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
Why can Fst vary among genes/loci in a genome?
Usually due to different selection pressures on the gene in the different populations/environments
(convergent evolution)
What is migration-selection balance?
Migration can introduce deleterious alleles into a population which selection acts to eliminate them
The deleterious allele will be maintained at a stable frequency
How can the frequency of the deleterious allele change (regarding migration-selection balance)?
The deleterious allele will be at a higher frequency if migration rate is high or selection against it is weak or at a lower frequency is mutation rate is low or selection against it is strong
What is genetic drift?
Change in the frequency of an allele in a population due to random chance
What is gamete sampling error?
For larger populations, sampled frequencies begin to look like expected frequencies (more representative of true frequency in the gene pool) compared to smaller populations
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
What is the probability that an allele will go to fixation or be lost due to genetic drift?
Pr(fixation) = frequency of allele
Pr(loss) = 1 - Pr(fixation)
What is the founder effect?
One example of drift
A subset (small # of individuals) of an original population “find” a new population
What is the bottleneck effect?
Another example of drift
When the population size is reduced due to a natural disaster (or some other thing)
Impact depends on the size of the reduced population & length of time in bottleneck
How are genetic drift and migration opposing forces?
Migration:
- Increases diversity WITHIN a
population (introduces new alleles)
- Decreases differences AMONG
populations (homogenizing, allele frequencies start to look the same)
Drift:
- Decreases diversity WITHIN a population (alleles are fixed or lost)
- Increases differences AMONG populations (each population has own path)
What is heterozygosity?
A measure of genetic diversity
The frequency of heterozygotes in a population
How does genetic drift and heterozygosity relate?
Genetic drift causes heterozygosity to decrease over time
What is effective population size (Ne)?
The size of an ideal theoretical population that lost heterozygosity (Hz) at some observed rate
Amount of genetic diversity a population has
How does drift and effective population size relate?
For larger populations and for populations with a larger effective population size, the strength of drift decreases (drift affects smaller populations more)
How does heterozygosity and effective population size relate?
Heterozygosity increases with Ne
How does drift and selection relate?
Both can cause a loss of genetic diversity
- Selection: beneficial allele goes to fixation
- Drift: random alleles go to fixation
Drift makes selection more “noisy” - adds random noise into the graph curves
How does drift work against selection?
Beneficial alleles can be lost randomly which with selection would normally go to fixation
Deleterious alleles can be fixed randomly
What is non-random mating?
Another mechanism of evolution
Mate selection is influenced by phenotypic differences which are caused by genotypic differences
What is assortative mating?
An example of non-random mating
Similar looking individuals mate with each other
“Like mates with like”
What is disassortative mating?
An example of non-random mating
Dissimilar looking individuals mate with each other
What type of mating is most common in animals?
Assortative mating
What is inbreeding?
An example of non-random mating
Relatives more likely to mate with each other (common)
- alleles share common ancestor
What is selfing?
Extreme form of inbreeding
Mating with yourself
What does non-random mating change?
Genotype frequencies not allele frequencies
Is non-random mating a force of evolutionary change? Why?
No, not by itself since allele frequencies aren’t changed although it does violate HWE
** Can be coupled w/selection on homozygotes
Why does inbreeding cause fitness in populations to decrease?
As heterozygosity decreases, recessive deleterious alleles are exposed which usually hide out in heterozygotes
Heterozygote superiority is masked reducing average fitness of a population
What is inbreeding depression? How to avoid it?
Decrease in fitness of a population due to inbreeding
Can be avoided with mate choice, self-incompatibility loci (gametes from same individual can’t create offspring), dispersal (mates from different population)
How can inbreeding increase the fitness of a population?
Through allele purging:
In larger populations, inbreeding depression can lead to purging of deleterious recessive alleles faster than random mating because they are exposed to selection when there are no heterozygotes to hide in
Do we expect it to be better to reproduce sexually or asexually? Why?
All else being equal, it would be better to reproduce asexually because they produce twice as many daughters
There is a 2-fold cost of sex:
- males
- only 1/2 the genes are passed on
Why does sex actually increase fitness in the long run?
New variation is created through recombination (the crossing over of chromosomes during Meiosis 1 creates new allele combinations)
What are Hill-Robertson Effects in asexual populations?
The fate of an allele depends on its genetic background so if a deleterious allele, a is closely linked to a slightly beneficial allele, B (genetic hitchhiking) then both allele frequencies would decrease due to their linkage (genetic load) even though B is slightly advantageous
What is clonal interference?
A type of Hill-Robertson effect
In asexual populations, beneficial mutations are competing with each other to win out since multiple beneficial alleles can’t be brought together via recombination (doesn’t exist in asexuals)
What is Muller’s ratchet?
Another type of Hill-Robertson effect
Once the most fit genotype is lost, it can’t be recovered
Why does sex “break” Muller’s ratchet?
The most fit genotype can be recreated through recombination in sexual populations
When is sex most beneficial?
When the environment is constantly changing, strong selection pressures
What is the Red Queen Hypothesis?
Organisms have to co-evolve with their environment whether that be abiotic (salinity, temperature) or biotic (viruses, parasites)
Harder to do this in asexual populations
How do asexuals get by?
Asexuality is present in many young species and may go extinct rapidly however being able to undergo sex every once in awhile can allow the species to survive
Can social behavior be heritable?
Yes! Altruistic, Cooperation, Spite alleles that have developed over time and can determine how the organism will behave
What is kin-selection?
A way to explain how an action/phenotype that decreases the fitness of the individual can be beneficial
Inclusive fitness depends on both your genes(direct) and your genes that have been passed on(indirect)
- 50% related to offspring
- 25% related to siblings offspring
What is Hamilton’s rule?
Br - C > 0
- B = benefit to recipient
- r = relatedness b/w actor & recipient
- C = cost to actor
Way to determine if the altruistic behavior should be done
Do animals know Hamilton’s rule?
Yes, they can identify kin and act accordingly
ex: squirrel adoptions
What are green-beard alleles?
A way to explain altruistic behavior
An allele that 1). produces a phenotype, 2). allows recognition of the phenotype in others,
3). initiates altruistic behavior to them
What are selfish genes?
Enhance their own transmission at the expense of other genes
What is an example of selfish genes?
Selfish genes in mitochondria can increase their own transmission at the expense of the nuclear genes resulting in decreased energy
Mitochondrial genes are also only passed through females so a mitochondrial allele that increases female fitness can be selected for even if it results in decreased male fitness or loss of function in males
What is eusociality?
The ultimate form of altruism
Some individuals give up their own reproduction completely so that one individual (or small group) can reproduce instead
ex: bees
What is the haplodiploidy hypothesis?
A hypothesis to explain how eusociality could evolve
Due to sex determination in some species, sisters are more closely related than to their offspring so there is a higher fitness for helping raise sisters than making your own offspring
Largely refuted due to many eusocial species not having haplodiploidy sex determination
What is the monogamy hypothesis?
A hypothesis to explain how eusociality could evolve
If you know your sibling are full siblings then they are as closely related to you as your own offspring
Also refuted
What is the ecology and life history hypothesis?
A hypothesis to explain how eusociality could evolve
All eusocial species have extensive nests and larvae that require a lot of care which is hard to do alone
Widely accepted
What is reciprocity?
Another way to explain altruistic behavior
Between non-kin, dependent on having social networks where you have the opportunity to punish if the altruistic behavior isn’t reciprocated
ex: vampire bats
Do humans follow Hamilton’s rule?
Yes. A population with altruistic behaviors has a higher fitness on average than populations with selfish behaviors
Kin-selection also happens in humans
Can Hamilton’s rule explain spite?
Yes, spite would mean that B is negative so r would have to be negative too (not related to recipient)
If it is better for the average individual to reproduce than the recipient then harming the recipient might increase the chances that the average individual (who is more related to you) reproduces
