evolution midterm #2 QFRS

Why are some alleles neutral?

no effect on fitness/surival/reproduction = natural selectivity

How do we define randomness? What are the two main random forces in evolution?

random = outcome can not be predicted individually

2 random forces → mutation and genetic drift

How do you define a Markovian process?

random memoryless process

previous event do NOT affect the next outcome

What is the main parameter that we need to know to predict the relative importance of drift in a population?

effective population size

How is fixation of an allele defined?

allele frequency reaches 100% → p=1

What is a deleterious mutation? Why is it more likely to fix in a small population?

harmful mutation

small populations = stronger drift overpowering selection

Being that drift is a random process, how can we make predictions as to how drift will proceed?

predict probablities mathematically and statistically

How do the results of natural selection and non-adaptive evolution differ?

selection = adaptive or nonrandom , favors beneficial alleles

drift = random or non adaptive , favors random alleles

What is the difference between actual population size and effective population size?

N = total individuals aka actual population size

Ne = indvidiuals actually breeding aka effective population size

Ne is always smaller than N

What factors can cause effective population size to be small (compared to actual population size)?

unequal offspring #s and sex ratio

population bottlenecks and fluctuations

anything reducing breeders lowers Ne

What are the ultimate fates of mutations that are neutral with respect to fitness?

evenetually fix or go extincit

0 or 1

Under what conditions is drift most powerful compared to natural selection?

small populations and small Ne

small population = STRONG drift

What is the probability of fixation of a new neutral allele in a diploid population?

1 / 2N

new mutation and is diploid

What is the probability of fixation of an established neutral allele in a diploid population?

equals current frequency → p

Pfix = p

What is the probability of getting a ‘heads’ on a coin flip after getting 10 ‘heads’ in a row?

50%

What is the main parameter that we need to know to predict the fixation rate of neutral alleles in a population?

mutation rate + effective population size matter

fixation rate mainly depends on MUTATION RATE

neutral fixation rate = mutation rate

What is an example of a founder effect?

small group colonizes new island and population

few foundes overrepresent alleles

 What are population bottlenecks and what effects do they have on populations?

population becomes very small temporaily

strong drift and loss of genetic diversity

alleles are randomly lost

What is heterozygosity and what does it tell us about a population?

probability of 2 individuals differing at same DNA site

measures genetic diversity

high heterozygosity = high diversity

What is dN/dS? What does it mean if this value is very low or very high?

ratio of nonsynonymous to synonymous substitutions

Low dN/dS (<1) = purifying selection

High dN/dS (>1) = positve selection

low = harmful mutations removed

high = adaptive evolution

What is a cline?

smooth change in trait mean or allele frequency across space

gradient across geography

What are some examples of clines?

whtie clover cynaide production

moose body size with latitude

geographic gradient

Are clines purely driven by natural selection?

no

gene flow also shapes clines

What other forces shape a cline?

gene flow and migration

mixing alleles b/w populations

For the white clover, why do some individuals produce cyanide but others do not?

cyanide protects from herbivores

cold environments select against it b/c frozen cells release posion

cyandie is harmful in cold

What explains the spatial range of a cline? Why are some wide and others narrow?

depends on balance of selection vs. gene flow

more gene flow = wider and smoother cline

gene flow spreads alleles

What is local adaptation and how can you test for it?

population is more suited to local evniorment than others

test by comparing fitness in different enivorments

best fitness is in home enviroment

For mammals, what explains the pattern that species tend to be larger in the north?

BERGAMANN’S RULE

large animals lose heat more slowly

cold favors larger bodies

How does gene flow affect local adaptation?

can weaken or erode it by introducing outside alleles

mixing reduces specilization

Can gene flow cause a population to become less adapted to their environment? How?

yes

by introducing maladaptive alleles from other enviorments

migration opposes local selection

What effect does gene flow have upon genetic differences between populations?

reduces differences b/w populations

homogenizes populations

 Is it possible for plants to disperse? How?

yes

pollen and seeds are dispersed by wind, etc.

gene flow is through pollen or seeds

How do we mathematically measure gene flow?

migration rate equation

delta p = m ( pm - p)

change in allele frequency from migrants

What is ‘m’ and what is ‘Fst’?

m = migration rate

Fst = genetic divergence among populations

high Fst = less mixing

 If the global Fst for humans is around 0.12, what does that mean in terms of mixing among different populations?

humans are gentically similar

considerable for mixing and gene flow among populations

have low differentiation

What does isolation by distance mean?

nearby populations exchange more genes than distant populations

farther apart = more genetically different

 How do selection and gene flow interact with each other?

selection INC local adaptation

gene flow counteracts by mixing alleles

selection vs. migration tug of war

Why is it useful to compare migration rate and strength of selection?

helps determine whether local adaption can persist

if migration is greater than selection then adaptation erodes

What is the ‘Essentialist view’ and how is it used to define species?

species are defined by matching an ideal “type“ based on phenotype or morphology
looks alike = same species

What are some key challenges to the essentialist view of determining species boundaries?

sister species = looking identical

SAME species = different looks

morphology can mislead

What are sister species?

closest relatives from common ancestor

closest evolutionary relatives

What is the ‘Biological Species Concept’?

species = actually or potentially interbreeding populations reproductively isolated from others

reproductive isolation

What are the limitations of the BSC?

hard to measure gene flow

problems w/ allopatric species

does not fit asexual organisms

can not test mating in separated species

What is the ‘Phylogenetic Species Concept’ and what is a key problem with this definition?

species = diagnosably different evolutionary clusters

key problem = tiny DNA differences can split species too much

PSC = too sensititve to small DNA changes

Given species such as the Grey Oak and Gambel’s Oak, are these two separate species or one? Why would you defend one answer or the other?

could argue separate b/c limited interbreeding

could also argue one b/c hydrbids or introgession occur

hybrid zone ambiguity

How is speciation defined?

origin of 2 speices from common ancestor

evolution of reproductive isolation

What are biological barriers to gene flow?

reproductive isolating barriers and mechanisms

prevent gene exchange

What does introgression mean? What effects does it have on a species?

genes from 1 species enter another species

hybridization + gene transfer

 How are pre-mating and post-mating barriers different? What are some examples?

pre mating = preventing mating and fertilization

post mating = hybrids reduced fitness

What are the differences between prezygotic and post-zygotic mating barriers?

prezygotic = before zygote forms

postzygotic = after hybrid forms

zygote formation cutoff

Why have some insects evolved ‘lock & key’ genitalia that only match their own species?

to help prevent mating w/ other species

mechanical isolation

What can we learn about speciation from the landmark study by Coyne and Orr in 1997?

reproductive isolation INC gradually w/ divergence time

speciation gradual

For the example of Mimulus cardinalis and Mimulus lewisii (monkeyflowers) what is the main cause of reproductive isolation between these species in nature? How do we know this?

pollinator differences + elevation and habitat differences

few hybrids in nature despite fertile lab hybrids

prezygotic isolation = STRONGEST

What is likely to evolve first, pre-zygotic or post-zygotic isolation?

prezygotic isolation

acta before costly hybrids form

What is allopatric speciation? How does it relate to ecological speciation?

speciation due to geographic separation

ecological differences can evolve afterward

physical barrier stops gene flow

 What hypothesis did the Dodd experiment on fruitflies test?

different envirometns and diets can cause reproductive isolation

ecological divergence leads to → speciation

How can reproductive isolation be caused by pleiotropy or hitchhiking?

1 gene affects adaptation + mating traits

nearby genes dragged along by selection

selection indirectly causes isolation

What kind of experiments show biologists that speciation is usually a slow, gradual process?

comparative studies across populations or species over time

more divergence = stronger isolation

How did Tilley (1990) show that populations diverge over time & evolve mating barriers?

salamander populations are father apart and had stronger mating barrieres

distance correlates w/ isolation

What is reinforcement?

natural selection strengthens prezygotic isolation to avoid unfit hybrids

selection is against hybrids

Can prezygotic isolation ever evolve by natural selection?

yes

b/c reinforcement favors avodiing costly hybridization

avoids low-fitness hybrids

What is hermaphroditism and how does it relate to assuring of reproduction?

1 indidivual produces both eggs and sperm

can self fertilize if no mate is available

selfing ensures reproduction

How do you define sexual reproduction?

combining genetic info from 2 individuals

fusion of gametes

How are sexes defined? Do plants have sexes?

defined by gamete size and type

YES plants have sexes

gametes define sex

What is anisogamy and what causes it to be common?

2 different gamete sizes = large eggs and small sperm

eggs maximize resources

sperm mmaximize number or mobility

large resource rich eggs vs. tiny mobile sperm

How are males and females defined?

male = small gametes

female = large gametes

What is sexual dimorphism? Why does it evolve?

males and females differ in traits

evolves to optimize reproduction and sexual selection

sex - specific traits

What are sex determination systems?

chromosomes

haplodiploidy

hermaphroditism

enviormental sex determination

many systems determing sex

What are primary and secondary sexual characteristics?

primary = gonads and gamete structures

2ndary = traits aiding mating success

direct reproduction vs. attraction and competiton

How do secondary sexual characteristics relate to sexual selection?

sexual selection acts on them to INC mating success

ornaments and weapon favored

How can sexual selection counteract natural selection? What are some examples?

traits improve mating but reduce survival

example:

• long tails

• loud frog calls attract predators

mating success vs. survival trade off

What is the Bateman principle? Do you think that it applies to humans? Why or why not?

male reproductive success INC w/ more mates

female success less dependent on mate number

cheap sperm vs. expensive eggs

How does variance in reproductive success vary between males and females?

greater in males than females

few males reproduce a lot

What is an operational sex ratio? Is it often biased towards one sex?

ratio of sexually available males : females

usually male - biased

more males competing

How can sexual role reversal occur?

happens when males invest more parental care

females compete for males

males parental investment reverses competition

What are some examples of male-male competition? Are there costs to this?

fighting w/ antlers or horns

costs = injury, energy, reduced survival

weapons used for access to females

Is it possible to have male-male competition without direct fights? How?

yes b/c indirect competition

displays, calls, territories and sperm competition

How do females choose males? What are some hypotheses about the evolution of such preferences?

prefer traits signaling quality or exploiting sensory bias

female choice drives ornaments

What is the good genes hypothesis for female preference of males?

females choose males w/ traits indicating high genetic quality

ornaments = healthy genes

 What is the perceptual bias hypothesis for female preference of males?

female prefernece evolves fro pre-existing sensory bias

males exploit female senses and preferences

How are life history traits defined?

traits affecting surivial and reproduction across life stages

survival + reproductive success

What is Lack’s theory of optimal clutch size?

natural selection favors clutch size producing most surviving offspring

maximize surviving young and not total eggs

Why are very big clutches (with many eggs) selected against?

too many offspring lowers survival and parental care per offspring

quantity vs. quality trade off

What are tradeoffs and how do they affect the number and size of offspring?

more offspring = smaller offspring

larger offspring = fewer offspring

limtied resources

What are some examples of tradeoffs that affect fitness?

growth vs reproduction

current vs. future reproduction

many seeds vs. large seeds

energy is allocated to 1 trait reducing another

What is the difference between semelparous and iteroparous reproduction?

semelparous = reproduce once then die

Iteroparous = reproduce multiple times

single vs. repeated reproduction

What is the cost of reproduction?

reproduction reduces energy and resources for survival or future reproduction.

reproduction trades off w/ another fitness traits

How can the cost of reproduction be experimentally measured?

manipulate reproductive tissues or output

example

• ovariectomy experiments

compare reproduction removed vs. normal

How do you calculate fitness in a semelparous population?

R = LM

L = survival to reproduction

M = offspring number

1 reproductive bout

 How do you calculate fitness in an iteroparous population?

R = sum (lx mx)

R= lifetime reproductive success

sum = sum total acorss all ages

lx = probability of surviving to age x

mx = average number of offpsring at age x

x = age class

iteroparous = reproduce multiple times over life

What are the two evolutionary hypotheses to explain why living organisms get old and die?

mutation accumulation

• harmful mutation affecting old age build up b/c selection is weak later in life

• late- acting harmful mutations presist

antagonistic pleiotropy

• 1 gene has multiple effects and is beneficial early in life

• harmful later in life

• early reproduction benfits

• late aging cost

Why does selection favor early reproduction?

early reproduction contributes to more genes before death

future survival is uncertain

Under what conditions is reproduction delayed?

when survival is high and larger size and resources improve future fitness

delay can increase future reproductive success

What did Reznick’s 1990 experiment teach us about age of reproduction in guppies?

high predation populations reproduced earlier

predation favors early reproduction

 How might predation risk affect age of first reproduction?

higher predation = earlier reproduction

reproduce before dying

What exactly does antagonistic pleiotropy mean?

1 gene is beneficial early in life but harmful later

early fitness benefit and late life cost

What predictions about the evolution of cooperation does Malthus’ Essay on Population promote?

limited resources = competition and conflict expected

struggles for existence

How do you define altruism?

helping another at the cost to yourself w/ no direct benefit

costly helping behavior

How are spite and competition similar in terms of interaction types?

both reduce fitness of interactors

(-,-) interaction

How does a group benefit when an individual joins a herd? Does the individual that joins the herd benefit too?

group safer from predators

individual also directly benefits

safety in numbers

byproduct mutualism