Evolution - Week 6

Question 1

What are the 4 evolutionary mechanisms?

Answer 1

mutation: the only source of completely new alleles

gene flow: movement of alleles from one gene pool to another

genetic drift: sampling error due to population bottlenecks, bottleneck event, founder effect, small population size

natural selection: leads to/maintains biological adaptations

Question 2

What is gene flow?

Answer 2

movement of alleles from one gene pool to another

• flow can be bidirectional, or unidirectional
• in either case, the alleles frequencies in the sink (ie. receiving) population will become more similar to the allele frequencies in the given source (ie. given population)

Question 3

What is a mutation?

Answer 3

the only source of completely new alleles

Question 4

What is genetic drift?

Answer 4

sampling error due to population bottlenecks, bottleneck event, founder effect, small population size

Question 5

What is natural selection?

Answer 5

leads to/maintains biological adaptations

Question 6

What is fitness?

Answer 6

lifetime reproductive success (# offsprings over lifespan)

Question 7

How do we measure the fitness of an individual?

Answer 7

the number of offspring they produce in a lifetime

Question 8

How do we measure the fitness of a trait?

Answer 8

the number of individuals in the next generation with that trait

Question 9

How do we measure the fitness of an allele

Answer 9

the number of copies of that allele that successfully make it into the next generation

Question 10

Does it matter where (ie. from what ancestor) a particular instance of the trait or allele originated?

Answer 10

no

fitness of A1 allele in a population is the total number of all A1 alleles in the population

Question 11

What are the prerequisites of evolution by natural selection?

Answer 11

• trait varies
• trait is heritable
• fitness varies
• fitness is associated with the trait: there is a particular relationship, that depends upon environment, between fitness and phenotype (trait)

OR

• trait must show heritable variation
• trait must be associated with differential reproductive success

Question 12

What does artificial selection do?

Answer 12

• change the frequency of alleles to favour those traits,
• works differently than in natural selection

ie. pure-bred dogs will often have diseases (from recessive alleles) that persist because dog breeders prefer to maintain a “pure” bloodline/pedigree of dogs

Question 13

Example:

Aliens come to earth and abduct everyone who has short hair. How does this affect hair length on planet earth?

Answer 13

all prerequisites are met EXCEPT ‘trait is not heritable’
- there is probably a little bit of heritable variation in hair length, but most of the time when people have short hair it’s because they cut it

expected result: removing individuals with short hair probably won’t affect hair length in the next generation, because individuals with long hair are probably just as likely to have children with short hair as they are to have children with long hair

Question 14

Example:

Aliens come to earth and abduct everyone who has red hair. How does this affect hair colour on planet earth?

Answer 14

consensus: red hair is more likely to be heritable than short hair, but the fact that some people dye their hair or wear wigs dampens the heritability of hair colour

to the extent that hair colour is inherited, we could see evolution in hair colour in this scenario

• however, evolution may not happen as quickly as it would if hair colour were 100% heritable: some individuals who were born redheads will still survive and reproduce because they changed their hair colour, and some non-genetic red-heads will be removed because they changed their hair colour
• these discrepancies will add “noise” to the effect natural selection has on the population

Question 15

Example:

Aliens come to earth and abduct everyone who has short hair. How does this affect eye colour on planet earth?

Answer 15

we could make arguments both ways

hair colour and eye colour are two separate traits: some people with dark eyes have dark hair, but other people have dark eyes and light hair

selection is acting directly on hair colour, not eye colour, so we shouldn’t necessarily expect to see evolution in eye colour as a result

but, we could also argue that directly selecting for hair colour might result in some indirect selection on eye colour

• in some cases, the same alleles that cause someone’s hair to be red can also cause their skin and eyes to be light
• whether or not selection on hair colour results in evolution of eye colour would depend on what sort of genetic variation was present in the population

Question 16

What are the prerequisites of natural selection vs. genetic drift?

Answer 16

natural selection:

• trait varies
• trait is heritable
• fitness varies
• fitness is associated with the trait

genetic drift:

• trait varies
• trait is heritable
• fitness varies
• fitness is unrelated to the trait

(last one is different)

Question 17

What is directional selection?

Answer 17

population before selection: shows variation in the value of a particular trait

after directional selection: population still shows variation in trait values, but the mean trait value is now different (increased or decreased)

Question 18

What happens in directional selection between each generation?

Answer 18

mutation (and also recombination and independent assortment) may introduce variation in trait values, resulting in individuals that differ from the original population

many of the mutant offspring will likely have low fitness, but some mutants may have high fitness

if directional selection continues in the same direction, the population mean could continue to move in the same direction, thanks to mutation

*shouldn’t assume that directional selection will continue in the same direction

Question 19

Example: Directional Selection in Swordtails

Answer 19

in many swordtail species, males have long, sword-like tails that help them attract males, and females in these species tend to prefer males with longer tails over males with shorter tails

in a few species of swordtails, neither males nor females have sword-like tails

• surprisingly, when females in these species are given a choice between long-tailed and short-tailed males, they prefer the long-tailed ones
• even though males with sword-like tails don’t currently exist in these species, if one arose through mutation there’s a good chance he would have higher reproductive success than other males

Question 20

What is natural selections vs. sexual selection?

Answer 20

natural selection: non-random variation in reproductive success

• due to variation in survival and/or reproductive success
• specific phenotypes have higher fitness = are selected for

sexual selection: non-random variation in reproductive success

• due to variation in mating and/or fertilization success
• specific type of natural selection

Question 21

In stabilizing selection, which individuals have the lowest fitness?

Answer 21

individuals with extreme trait values (too big or too small)

Question 22

What happens in stabilizing selection?

Answer 22

after stabilizing selection, the population mean trait value has not changed, however variation in trait values has been reduced

Question 23

What can continue to introduce genetic variants that have low fitness during stabilizing selection?

Answer 23

mechanisms like mutation and gene flow

the population may never reach a point where all individuals are perfectly adapted to the environment

Question 24

How can something that starts as directional selection end up as stabilizing selection?

Answer 24

fitness trade-offs (such as those between attracting mates and avoiding predation) can limit the extent of directional selection

such trade-off can be thought of as natural selection “pushing on a trait from both sides”

the result is stabilizing selection

Question 25

Why might there be a limit to the “bigger is always better” rule?

Answer 25

fitness costs might be associated with being too big

ie. if a male swordtail’s tail is too long, he might have trouble swimming away from predators

Question 26

Which individuals have higher fitness during disruptive selection?

Answer 26

individuals with lower mean trait value

Question 27

What happens during disruptive selection

Answer 27

under strong selection, only individuals with very specific traits survive and reproduce (everyone else doesn’t)

Question 28

What happens if natural selection is weaker during disruptive selection?

Answer 28

it’s still the case that certain traits are more likely to survive and reproduce than others, but the relationship between fitness and phenotype is less pronounced

Question 29

All else equal, the stronger selection is…

Answer 29

the more likely it is to result in evolution (= changes in population allele frequencies between generations)

Question 30

How can natural selection act as a filter?

Answer 30

imagine the large particles passing through the filter are individuals/alleles, and natural selection is the filter

the same in which we could have multiple filters lined up, filtering out different types of particles, we can also have simultaneous selection for/against different types of traits

Question 31

How does natural selection act as a filter for a large population that is NOT experiencing genetic drift?

Answer 31

• a steady, streamlined flow of particles through the filter
• it ensures that most of the undesirable particles remain on this side, and most of the desirable particles make it to the other side

Question 32

How does natural selection act as a filter for a small population that is experiencing genetic drift?

Answer 32

natural selection is a bit like trying to filter turbulent water - random fluctuations in flow mean that some of the desirable particles might never make it through the filter

Question 33

How does selection often change allele frequencies?

Answer 33

by removing individuals/alleles

Question 34

Is selection specific, or not very specific?

Answer 34

can be either

Question 35

Why can natural selection only act on what is already there?

Answer 35

• just like a filter, natural selection has no control over which alleles/particles are present
• all natural selection can do is act on pre-existing genetic variation (which arises through random mutation)

Question 36

What does a fitness landscape assume?

Answer 36

• assumes that all phenotypes are possible

- assumes that the environment does not change

Question 37

In real life, why do not all phenotypes exist, and even might not be possible?

Answer 37

ie. pre-existing variation, developmental constraints, and/or other limitations

in that case, it would be impossible for certain traits to evolve, even if they would theoretically result in high fitness in the current environment

Question 38

Why does a fitness landscape assume that all phenotypes are possible?

Answer 38

if all combinations of traits are possible, then given enough time (and enough mutations), it should be possible for the population to evolve up to a fitness peak

Question 39

Why does a fitness landscape assume that the environment does not change?

Answer 39

if it changes, the fitness landscape may change

if the environment changes suddenly, then a population that was previously well-adapted to its environment may suddenly be very poorly adapted to its environment
- this could lead to evolution (but more likely death of many individuals)

Question 40

What type of population will be under stabilizing selection?

Answer 40

a population that is currently at fitness “peak” (ie. selection to stay the same = selection against change)

Question 41

Describe conditions under stabilizing selection?

Answer 41

• mutant will have lower fitness than wild type
• population is currently well-adapted to its environment
• any changes in phenotype due to mutation will be selected against

Question 42

Under stabilizing selection, what factors would limit a population from evolving towards a higher peak than a lower peak?

Answer 42

• note: whether the population “climbs up” (evolves) towards lower or higher peak depends on what sort of variation exists in the population
• if there is sufficient heritable variation, we should expect the population to evolve toward the higher peak
• however, if trait variation is biased toward smaller trait values (ie. those are the only ones that exist in the population), the population would instead evolve towards the lower peak

Question 43

Under stabilizing selection, what population is under selection to change?

Answer 43

a population that is currently in a fitness “valley”

Question 44

Under stabilizing selection, which traits are selected against?

Answer 44

changes in traits that result in lower fitness

• a population could get ‘stuck’ at local fitness optimum (a small hill), and never make it to the global fitness optimum (largest hill)

Question 45

Under stabilizing selection, how could complex traits evolve gradually?

Answer 45

each subsequent step results in higher fitness than the last (if the population is at the foot of an optimal “hill”)

Question 46

What are limiting factors?

Answer 46

• heritable phenotypic variation

- relationship between variation and fitness

Question 47

What are heritable variation in traits limited by?

Answer 47

limited by: 
- pre-existing variation 
(quantity of genetic variation, specific type of historic variation)
- developmental constraints 
- phenotypic correlations 
(two traits are correlated/links)