are less mobile.
- Zygote survival probability increases with size, which depends on the sizes of the fusing gametes.
- Intermediate-sized gametes will diminish to zero frequency when zygote size and fitness correlate in an S-shaped manner
Why an Even Sex Ratio?
- Many species, including humans, exhibit an approximately even sex ratio (1:1)
- The question addressed is why natural selection favors this ratio.
- Chromosomal sex determination mechanics in mammals (XX females, XY males) result in roughly equal proportions of X-bearing and Y-bearing sperm.
- The key question is why this system evolved over others producing different sex ratios, and why other sex-determination systems also result in 1:1 ratios.
- Ronald A. Fisher's model (1930) explains the evolution of even sex ratios.
- The model assumes sex ratio is under genetic control and that the fitness of each sex depends on its frequency in the population.
- Parents influencing offspring sex ratio affect the reproductive success (grandchildren) rather than the survival (offspring).
- When one sex is less common, it has better mating prospects, leading to more grandoffspring for parents producing that sex.
- Genes for producing the rarer sex spread until a 1:1 ratio is reached, at which point the advantage disappears, leading to an equilibrium.
Mathematical Model of Sex Ratios (Box 16.2)
- Regardless of the sex ratio, each offspring must have one mother and one father
- Total reproductive success of males equals that of females.
- Each male has N/m offspring and each female has N/f offspring (where N is total offspring, m is males, and f is females).
- The number of grandoffspring a parent has will be k(N/m) + (1 – k)(N/f) (where k is the fraction of sons offspring).
- When there are more females than males (f > m) parents with high k values will have more grandchildren
- When there are more males than females (f < m) parents with low k values will have more grandchildren
- Natural selection will drive the sex ratio to an even 1:1 ratio.
Testing Fisher's Sex Ratio Model
- Fisher's model predicts that deviations from a 1:1 sex ratio will result in strong selection for genetic changes that restore it.
- Samoan blue moon butterflies (Hypolimnas bolina) were used to test this prediction.
- In 2001, 99% of these butterflies were female due to male mortality from Wolbachia bacteria infection.
- The Wolbachia infection killed most males during larval development.
- The prediction was that if a genetic variant arose that could overcome Wolbachia's effects, it would spread rapidly and restore the 1:1 ratio.
- By 2006, a genetic variant from another area had spread, returning the sex ratio to approximately 1:1, even with the continued presence of Wolbachia.
- Researchers introduced the Samoan Wolbachia strain into blue moon butterflies from Moorea (Tahiti). The Moorean butterflies produced only female offspring.
- A genetic region of the butterfly genome responsible for the change was located
Implications of the Butterfly Study
- This rapid shift in sex ratio demonstrates the predictive power of Fisher’s model.
- It illustrates the speed at which evolution by natural selection can alter a population.
- This stresses the importance of genetic variation for natural selection to act.
- Before the arrival of genetic variants suppressing male killing by Wolbachia, the biased sex ratio persisted.
- The arrival of a genetic variant allowed selection to act; the variant spread rapidly, restoring parity.
Differential Selection on Males and Females
- Natural selection favors different traits in males and females.
- Competition for mating access is stronger among males due to fundamental differences in gamete production.
- Females produce fewer, larger, and more valuable eggs.
- Males produce numerous, less energetically costly sperm.
- Male reproductive success is limited by the availability of eggs, leading to competition for mating opportunities.
- Males chosen by multiple females may have high reproductive success, while others have very few opportunities, leading to high variance in male reproductive success.
- Female reproductive success has relatively low variance due to the costs of egg production and gestation.
Bateman Gradient Definition
- Bateman gradient maps the number of mates (mating success) against reproductive success
- Often used to test the prediction that male reproductive success is tied to number of mates.
Female Choice
- Females are often the choosier sex due to the expense and scarcity of eggs
- Females are more discriminating in mate selection because of the higher replacement value of eggs
- In species with internal gestation, females invest significant energy in offspring and choose good mates to secure investment.
Sexual Selection
- Focuses on animals, but the same logic applies to gametes produced in plants.
- Examined in the context of the wind-pollinated plant Mercurialis annua.
- Tonnabel and colleagues found that variance in reproductive success was greater in males and that the Bateman gradient was steeper in males than females.
Darwin and Sexual Selection
- Darwin noted extravagant traits in males (plumage, songs, horns) and wondered how they could be favored by natural selection.
- He proposed that such traits evolve through sexual selection, which increases mating success rather than survival.
- Sexual selection depends on the advantage certain individuals have over others of the same sex in relation to reproduction.
- Sexual selection can lead to sexual dimorphism, where traits manifest differently in males and females.
- Sexual selection is divided into intersexual selection (mate choice) and intrasexual selection (competition for mates).
- Mutual mate choice occurs when both sexes are selective in choosing partners.
Light Pollution
- Artificial light interrupts courtship in fireflies.
- Fireflies (Photuris versicolor) use light signals for courtship; males and females flash light signals to one another
- Increased artificial light plots were “sinks” that drew in more fireflies.
- Artificial light completely eliminates the normal courtship ritual of this firefly.
Historical Research Bias
- Research on sexual selection historically focused on male–male competition rather than mate choice.
- This bias may have resulted from the gender of the researchers, the ease of observing male–male competition, and the belief that mate choice was unimportant.
- Research focus has shifted to intersexual selection.
Sexual Selection
- Can lead to increased rates of speciation.
- It may favor different traits across allopatric populations, accelerating divergence.
- It may promote assortative mating within sympatric populations.
- Stronger sexual selection, especially on males, is associated with higher species richness.
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