Midterm 2 (Sexual Selection - Molecular Evolution) Flashcards
How do we define sex?
By gamete size
Females have larger gametes, males have smaller gametes
What is sexual dimorphism?
Differences in the size/ appearance or differences in sexual organs between the sexes of a species which can be due to natural or sexual selection
What is an example of sexual dimorphism in humans?
Height,
Men on average are taller than women
What is sexual selection?
A subset of natural selection
Variation among individuals in getting mates leads to differential reproduction (those with access to the most mates will tend to reproduce more & pass on their genes)
What are females limited by? Why?
Since females have the larger gametes, they invest more energy into offspring and therefore are limited by the amount of resources they can get to produce eggs
(number of gametes they can produce)
What are males limited by? Why?
Since males have smaller gametes, and it doesn’t require as much energy to produce them, they are limited by the number of mates they can get
What is the correlation between the # of mates and fitness in males?
Strong, positive correlation
What is the correlation between the # of mates and fitness in females?
Weak but positive correlation
What is intra-sexual selection?
Competition between one sex (typically males) to gain access to mates
What are examples of intra-sexual selection?
Male-Male competition
Alternative mating strategies
Sperm competition
Infanticide
Explain Male-Male competition.
Combat b/w males
ex: male elephant seals battle each other; usually the
largest/dominate ones win out and get to mate
Explain Alternative mating strategies.
Big/dominate males vs. a sneaky male: males phenotypically look like females & trick the
larger/dominate male & get a chance at mating they
otherwise wouldn’t have
- can be genetic: sneaker vs dominant phenotype
- can be environmental: access to food while young so not
larger in size
- negative frequency dependent: if a population has more dominant males, sneaky allele has higher fitness (& vice versa)
Explain Sperm competition.
An example of post-mating competition
- Production of more sperm to overload & win out
- Faster/better sperm
- Removing other’s sperm: the penis shape can scoop out
other sperm
- Preventing others from getting in
Explain Infanticide.
An example of post-birth male-male competition:
killing off offspring that isn’t yours
ex: lions, dolphins
What is intersexual selection?
Members of one sex choose (typically females) members of the other sex: “choosy females”
What is the sensory bias model?
A model to explain female choice
Females prefer a stimulus not related to reproduction that males take advantage of to mate with them
What is the resource acquisition/ direct benefits model?
Another model to explain female choice
Males provide resources (food, shelter, protection) to females in exchange for mating
Males that provide the best resources, mate more
What is the good genes model?
Another model to explain female choice
Traits are markers that indicate to the female that the males have good genes
ex: brightly colored males, length of calling sounds
What is the null model?
Another model to explain female choice
No real reason why females prefer a trait (traits are arbitrary)
What is runaway sexual selection?
Falls under the null model to explain female choice
If males expressing the phenotype and females expressing their preference are correlated, then a disturbed equilibrium (natural selection) can cause male traits to be exaggerated
How does sexual selection affect females?
Females having more mates can benefit them due to them receiving more resources/ direct benefits (shelter, territory, food, protection) from their mates and their offspring having enhanced genetic diversity
Does sexual selection act in humans?
It’s hard to determine the importance of sexual selection in shaping human evolution due to:
- cultural differences among groups
- the lack of data on ancestral populations
- the infeasibility of doing experiments on human mate choice/breeding
- however, most other primates show sexual selection on various traits so it could be possible in humans
How strong is mutation?
Mutation by itself is a weak force of evolutionary change since the rate of mutation is very low (A–> a ~0.0001)
With only mutation it would take a long time for an allele to be lost or fixed in a population
How can mutation become a strong force of evolutionary change?
When mutation is combined with selection, mutation can be a strong force of evolutionary change
If mutation is constantly introducing beneficial alleles, those beneficial alleles will be driven to high frequencies
Explain the mutation-selection balance.
Mutation creates deleterious mutations (most mutations are deleterious or neutral) while selection acts to eliminate the deleterious ones
Explain the equilibrium frequency and the relationship between the mutation rate of an allele, the selection parameter, and the frequency of the allele.
The equilibrium frequency is: p̂ = μ/ s
- p̂ = the frequency of allele
- μ = the mutation rate of allele
- s = selection against allele
The relationship:
- The equilibrium frequency is directly proportional to the mutation rate meaning the deleterious allele will be at a higher rate if the mutation rate is higher
- The equilibrium frequency is inversely proportional to the selection against the allele meaning weakly deleterious alleles will be at higher proportions in the population than extremely deleterious alleles
What is mutational load?
Mutations create a mutational load in the population - the amount of fitness the populations loses due to deleterious mutations constantly being introduced
- L = 1 - e^-u
where u = the number of new mutations added to the genome each generation
LOAD ONLY DEPENDS ON MUTATION RATE NOT THE FITNESS CONSEQUENCES OF INDIVIDUAL MUTATIONS
Why does mutational load depend only on the mutation rate?
Because of the mutation-selection balance
Assuming equilibrium, weakly deleterious & very deleterious mutations affect the population’s fitness the same since weakly deleterious mutations decrease fitness a little but are present in many individuals in a populations while strongly deleterious mutations decrease fitness a lot but are only present in a few individuals in the population
What’s the difference between migration and gene flow?
Migration is the physical movement of individuals from one population into another population
Gene flow is the movement of genes from one population into another
Knowing that migration can happen without gene flow, can gene flow happen without migration? Explain.
Yes, mating is required for gene flow so species that can’t physically move (migrate) can still reproduce
- ex: plants that can’t move reproduce with the help of pollinators
Is migration a strong evolutionary force in populations? Explain.
Yes, migration can be a strong force of evolutionary change based on the number of individuals/alleles moving between populations and the size of the populations
Migration has larger effects in smaller populations
How is migration a homogenizing force AMONG populations?
Migration causes allele frequencies to look the same between populations
With enough migration and time, 2 populations sharing genes will have the exact same allele frequencies
In the island model, the population that is introducing new alleles will eventually “swamp out” the alleles in the native population, which will be lost
How is migration a diversifying force WITHIN a population?
New alleles are introduced in a population via migration
What is Fst?
A statistic that describes how similar allele frequencies are between populations
Explain what Fst = 0
0 < Fst < 1,
and Fst = 1 mean.
If Fst = 0, two populations have the exact same allele frequencies due to excessive gene flow
If 0 < Fst < 1, there is a moderate amount of migration, sharing alleles
if Fst = 1, two populations do not share any common alleles (the populations are fixed for different alleles)
** Fst decreases as migration among populations increases
Explain what this graph is conveying in regards to genetic drift.
Genetic drift is strongest in smaller populations so alleles that go to fixation are random
Explain what this graph is conveying in regards to genetic drift.
Genetic drift can still affect larger populations (alleles will eventually be fixed or lost but will just take much longer) however, in super large populations, selection is more efficient so beneficial alleles will increase in frequency
Explain what these graphs are conveying in regards to genetic drift.
Genetic drift increases variation among populations all based on chance
Explain what this graph is conveying in regards to genetic drift.
In small enough populations, with drift alone, alleles will go to fixation or be lost
