Evolution Quiz 2

In order for evolution by natural selection to occur Darwin said there must be:

heritable variation

differential survival/ reproductive success

Quantitative genetics allows us to

measure heritable variation

measure differences in survival/ rep. success

Predict the response to selection

Which equation is used to determine the extent to which a trait is heritable

VP = VG + VE

VP=

total phenotypic variation in a trait

VG meaning

Genetic variation (variation among individuals due to genes)

VE=

environmental variation (variation among individuals due to variation in their environment)

Broad sense heritability

proportion of variation in the trait due to variation in genes

Broad Sense heritability equation

VG/VP

If variation among individuals is due to variation in genes then offspring should..

resemble parents

VG=

VA + VD

VA=

additive genetic variance (variation due to the individual effects of alleles)

VD=

dominance genetic variation (variation due to allelic interactions)

Since interactions are not inherited (VA or VD) causes resemblance between parents and offspring

VA

Narrow sense heritability equation (h2) =

VA/VP

Why do we use narrow sense heritability

to estimate the response to selection

If neither allele is dominant over the other (codominance) then

the line of best fit explains all genetic variation

VG=VA

If there is no line of best fit, use the equation

VG= VA+ VD

Selection differential (S) =

the difference between the trait mean of the selected individuals and the trait mean of the entire population

Selection gradient (B) =

more broadly applicable measure of selection

Selection gradient steps

• take absolute fitness (ex. total # of offspring an individual has) and divide it by the total population’s mean fitness to calculate relative fitness

• then plot the value of the phenotypic trait (x-axis) vs. relative fitness (y-axis)

• the slope of the best-fit line through this plot is the selection gradient

Selection gradient (B) equation =

S/(var t)

var t =

variance in the trait

Selection differential equation =

mean of selected individuals - mean of all individuals

The slope of the line of best fit is the

selection gradient

The breeder’s equation predicts

the response to selection

The breeders equation

R=h²S

R defined=

response to selection

h²=

heritability

S =

selection differential

How to solve for R =

the difference between the mean trait value of offspring from selected parents and the population mean

The breeders equation can be used when

we know two variables

ex.) if we know heritability and selection differential we can predict the response to selection

Ex. Question that prompts use of breeders equation

Is the difference in flower size a result of natural selection on bumblebees

If h² is between 0.2 and 1.0 this suggest -% of the variation in flower size is due to additive genetic variance

20

axes for heritability (h²)

offspring flower petal distance (y)

maternal flower petal distance (x)

S = (equation rearranged)

B x (var t)

R is used to calculate

response to selection

If R is 0.01 this means

within a single generation, selection by bees should produce an increase in flower size by at least 1%

Given R what do you predict the beak depth for the next generation is

mean beak depth + r

Direction selection defined

fitness consistently increases or decreases with trait fitness

In continuous traits directional selection changes the

means and lowers the variation

directional selection graphs

stabilizing selection definition

intermediate phenotypes have the highest fitness

In continuous traits, stabilizing selection does not alter the - but decreases - by “trimming the tails” of the distribution

mean, variation

stabilizing selection graphs

Disruptive selection defined =

extreme trait values have the highest fitness

In continuous traits disruptive selection does not alter the population - but “trims the tops” of the distribution to - variance.

mean, increase

Example of disruptive selection

small or large beaks lead to better bird survival than birds with medium beaks

Heritability is a ratio of VA and VP within a population under a particular set of environmental conditions. It is a -

within population measure

Heritability (does or does not) tell us anything about the causes of differences among populations living in different environments

does not

Differences among populations might be due to - (since heritability estimates depend on both)

very different environments

Low heritability (does or does not) necessarily mean a trait has little underlying genetic variation

does not

Can the heritability of a trait in a population change?

yes

- estimates are specific to a particular population in a specific environment.

Heritability

h² being low could mean

• absolute value of VA is low

• VP is large (due to large VE), relative to VA

A low h² means -. It doesn’t mean that the trait is -

a small response to selection from one generation to the next.

not capable of long-term evolution.

h² can change as the effect of - on the trait changes.

VE

Clausen, Keck, Hiesey’s study on Achillea, a perennial plant

two cuttings (one from top half one from bottom half) from 7 different plants and grew the cuttings in different environments (mather and stanford) same soil, sun, etc.

In the Clausen, Keck, Hiesey experiment since all cuttings were reared in the same environment - = 0 and any differences between plants in a garden was due to genetic variation. heritability =

VE, 1

Results of Garden plant experiment

plants raised at the stanford garden were taller

Interpretation of stanford garden results

since the cuttings were genetically the same even though heritability was high within each population this tells us nothing about differences between populations raised in different environments.

Quantitative Trait Loci (QTL) =

the loci that influence quantitative traits

QTL mapping =

use of marker loci to scan chromosomes and identify regions containing genes that may contribute to a quantitative trait

How can we detect the presence/location QTLs by

crossing parents from populations with fixed differences then comparing individuals of the F2 generation

If the distributions of phenotypes overlap then the marker (is or is not) linked to a QTL

is not

Genetic variation (is or is not) a requirement for evolution

is

_ and - reduce genetic variation

NS and genetic drift

_introduces variation slowly

mutation

Balancing selection =

selective processes by which multiple alleles are maintained in a population

Mutation-selection balance

if the fitness differences between alleles are small (i.e. selection against a lower fitness allele is weak), then selection would remove the less fit allele very slowly

Mutation adds the allele back into the population at the - rate it is lost via selection

same

If selection is weak and similar to the rate of mutation

a less fit allele can be maintained at higher frequencies

equilibrium frequency

Heterozygote advantage

heterozygotes have a higher fitness than either homozygote

How does heterozygote advantage maintain genetic diversity at a locus

at equilibrium the advantage lethal/deleterious alleles in heterozygotes is balanced the obvious disadvantage experienced in homozygotes

Sickle cell anemia is an example of - since -

heterozygote advantage, heterozygotes are resistant to malaria.

_ selection selects for the intermediate phenotype regardless of underlying genotype

stabilizing

_ selection may not maintain multiple alleles at a locus

stabilizing

_ selects for heterozygous genotypes regardless of the phenotype they produce

heterozygote advantage

_ favors different alleles in different environments, or different alleles over time (if the environment changes over time)

selection

Selection can maintain - but requires fairly extreme amounts of variation or high migration

polymorphism

Frequency dependent selection

the relative fitness of a genotype is dependent on how common it is (its frequency) in the population

Negative Frequency dependent selection

rare genotypes have the selective advantage

Negative FDS example

rare clonal freshwater snails are attacked by parasites less

tradeoff

two different selective forces acting on phenotype (opposite ways)

Example of a tradeoff

being big means a better territory and more mates, but being big is also costly in terms of energy expenditure

Even if there is a tradeoff variation in the traits can only be maintained if different combinations have - fitness

equal

Example of tradeoffs in crickets

alternative male mating strategies

males get attacked by parasitoid flies. the flies are attracted to the male cricket call

male crickets employ two strategies for mating

some males call to females

other males use a “sneaker” strategy and don’t call

Tradeoff: mating, parasite load

• variation is only maintained if the two strategies have equal fitness

• all have different mating strategies but same fitness (same # offspring produced during lifetime)

If fitness is not exactly equal (one strategy has a slight advantage) selection will favor that strategy and it will -

reach fixation

example of tradeoffs reaching fixation

if calling 100% of the time gives males slightly more mates, this will be selected for

Trade-offs can interact with - selection

frequency-dependent selection

Trade-offs alone don’t maintain variation but trade-offs resulting in equal fitness can maintain -

variation

What is the smallest evolutionary independent unit

a species

evolutionary independence occurs when

mutation, selection, gene flow, and drift operate o populations separately

evolution (a change in allele frequencies) can occur

speciation

a lack of gene flow when populations are in contact with each other

Morphospecies concept

criterion for different species : unique morphological traits

Advantages to the morphospecies concept

widely applicable

can be applied to extinct or currently living organisms, asexual or sexual organisms.

Disadvantages to the morphospecies concept

× Species definitions may be arbitrary

× Different researchers may use different criteria

× May not accommodate polymorphism

× Difficult to identify important morphological traits the

fossil record

× Ignores strongly diverged non-morphological traits (e.g.

mating songs, drought tolerance, habitat preferences)

Biological species concept (BSC)

Criterion for different species: reproductive isolation. Different species do not hybridize in nature or fail to product fertile offspring if they do

Biological Species Concept Advantages

confirms lack of gene flow (the essence of speciation)

legal definition used by the endangered species act

Biological Species Concept Disadvantages

× Difficult to apply. What if two populations never meet? We can

hypothesize how they might reproduce but we don’t know for sure.

× Irrelevant for fossils, asexual organisms, and many plants (distinct

groups regularly hybridize)