Evolution by natural selection The Modern synthesis restated Darwin’s 4 postulates: (1)Individuals...

Evolution by natural selectionThe Modern synthesis restated Darwin’s 4 postulates:

(1) Individuals in a population are variable for most traits, because mutation creates new alleles and sexual reproduction creates new allele combinations in every generation

(2) Individuals pass their particular alleles to their offspring

(3) More offspring are produced than can survive

(4) Individuals that survive, or reproduce the most, have allele combinations that best adapt them to their environment

Natural selection is the sole process that produces adaptation to the environment

- an adaptation is a trait that makes you better suited to your ecological niche, and increases your fitness

Alleles or allele combinations, and the traits they produce, determine fitness of an individual: # of offspring that survive to reproduce

- if you live forever but produce no offspring, your fitness = 0

Allele combinations resulting in higher fitness are passed to more offspring, and thus those alleles rise in frequency over time (becoming more common)

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Natural selection & fitness

Two general points to consider:

1) there is a trade-off between offspring size and number

- the same amount of resources can be divided into more but smaller offspring, or fewer, bigger offspring


maternalenergy

(e.g., available egg yolk)

If life is easy, and offspring survival rates are high, which strategy do you predict selection will favor?

Two general points to consider:

1) there is a trade-off between offspring size and number

- the same amount of resources can be divided into more but smaller offspring, or fewer, bigger offspring

- measuring “fitness” is not trivial; it’s not always just # of babies made

2) an individual’s fitness is determined by lifetime reproductive output, not just the # of offspring laid in a given year


Lack’s hypothesis: How many offspring?

Assume the more eggs you lay, the lower the odds of surviving are for each individual offspring

Multiplying # of offspring (= clutch size) by odds of each offspring’s survival gives the optimal clutch size (Lack’s hypothesis)

Life-history evolution: How many offspring?

Data from study on birds showed that the mean # of surviving baby birds was highest for clutches of 12 eggs

However, birds laid 8-9 eggs per clutch in any given year, to save energy for future reproduction

In a given environment, natural selection may act strongly on:

1) one genetic locus (physical spot on chromosome)

2) multiple, adjacent loci

3) one trait in isolation

4) the correlation between two or more traits

Natural selection acts at many levels

molecular evolution

phenotypic evolution

Selection favoring one allele at a given locus is termed purifying selection

- “purifying” because at this level, selection acts against all but the most favorable allele

- will decrease genetic polymorphism at that locus

Selection on one locus

A B C D E F

a b c d e f

if selection strongly favors “C” allele of the C gene...

a b c d e f

a b c d e f

Selection favoring one allele at a given locus is termed purifying selection

- “purifying” because at this level, selection acts against all but the most favorable allele

- will decrease genetic polymorphism at that locus

Selection on one locus

A B C D E F

Purifying selection reduces polymorphism at the C gene, by removing other alleles from the population

A B C D E F

A B C D E F

A B C D E F

A B C D E F

A B C D E F

Selection favoring one allele at a given locus is purifying selection

Case 1: Bacteria (~ no recombination during reproduction)

Indirect selection on nearby loci

A B C D E F

a b c d e f

What effect will purifying selection on the C gene have on level of polymorphism at the D gene?

a b c d e f

a b c d e f

Selection favoring one allele will also tend to drag alleles at nearby or linked loci to high frequency

...it will also tend to favor “B” and “D” alleles, if they happen to be linked to “C” on a chromosome

in asexual organisms, alleles get inherited as a team due to linkage; what’s good for one allele is good for its neighbors

genetic hitch-hiking: neighboring alleles benefit from purifying selection, even if they aren’t under selection at all


if selection strongly favors “big C” allele of the C gene...

A B C D E F

a b c d e f

Case 1: Bacteria (~ no recombination during reproduction)

What effect will purifying selection on the C gene have on polymorphism at the distant Q gene?



A B C D E F

a b c d e fQ

q

Selective sweeps

A B C D E F

Bacteria (and mitochondria, their descendants) have circular chromosomes and limited recombination

Are therefore prone to selective sweeps

1) purifying selection favors one allele of one gene, pushing it to fixation (100% frequency in the population)

2) at the same time, selection reduces polymorphism at all genes throughout the genome

3) whole population ends up with reduced (if any) polymorphism, until mutation or migration introduces new alleles

Selection favoring one allele will also tend to drag alleles at nearby or linked loci to high frequency

A B C D E F

a b c d e f



...all these alleles will be lost, unless they can get onto the “winning team” i.e., any chromosome with a C allele


Case 2: Eukaryotes (recombination during reproduction)


What effect will purifying selection on the C gene have on level of polymorphism at (1) the D gene?... (2) the F gene?

A B C D E F

a b c d e f

A B C D E F

a b c d e f

recombination allows linked loci to escape the effects of purifying selection on nearby genes

crossing over “breaks up the team”

A B C D E F

a b c d e f

Even if selection strongly favors C allele... alleles of other genes can cross over onto C chromosomes

A B C d e f


A crossing over event can rescue the “d” allele by moving it next to the winning “C” allele, favored by purifying selection

A B C D E F

a b c d e f

A B C D E f


However, a crossing over event becomes more likely as you move farther away on the chromosome

“f” is more likely to get recombined onto a “C” chromosome than “d” because the F gene is farther away


Case 2: Eukaryotes (recombination during reproduction)

In a given environment, natural selection may act strongly on:

1) one genetic locus (physical spot on chromosome)

2) multiple, adjacent loci

3) one trait in isolation

4) the correlation between two or more traits

Natural selection acts at many levels

molecular evolution

phenotypic evolution

Modeling selection on quantitative traits

How would you model the relationship between weight and fitness, given these data?

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weigh t


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y = 1.6 (x) - 1 .9

weigh t

A model is an equation expressing the relationship between a trait and some measure of fitness

For each unit increase in body size (weight), you get ~1.6 offspring

- the slope (coefficient associated with trait value) tells you how strongly that trait affects fitness


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y = 1.6 (x) - 1 .9

weigh t

A model is an equation expressing the relationship between a trait and some measure of fitness

How would you expect this trait (size) to evolve over time?

Directional selection

A linear relationship between the value of a trait and fitness is termed directional selection

The model is the equation of the best-fit line through the data

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tra it

“fitness” is generally estimated as probability of adult survival, or # of offspring produced over some interval (ideally, lifetime)

The mean value of a trait under directional selection should evolve in one direction over time... - mean value should increase if slope (m) is positive

y = mx + b

Directional selection

Directional selection can also be inferred when the distribution of trait values in a population shifts in one direction over time

Remember: directional selection

a) changes the mean value of a trait (+ or -)

b) decreases the variance around the mean value of a trait

(trims one tail off the distribution)# of ind ividua ls

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trait value

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y = -0.1x2 + 2.3x + 1.4

Stabilizing selection favors individuals with trait values close to the population mean

Model is non-linear, meaning not a straight line (curved)

Quadratic equation includes (a) the trait value (x), and also (b) the square of the trait value (x2), which is what makes it a non-linear relationship

Stabilizing selection (non-linear)

Stabilizing selection (non-linear)

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Stabilizing selection maintains trait values over generations close to the same mean

Remember: stabilizing selection

a) no change to mean value of trait

b) decreases variance

(trims both tails off distribution)

# o

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trait value

Model is also a quadratic equation, but now: (a) coefficient associated with the trait value (x) is negative (-2.3) (b) coefficient of squared term (x2) is positive (0.1)

Disruptive selection (non-linear)

weigh t

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y = 0.1x2 - 2.3x + 27

Disruptive selection favors individuals with trait values far from the population mean


Disruptive selection splits one group into two sub-groups of different ecotypes or specialists

Remember: disruptive selection

a) no change to mean value of trait

b) increases variance

(shifts distribution towards tails)

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Disruptive selection (non-linear)Disruptive selection is of special interest as a potential agent of ecological speciation

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A generalist species of bird will feed on seeds of many sizes

If the most common plants produce either large or small seeds, this can impose disruptive selection on beak size

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Disruptive selection may thus split one generalist species into two specialist ecotypes that over time, under the right conditions, may evolve into separate species

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big beaks = crack large seeds

tiny beaks = handle tiny seeds

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trait value

Selection-mutation balance

standing geneticvariation in a population

mutation

migration

selection

(usually reduces polymorphism)

Multivariate selection

Rather than acting on one trait in isolation, natural selection often acts on suites of traits that contribute to the overall phenotype

- multivariate meaning multiple traits (variables)

We model multivariate selection using equations that include the variables themselves (trait values), as well as the correlation between two variables

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tra it 1

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positive correlation

negative correlation


Example: In the sea slug that my lab studies, finger-like projections on the back of the slug (cerata) convulse to circulate body fluid, instead of a beating heart

cerata Under normal conditions, selection favors a negative correlation between (a) # of cerata, and (b) the rate at which cerata beat

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# o f ce ra ta

few, fast-beatingcerata

many, slow-beatingcerata

two different ways to achieve same basic “heart rate”

mosteggs


When it rains, slugs with many cerata have large surface area over which freshwater rushes into body; they explode

Slugs with fast circulation (high beat rate) also take on too much water, and explode

Rainfall selects for a positive correlation between # of cerata, and beat rate

- favors few & slow- beating cerata

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# o f ce ra ta

few, slow-beatingcerata confershigh fitness

(no slugs with many, fast- beating cerata; all dead)


Marshall & Munroe (2012) studied how competition affected multi-variate selection on colonies of a marine bryozoan

- bryozoans grow as colonies like a honeycomb, where each unit is an individual that (a) can reproduce, and (b) will at some point senesce (stop reproducing; basically die)

- colonies differ in: (a) size of individual units; (b) offspring size;

(c) how fast units go into senescence

senesced (dead) units

Multivariate selectionUnder normal conditions, colonies with intermediate trait values produced the most offspring (= had highest fitness)

- medium unit size- medium offspring size- medium death rate

= stabilizing multivariate selection

Multivariate selectionStrong competition from neighboring colonies changed the shape of selection on trait correlations

crowding favored a negative correlation between offspring size and senescence rate, regardless of unit size

= disruptive selection

Correlated selection

Consider: Suppose fish like to bite off cerata, which look like worms (fish bait). This harms the bitten slugs. Studies suggest slugs with more cerata get attacked by fish more often.

cerata 1) What kind of selection does fish predation impose on cerata #, as a trait in isolation?

2) How would you predict cerata # to evolve?

3) Would you expect cerata beat rate to evolve in response to fish predation? Why, or why not?

Evolution by natural selection The Modern synthesis restated Darwin’s 4 postulates: (1)Individuals...

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