Lec 12 Flashcards
Sex vs. gender
Sex: What type of gametes does na individual have - large or small? Large gametes = females, small gametes = males
Gender: non-binary concept related to how individuals view themselves within societal constructs, often related to, but not always the same as, an individual’s biological sex
When we use the terms “male” and “female” in this course, we are referring to the TYPE OF GAMETE that individual organism produces
Is selection the same on males and females?
We look at how many gametes can be produced for a given amount of resources
For the same amount of resources, the female can produce 2 gametes and the male can produce 2000
If the goal is to have as many offspring as possible, what mating strategy should MALES pursue? What mating strategy should FEMALES pursue?
If the goal is to have as many offspring as possible, what mating strategy should MALES pursue? What mating strategy should FEMALES pursue?
Males mate with as many individuals as possible, females choose mates carefully
Fitness
Relative genetic contribution of individuals to future generations
2 components of fitness
Survival
Reproduction
For your genes to make it to the next generation, you need to SURVIVE and PRODUCE OFFSPRING
Selection operates ______________ on males and females
DIFFERENTLY due to differences in investment in reproduction
What are the consequences of anisogamy for selection and evolution in males vs. females?
Darwin’g theory of sexual selection
-Sexual selection “depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring”
Traits invovled in reproductive fitness
Sexually selected traits
Anisogamy leads to sexual selection
By definition, males produce many tiny gametes (sperm), females produce fewer, large gametes (eggs)
Sexual selection in males and females is then driven by differences in investment (time, effort, resources) parents devote to each offspring
Which is a consequence of anisogamy?
Females are very choosy about their mates, and males evolve to be attractive to females
Consequences of anisogamy and differential investment
Differences in the way males and females maximize fitness lead to differences in behavior, morphology, and physiology
Females have more to lose from mating “wrong”, so they are MORE SELECTIVE about choosing mates
Since females are choosy, and males mate multiply, males compete for access to females
Mechanisms of Sexual Selection
Selection that acts on an organism’s ability to obtain a mate
Intrasexual selection (male competition);
- WITHIN the sexes
- Flashy, competitive males
- “Male-male competition”
- Selects for traits that aid in combat, territory defense
Intersexual selection (female choice)
- Between sexes
- Drab, choosy females
- “Female choice”
- Selects for traits that increase attractiveness
What are males competing for and why?
Males compete for access to females, or monopolization of resources that attract females
Anisogamy = Skew in OSR
OSR = Operational sex ratio = ratio of AVAILABLE males to females
Smaller gametes and less parental care means males are more available in the mating pool
Reproductive Skew:
When more males than females are available to mate, competition for mates INCREASES
-A lot of males must compete for few females
there is not a perfect correlation between reproductive skew and competition because male mating success depends on:
1) Pool of available females
2) Ability of a male to monopolize access to females
For males to reproduce, he needs to mate with a lot of females
When there are more males than females available to mate what happens to competition for mates?
Increases
Intrasexual selection in elephant seals
Elephant seals breed on small beaches at particular times of year (you can go see them on the Central Coast)
Females are clumped on small beaches
-Females come give birth on land and mate immediately after
This allows males to monopolize access to females
-Males spen ALL of their time during short breeding season to monopolize females, do NOT assist in raising young
Males provide NO parental care or investment in offspring
Only about 1% of males EVER mate
-ONLY the biggest, strongest, most territorial
Territory defense in kittiwakes
Males find a nest site
Males and females jointly build a nest on a cliff face
Males and females both incubate the eggs and feed chicks
Both males and females participate in nest defense
Kittiwakes are monogamous and the same pairs will often stay together for many years
In which group is INTRASEXUAL selection stronger?
Elephant seals, because only a few males are able to mate so there is intense competition and the evolution of competitive traits
Male competition (intrasexual selection) summary
Males invest less per gamete
They maximize thier fitness by mating wtih as many females as possible
Males compete with each other for access to mates or matings
Strength of competitions depends on OSR
A skew in OSR (ratio of available males : females) typically means stronger male competition
Competition is also influenced by how effectively males can monopolize access to females
Female choice has been harder to understand than male competition
The female could in most cases escape, if wooed by a male that did not please or excite her; and when pursued, as so incessantly occurs, by several males, she would often have the opportunity, whilst they were fighting together, of escaping with, or at least temporarily pairing with, some one male
Nevertheless, when we see many males pursuing the same female, we can hardly believe that the pairing is left to blind chance - that the female exerts no chocie, and is not influenced by the gorgeous colors or other ornaments with which the male alone is decorated
Which male obtained the most nests in his territory?
Elongated tail
Why do female preferences for costly male traits evolve?
Darwin originally argued that females have an “aesthetic sense” - these are now termed “arbitrary preferences”
Later work suggested that some female preferences are tied to benefits that females gain from mate choice
Male signals evolve to advertise benefits to females
What are some benefits females could look for in a mate? 2 main models of female mate choice
Models based on benefits to female AND underlying genetic mechanisms of mate choice
Direct benefits
Indirect benefits
Direct benefits
Good resources
Females get direct fitness benefits from mating with a particular mate
Males advertise territory quality, access to resources, or parental care
Choice of mate increases female fitness
Females prefer more dominant males
Dominant males provide more food to females
Dominance status may therefore advertise direct benefits of mating to prospective females
Dominance status and competitive traits can be PROXIES for certain types of direct benefits
Yellow tail may signal to female that male has traits she wants and to other males that he is stronger
Indirect benefits
Good genes
Females get genetic benefits from male - affect the fitness of her offspring
Attractive sons
High-quality offspring (good immune system)
Models of indirect benetifs:
1) Fisherian Runaway
2) Handicap/good genes hypothesis
Evolution of direct benefits mate choice is ____________
Easy
Female just needs to optimize her own fitness
Fisherian runaway: Females prefer high-quality males
Starting point: Male has some trait that confers survival advantage (Fisher)
Males with preferred trait have higher mating success.
If trait is beneficial, now males benefit because they are better AND attractive to females
Females will benefit because their offspring will be better at surviving AND their sons will be attractive
Trait will continue to get bigger, and preference will get stronger
What happens to size/frequency of a preferred male trait over time? What happens to female preference for that trait?
Trait becomes larger/more common, preference becomes stronger
Fisherian runaway: Females have arbitrary preferences
Starting point: Male has a trait the females arbitratily like
-Doesn’t confer survival advantage, just like a trait
Males with preferred trait still have higher mating success
Female preference spreads
Males benefit because they are attractive to females
Females will benefit because their sons will be attractive
Trait will continue to get bigger or more elaborate
When does Fisherian runaway stop?
Eventually the cost of the elaborate trait in terms of survival is greater than the benefit to reproduction and the trait stops getting bigger
Fisherian runaway: Females gain attractive sons
Starting point: male has a trait the females like (either arbitrarily or because it confers a survival advantage)
Males with preferred trait have higher mating success
Female preference spreads
Males benefit because they are attractive to females
Females benefit because their sons will be attractive
Trait will continue to get bigger or more elaborate, preference will continue to spread
Negative genetic correlation between male sexual attractiveness and survival
When does Fisherian Runaway stop?
When the cost of the trait to survival is greater than the benefit from mate choice
Fisherian runaway generates (and requires) linkage disequilibrium
There must be a genetic basis to the female preference and the male trait
Over time, alleles for stronger preferences become associated with alleles for more elaborate trait
As long as the males with the more elaborate trait have higher fitness, the loci will stay in association
In heliconius butterflies, the gene for mate preference is located next to the gene for color patterning on the same chromosome
Assortative mating occurs because the preference for wing color is located right next to wing pattern gene\
Linkage disequilibrium is difficult to maintain over time
Handicap hypothesis proposed due to criticism that Fisherian Runaway would break down over time
The Handicap hypothesis/condition-dependent indicators
Females prefer male traits because they are costly
Only the highest-condition males can produce the biggest traits
If there is a genetic basis to condition, offspring benefit from having hihg-quality fathers
-Males must be healthy and strong in order to produce long tails/bright colors
Traits must be condition-dependent rather than fixed
Only the highest-condition males can produce the biggest traits
It is energetically costly to carry a big tail around
Better quality males have more energy to invest in their displays, so their displays are larger
-Good at finding resources, no parasites or diseases, etc. = can have larger displays
If low-quality males invest beyond their abilities, they die (which is bad for fitness)
Handicap hypothesis: Costly traits advertise good genes
In peacocks, males with larger tail spots had more surviving chicks, suggesting they have better genes than males with small eyespots
Why would a female preference for a trait that has no fitness value or benefit lead to that trait increasing in frequency in the population?
Because the sons of males with the trait are attractive to females and have higher reproductive success
Indirect benefits: THe Hamilton-Zuk hypothesis
The expression of traits is associated with resistance to parasites or pathogens
Hamilton and Zuk (1982) proposed that females choose mates that are resistant to disease
General good health and freedom from parasites are often strikingly indicated in plumage and fur, particularly when these are bright rather than dull or cryptic
Thus only resistant to parasites will be able to display conspicuous traits
Threespine sticklebakcs
Males develop bright red coloration and build nests where females come to spawn
Males also care for eggs and fry
What is the main difference between the handicap and runaway models?
IN the handicap model, the preferred trait indicates some other preferred aspect of quality, whereas in the runaway model the preferred trait is the only target of the preference
Comparing models of indirect benefits
In BOTH females are NOT getting any direct resources, only get benefits that accrue in offspring
Fisherian: Trait evolves ONLY because it confers a mating advantage
-Looking for benefits for the SONS
Handicap hypothesis: traits chosen because they confer some OTHER advantage - they advertise general health/condition/quality
-Advertise general health/quality (i.e. ability to find food, fight off parasites, etc.)
Fisherian traits become indicators over time
Once a trait is very costly, it can act as an indicator of condition
We therefore have to know how the evolution of the trait STARTED, which is hard to do
Which is a stronger source of selection - direct or indirect benefits
DIRECT benefits are typically STRONGER because they directly impact fitness (as opposed to indirectly)
Indirect benefits are harder to evolve because they require strong linkage disequilibrium between traits and preferences
Models are NOT mutually exclusive! Environmental variability means females often evalueate multiple pieces of information about male quality
Female choice summary
Females often choose mates based on benefits gained from mating with a particular male
These can be direct benefits (increase female fitness) or indirect benefits (increase fitness of offspring)
Males advertise beendfits to females via costly dipaly traits
Costly display traits in systems with no direct benefits can evolve via Fisherian runaway (genetic correlation between trait and preference ) or Handicap principle (condition dependent signal of quality)
How many males should females mate with?
According to our predictions under anisogamy, females should mate with the single best male
However, multiple mating by females is widespread
Why would females mate multiply?
Benefits of multiple mating for females
Sperm replenishment (sperm stores depleted)
Direct, material benefits (resources)
Genetic benefits (trading up, genetic compatibility)
Convenience (minimize harassment)
Correlated evolution (with selection on males)
Genetic benefits: Acquiring genes from superior mate
Most songbirds form social pairs where both parents raise chicks
Female songbirds solicit extra pair copulations (EPCs) from males who are not their social partner
More extra pair paternity in ornamented systems
Females prefer more elaborate males as extra-pair fathers
Females socially paired to least attractive males seek out more EPCs
What are the evolutionary consequences of females mating multiply?
Continued competition among male sperm for access to female eggs
Continued “choice” by females for the best male after mating
Sperm competition
Competition between sperm of 2 or more males for the fertilization of ova
One of the many forces that have led to incredible variability in sperm morphology
Multiple mating and differences in investment set up sexual conflict
Optimal outcome of mating is different for males and females
Can traits that increase male reproductive success but harm females (and vice versa) be selected?
Yes, because all individuals are just increasing their own genetic fitness and the individuals they mate with don’t share their genes
Multiple mating and differences in investment set up sexual conflict
Males will evolve strategies to maximize paternity, often at a cost to females, while females evolve defenses to maintain choice, often at a cost to male gametes
Intromittent organs
Male genitalia evolves to remove rival sperm
These can harm females, reducing their survival
Accessory gland proteins
Many insects inject proteins along with sperm
These influence female behavior and physiology:
- Incapacitate rival sperm
- Shield sperm from female reproductive tract
- Increase rate of egg laying
- Decrease propensity to remate
These adaptations increase male fitness but decrease female fitness
Very fast evolution of ASPs as females evolve defenses to male tactics
Cryptic female choice for nesting males
2 male strategies: nesting (preferred by females) and sneaking (try to steal copulations) in ocellated wrasse
Nesting males have faster sperm, sneaking males have more sperm
Female ovarian fluid favors the faster sperm, increasing success of nesting males
Waterfowl are a particularly dramatic example of coevolutionary sexual conflict
Male ducks often attempt forcible copulation
Female ducks have evolved a corkscrew-shaped vagina to make it hard for males to inseminate them
In turn, males have evolved a corkscrew-shaped phallus
there is a strong correlation between the length of the male phallus and the umber of pouches (dead ends) and spirals in the female vagina
These elaborate structures only occur in species with forced copulations
Why does sexual conflict occur?
Males and females have fundamentally different reproductive strategies, with males trying to mate with as many females as possible and females trying to mate with only the best one or few males
Sexual selection summary
Anisogamy causes males and females to pursue fundamentally different reprodcutive strategies
Because female gametes are typically rare, males compete for access and females choose the best quality mates
Males and females can be in conflict, expecialy when females mate multiply, with males evolving to maximize paternity and females evolving to exert choice
