Sexual Selection I Flashcards
What was Chuck’s discussed enigma of extreme traits in males? What was his solution? In what two ways is sexual selection viewed?
- Males had traits that didn’t necessarily promote survival because they were not found in females.
- Traits evolved solely for male competition.
- Separate from natural selection, and a subset of natural selection
What is the difference between intra- and intersexual selection? What is a general example of each?
- Intra = between individuals within the same sex (intra = within) in the form of driving away or killing rivals
Two male kudus fighting to the death
- Inter = between same sex to charm/excite indvs. of the opposite sex (inter = between)
Two male birds of paradise charming a female
What is the first question needed to ask when thinking about sexual selection? What did Geoff Parker have to say about it? What kind of relationship do sperm have with eggs? What traits are selected for to make it this way?
- Why are male and female gametes (sperm and egg) different?
- He suggested that the ESS = one gamete large + energy packed, and the other small + energy free
- Parasite/host
- Egg - large to support zygote growth, sperm - small size (many sperm used) and ability to find the egg (disruptive selection on size)
What causes a stronger selection on sperm traits?
Limited life span => increased variability between sperm
High mortality rates => stronger selection on sperm
What is an example that supports Parker’s view that there is disruptive selection of gamete size?
- Volvocales are either isogamous (same size) when food is unlimited or anisogamous (unequal size) when food is limited
Which sex invests more in parental care, and how? In what species do males contribute to parental care?
- Females: in mammals, pregnancy and lactation are female-only
- Primates/carnivores (lions)/zebra
How does parental investment influence sexual selection? How do males typically maximize their fitness? What is an example of an odd case? What makes it special?
- When one sex invests more, the less-invested sex with compete for the more-invested (as they tend to be more rare)
- Mating with as many females as possible
- Kittiwake gulls - max. # of offspring throughout lifetime is about the same
- monogamous with equal parental care (bi-parental)
Why do females typically invest more than males? (2 reasons)
- If females multiply mate, then r of males –> offspring = lower so males may have less incentive to help care for young
- If females invest more then operational sex ratio (OSR) is skewed to males so selects for more active mate search in males and reduction in investment
What is the operational sex ratio (OSR)?
The proportion of actively reproductive females to males.
Under what situation is there selection on males to maximize mating rates in lieu of parental care?
If a subset of males are better at obtaining mates.
Which sex has a higher variance of reproductive success? What does this lead to?
- Males
- Multiple mating in males
What are two consequences of sexual selection?
- Success through combat between males (intra)
- Success through mate choice by females (inter)
What is an example of combat (intrasexual selection)? Elaborate. How does age-dependent mating behavior affect the OSR?
- Southern elephant seals - females group on the beach and males monopolize on mating opportunities (are 3-7x heavier than females and breed later in their lifetime (time to grow))
- Should even it out because the limited females will be ready first, while the “unlimited” males will be ready later.
What is an example of female choice? Elaborate. What are key traits about this species?
- Long-tailed widowbird males have really long tails (1/2 a meter long).
- Polygynous without paternal care
What experiment was done to look at female choice in widowbirds? What was the usefulness of each control? What were the results? What were problems with the graphs produced and the data results (one of each)? What does this study tell us about current sexual selection in the population? If not, what does it look at then?
- 36 males –> 4 groups (cut tails, pieces added to tails, untouched tail (control), and cut tail then reglue without changing length (control))
- Untouched - control for tail length that is not manipulated. Cut and reglue - control for any changes that gluing on a tail could have done
- Results:
Before experiment: no difference in # females/male in groups
After experiment: females preferred the extra long tails - No standard error bars, and the shortened tail birds did just as well as the untouched birds.
- Absolutely nothing, because the variation in tail length was not in the natural range for that time.
- It looks at the trade-off between female choice driving selection, and how selection can halt this directional selection based on other factors (such as survivorship)
What kind of study was done with red-collared widowbrids? How did this differ from the previous study? What were its results?
- Pryke and Andersson cut the tails on some males and left others alone.
- This differed from the previous study because extra length was not added to the birds (and standard error bars were included).
- The results of this study were that normal length tails had more nests than shorter length tails.
Does tail length indicate anything about the quality of the male? Or do the females just like it? What happens as the breeding season goes on? Why? What is an alternative explanation we talked about in class (not on slides, and probably not on exam)?
- Yes, increased body size
- As breeding season goes on, longer tailed individuals lost weight. This is thought to be because there is an energy cost to making the tails.
- The males are expending energy signaling to females instead of foraging for food.
What are the important conclusions about the red-collared widow birds from Pryke and Anderssons experiment (2)?
- Natural variation in tail length is correlated with:
of active nests
time of year when 1st egg is laid (earlier = more time to grow = better) –> bigger body size for males/longer tails earlier in the season
What are two factors of female choice? What do these do?
- Good resources + good genes
- Increase her fitness
What kinds of resources are “good resources” in regard to female choice?
Eggs/nests/territory/offspring/female herself
What are two examples of things for eggs being the “good resource” in female choice? What are the nuances of the nuptial gift example?
- Bullfrogs - females want high quality habitat for eggs, males compete for high quality (warmer and low predation rates)
- Hanging fly - males offer a large nuptial gift for females
- Nuances include: increased copulation time for males (leads to increased fertilization success), increased nutrients to female to produce eggs
What is an example of “good resources” for offspring? How can females tell?
- Widowbirds - males fight over high quality territories (good nest sites and more food for offspring but male doesn’t stay so it’s not a benefit for her)
- Tail length
What do females get out of the “good genes” reason? What is an example?
- Males transfer alleles in sperm to the offspring, which is important for her fitness and the survivability of her children.
- Bower birds - males make a pretty bower and decorate it. The female does not use the territory and there’s no parental care = only genes are given by male.
What are the two hypotheses why females should chose males without direct benefits? Explain each and given an example.
- Fisher’s runaway hypothesis - starts out that female does get something. However, the runaway part is the positive feedback loop where the females select for a trait, then that trait selects for female preference for the trait.
Example: stalk-eyed flies (males have long stalks for eyes)
- Good genes and extreme traits (Handicap hypothesis) - the traits hinderance on day-to-day survival may show that male is a good fit (high quality)
Example: Peacocks
What is Fisherian selection also called?
Sexy son hypothesis
What was an experiment done to test Fischer’s on stalk-eyed flies? What were its results?
- Experiment = artificially select for different stalk sizes (short and long) then introduce them to females to see if it changes female preference.
- Females preferred long stalks when they were the only ones available AND females preferred short stalks when they were the only ones available.
