Bios 3300 Exam 2

artificial selection

an evolutionary process in which humans consciously select for or against particular features in organisms

what is natural selection?

a difference, on average, between the survival and reproduction of individual with certain phenotypes compared to individuals with other phenotypes

is evolution survival of the fittest?

no

evolution is not a force, but a _______-______ process

multi-step

what are the 3 steps of natural selection?

1. individuals vary in phenotype

2. some of this phenotype variation is heritable

3. the variable, heritable traits affect an individual’s reproductive success (“selected” by nature)

how can individuals vary in phenotype?

variation could be due to genetic factors, environment factors, or their interactions

what is the struggle for existance?

not all individuals reproduce, or reproduce maximally due to struggle for existance

what is evolution by natural selection?

genetic composition of the population changes across generations

what are two examples of natural selection?

industrial melanism, darwin’s finches

industrial melanism (natural selection example)

before 1850: light morph of moth at nearly 100%, small amount of dark morph

coal pollution darkened tree trunks and killed the lichens

within about 50 years, dark morph increased to nearly 100%

clean air legislation passed in 1950; today light morph ~ 70%

when environment was darker, dark morphs were selected for, and when the environment was lighter, light morphs were selected for

how did darwin’s finches meet the requirements for natural selection?

varied in beak depth (phenotype), and beak depth is heritable

explain what happened to darwin’s finches as a result of the 1977 drought

84% of finches died

seed abundance dropped dramatically, causing a struggle for existance for seed eaters

larger seeds (more easily processed with a deep beak) were more common compared to smaller seeds (more quickly processed with a shallow beak)

after the drought the average beak depths had increased compared to before the drought; this caused evolution by natural selection

explain what happened to darwin’s finches as a result of the 2004 drought

large finches had colonized the island and dominated access to larger seeds, causing only smaller seeds to be available

this caused selection for smaller beak sizes in the medium finches

traits under natural selection only impact the _________ of survival and reproduction

probability

what does natural selection act on? what does evolution (by natural selection) act on?

natural selection operates on the phenotypes of individuals within a population, while populations evolve across generations

can natural selection operate if the character under selection is not heritable? what happens if it can?

yes, the population will not change in its genetic composition across generations

natural selection is a process composed of _________ components

testable

can natural selection predict the future? explain

no, every generation reflects the impacts of natural selection on the previous generation

natural selection is not _______, and does not lead to _______

progressive, perfection

how can new traits evolve?

by natural selection

how must an adaptation evolve?

by increasing the reproductive success of its possessor

what determines what genes make it to the next generation?

reproduction, not survival

what is the alternate definition of natural selection?

the non-random survival and reproduction of variants randomly generated with respect to need

is natural selection random? how can it be predicted accurately?

no; with knowledge about ecology and heritability

can natural selection favor a trait “for the good of the species?” why or why not?

no, it can only act on individuals and proceed without foresight

what can natural selection do in regards to the mean fitness of a population?

it can only improve the mean population fitness, it can never operate to lower mean population fitness

evolution is a tinkerer, so it operates step-by-step, which imposes ______________ __________

evolutionary constraints

evolution ______ select on variation that does not exist, and ______ in the production of variation can influence the evolutionary path taken by a population

cannot, biases

fitness

the extent to which an individual contributes genes to future generations

what is an ideal measure of fitness?

number of offspring produced by an individual in its lifetime

what is fitness usually applied to?

phenotypes

explain the relationship between global and local optimum in regards to fitness

they both exist, cannot move between the if it requires a decrease of mean population fitness

what is the Hardy-Weinberg equation?

(p+q)² = p² + 2pq + q² = 1

what are the 8 assumptions of HWE?

1. diploid organism

2. sexual reproduction

3. non-overlapping generations

4. infinite population size

5. no migration

6. no mutation

7. no natural selection

8. mating is random

what are allele frequencies given by (when in HWE)?

p and q

what are genotype frequencies given by (when in HWE)?

p², 2pq, q²

will allele frequencies change from generation to generation within a population if it is in HWE?

no, unless one of assumptions 4-7 is violated

violation of assumption 8 of HWE (mating is random) changes what?

changes genotype frequencies, but not allele frequencies

how many rounds of random mating is required to re-establish HWE?

1

absolute fitness

the fitness of a phenotype is the average lifetime reproductive success of individuals with that phenotype

fitness component

an individual’s score on a measure of performance expected to correlate strongly with the genetic contribution to the next generation

re

what is the maximum relative fitness equal to?

1

how do you calculate relative fitness?

divide all absolute fitness values by the maximum absolute fitness

fitness is a _________, not an ___________

description, explanation

is “survival of the fittest” an accurate definition of natural selection? under that definition what would natural selection be?

no, natural selection would be a tautology; fitness is not an explanation, as the causes of fitness differences can/should be studied

how can an advantageous trait causally contribute to the organism’s survival and reproductive success?

one trait may provide higher fitness, but it does not give an explanation as to why; trait could be genetically correlated with another trait under selection

how is fitness context dependent?

the phenotype of an organism in relation to the environment it inhabits determines how fit that organism is

what does Δq represent in the general selection model?

rate of change in allele frequency

what do W₁, W₂, W₃ represent in the general selection model?

relative fitness of genotypes

what are the 3 components that impact the rate of evolution?

genetic variance, mean population fitness, average effect of an allele substitution

genetic variance (pq) - what does a larger pq represent? when is pq the largest?

the larger pq, the greater rate of of evolution; pq is largest when p=q=0.5

mean population fitness (W̄) - what does a lower W̄ mean in regards to rate? what does a higher W̄ mean in regards to rate?

lower W - faster rates of evolution

higher W - slower rates of evolution

average effect of an allele substitution (q(W₃ - W₂) + p(W₂ - W₁))

quantifies the interaction between relative fitness and allele frequency; the greater difference between the relative fitnesses, the greater rate of evolution

selection coefficient (s)

quantifies the difference in fitness

directional selection

a mode of natural selection in which a phenotype is consistently favored, causing allele frequencies to shift over time in the direction of that phenotype

what will directional selection always increase?

mean population fitness

what happens in selection with dominance when the fitness of the homozygous recessive = 0 to the recessive allele frequency?

the recessive allele frequency decreases rapidly at first when it is common, but very slowly when it becomes rare

eugenics

the study and practice of control over the evolution of human populations in an effect to decrease the frequency of undesirable traits; does not work

in selection with overdominance, heterozygotes have a(n) _________, and the ________ fitness

advantage, highest

what does selection with overdominance lead to?

an equilibrium frequency of the A & a alleles

balancing selection (balanced polymorphism)

forms of natural selection that maintain variation within a population

in selection with overdominance, what do p̂ and q̂ provide?

the frequency at which the A & a alleles reach equilibrium

in selection with underdominace, heterozygotes have a(n) _________, and the ________ fitness

disadvantage, lowest

in selection with underdominance, the selection coefficient (s) is ________

negative

what happens if you deviate from equilibrium in selection with underdominance?

directional evolution will fix for a specific allele

what are the stable equilibria in selection with underdominance?

0 and 1

is underdominance an example of balancing selection?

no, variation is not maintained

are mutations completely random?

they have causes, but are random with respect to adaptive “needs”

true or false: the environment does not induce adaptive mutations

true

purifying selection

selection against deleterious mutations

in purifying selection, how are deleterious mutations removed if they are dominant or co-dominant?

removed by selection

in purifying selection, how are deleterious mutations removed if they are recessive?

through mutation-selection balance

mutation without _______ will __________ the frequency of the a allele

selection, increase

selection without _________ will _________ the frequency of the a allele

mutation, decrease

what will happen to the a allele at the mutation-selection balance?

it will achieve an equilibrium

what does q̂ represent in the mutation-selection balance?

predicted equilibrium frequency of q when mutation and selection are both operating, which is normal

why are debilitating diseases not eliminated by natural selection?

due to the mutation-selection balance, as mutation and selection both operate at the same time

genetic drift

a change in allele frequencies across generations due to random error in the sampling of gametes (random genetic changes)

binomial distribution

probability distribution of the number of success in a sequence of N independent yes/no experiments, each of which yields success with probability p (frequency of the A allele) or failure with probability q (frequency of the a allele)

what does genetic drift lead to, in terms of genetic variation?

the loss of genetic variation within a population

the amount of possible change is allele frequency per generation is ________ to the population size

proportional

in genetic drift, what size of population will fix for a specific allele quicker?

smaller populations

what is the probability of fixation of an allele equal to?

initial frequency of the allele

what type of alleles is genetic drift more likely to remove?

low frequency alleles

what does genetic drift cause among populations?

differences, a higher variance among populations

in genetic drift, genetic diversity is _______ within populations, while diversity between populations ___________

lost, increases

true or false: genetic drift changes the mean allele frequency among populations

false

what does genetic drift lower? (hint: genotype)

heterozygosity, and the loss of heterozygosity is more pronounced in smaller populations

what type of effects can genetic drift produce?

founder effects

founder effect

change in allele frequencies that occurs during the establishment of a new population due to sampling error in drawing founders from the source population

migration (gene flow)

the movement of alleles between populations

migration-selection balance

the equilibrium reached when natural selection favors local adaptation while gene flow (migration) continuously introduces maladapted alleles from other populations

gene flow ___________ populations genetically

homogenizes

gene flow ______ genetic variance between populations, and _________ genetic variance within populations

decreases, increases

can gene flow and selection work in opposite directions?

yes

what causes the opposite effect of gene flow/migration?

genetic drift

N_e (effective population size)

the size of an idealized population in which the rate of genetic drift is the same as in the actual population

N_e is usually ______ than the census N

less

why is N_e usually smaller than the census N?

variation in the number of progeny from each individual, unequal numbers of male and females, variation in population size