lec20: population structure, gene flow, and genetic drift

Question 1

population

Answer 1

a group of individuals of the same species occupying a given area at the same time

Question 2

migration

Answer 2

the movement of individuals from one population to another

Question 3

gene flow

Answer 3

any mechanism that allows genes to flow from one population to another

Question 4

What are Key questions for evolutionary studies of variation within populations? (2)

Answer 4

How many of these variations are genetic in origin?

Does variation contribute to fitness differences among organisms?

Question 5

What are Key questions for geographical differentiation? (2)

Answer 5

What proportion of genetic variation in a species is found within a population vs between populations?
How is diversity distributed within and between populations?
Are the differences between populations heritable and adaptable?

Question 6

considering the “effects on population divergence” what is the job of gene flow

Answer 6

acts to homogenize to make populations more similar each other

Question 7

how does selection and drift affect populations?

Answer 7

selection:
- natural selection will act the populations to drive apart
- if have these two populations in the same environment, natural selection might keep them more similar; but if have different environments that these populations occur in, natural selection drive them to be more differentiated

drift:
- causes the populations to be different

Question 8

what are ways to measure gene flow? (2)

Answer 8

Experiments

most direct way: use neutral genetic markers (seeing if variant is moving between populations)
- Polymorphic genetic variants that are not direct targets of selection

Question 9

If two populations have different genes, one fully F and only fully S, and we want to find out how much gene flow occurs between them, what do we do?

Answer 9

score FS heterozygotes in offspring
- frequency of heterozygotes = an estimate of gene flow

Question 10

do most gene flow occur over a long or short distance?

Answer 10

short but a small amount occurs as far as 1km. so further distance away = less gene flow, but there’s evidence that these genetic markers are getting into these populations

Question 11

If there is a field of crop and weed sunflowers, what happens to gene flow the further these are from each other?

Answer 11

Less gene flow

Question 12

genetic drift

Answer 12

Random fluctuations in alleles

other words: the change in frequency of an existing gene variant in the population due to random chance

it’s like rolling the dice

Question 13

Does genetic drift lead to particular direction of evolutionary change?

Answer 13

no

Question 14

stochastic

Answer 14

unpredictable or random

Question 15

What are stochastic evolutionary forces? MRG

Answer 15

mutation, recombination, genetic drift

Question 16

deterministic

Answer 16

predictable or non-random

Question 17

What is a deterministic evolutionary force?

Answer 17

natural selection

Question 18

stochastic vs deterministic in terms of dice

Answer 18

drift is like rolling the dice (a unloaded dice) and with natural selection is like a loaded dice (like an unfair dice)

Question 19

genetic drift is “by chance” or due to “genetic differences”

Answer 19

Unlike natural selection, genetic drift does not depend on an allele’s beneficial or harmful effects. Instead, drift changes (stochastic changes) in allele frequencies purely by chance (to random variation) in fecundity and mortality and can lead to these random losses of alleles leading to loss of diversity

Question 20

What are Stochastic changes in allele frequency due to (don’t say the evolutionary forces) ?

Answer 20

random variation in fecundity and mortality

Question 21

What type of population size do stochastic changes affect?

Answer 21

small population

Question 22

what are the 3 kinds of stochastic processes that play a role in loss of diversity?

Answer 22

1. genetic drift
2. population bottlenecks
3. founder events

Question 23

population bottlenecks

Answer 23

an event that drastically reduces (a single sharp reduction) abundance (the size of a population) causing a loss of diversity

Question 24

founder events

Answer 24

occurs when a new population is established from a small number of individuals drawn from a large ancestral population

other words: colonization by a few individuals that start a new population leading to loss of genetic diversity because colonizing group contains only limited diversity compared to the source population

Question 25

Which type of population size is genetic drift more common in?

Answer 25

Genetic drift more prominent in small populations

eg. starting at 50% of allele frequency; 9 individuals, 18 gene copies vs 50 individuals, 100 gene copies

Question 26

What are the effects of genetic drift in small populations? (4) FDAC

Answer 26

More drastic fluctuations each generation

Rapid loss of genetic diversity

Faster allele fixation or loss

Less consistency across replicate populations

Question 27

draw a diagram of genetic variation + founder events + continued population bottleneck. what does it represent?

Answer 27

over time, if the population expands it’s going to start accumulating new genetic variation but over a long time you get a loss of genetic diversity and as you get these founder events, you get recurrent loss of this genetic variation

Question 28

how does genetic drift differ in populations/countries that are closer or further apart from each other?

Answer 28

populations that are closer to each other like CAN-US populations has more genetic variation THAT’S SHARED.
CAN ad SWE, we see a lot less of the genetic variation is shared, a lot more of it is unique and we start to see fixed differences between the populations

so populations that are further away from each other has experienced more genetic drift that’s differential between them, less gene flow, and so they’re more differentiated than populations that are closer to each other

Question 29

between countries, we can see fixed differences between the populations due to the distance. what about in human population (between humans) differentiation?

Answer 29

very super duper rare to find fixed differences between human populations. so we have a lot of allele sharing across populations and that’s because we’ve experienced relatively recent origins and reasonably high levels of gene flow around the globe.
certainly much greater allele sharing than the plant population

most genetic variation that you see is found both within and between populations, in other words, we share most of our alleles around the world

Question 30

explain why there’s human population differentiation from East Africa with increasing distance

Answer 30

population that colonize further away experience more genetic drift and potentially less historical gene flow from East Africa allowing us to see differences in allele frequencies across human populations even though allele sharing is extremely high

this is because of the Neanderthals that originated from East Africa and their relation to modern humans.

Question 31

draw a diagram of the gene flow between humans and neanderthals + what does it illustrate

Answer 31

there’s higher levels of neanderthal ancestry in Eurasia than there is in Africa

human populations out of Africa is consistent with ancient interbreeding averaging about 2% of the genome that have short stretches of neanderthal-derived DNA

Question 32

founder events/bottlenecks and lower/higher gene flow with increasing distance from East Africa.

Answer 32

founder events and lower gene flow with increasing distance

Question 33

genetic differentiation among populations is often observed across a _______ range + explain

Answer 33

geographic range

meaning phenotype across the geographic range differs (populations differ in what they look like across the geographical range)
- eg. the rat snakes on the first slide

Question 34

phenotypic differentiation may be ____ or due to ____ or _____

Answer 34

adaptive (“local adaptation”): local adaptation to different environmental conditions driving these differences in phenotype between populations)
OR due to genetic drift (random chance)
OR phenotypic plasticity

Question 35

phenotypic plasticity

Answer 35

ability of a genotype to modify its phenotype in response to environmental changes

Question 36

in a phenotypic plasticity, does that mean that our genotypes are changing, changing our phenotype? if not then how does it occur?

Answer 36

not changes in our genotype that are happening but changes in the expression of genes in epigenetic regulation depending on the environment we’re exposed to

occurs through modifications to growth and development and behaviour
- phenotypic plasticity is under genetic control
(there are genes built in that allows us to respond to the environment but the key thing to remember is that it’s not genetic differences that are causing the difference in phenotype, it’s under genetic control, but those genes are allowing the phenotype to differ even though the genotype has stayed the same )

Question 37

what factors make a phenotype?

Answer 37

genomic and environmental factors

Question 38

do genotypes grow the same in every environment? + ex

Answer 38

no

Question 39

can 2 species with exact same genotype, no difference in the genetic composite, look very differently due to the different environment each are in

Answer 39

yes such as the aerial leaves (terrestrial phenotypes) and submersed leaves (aquatic phenotypes) which are both arrowheads species

Question 40

what organism is phenotypic plasticity most common in? (2)

Answer 40

common in sedentary sedentary organisms (eg. plants and corals)
- eg. seed may germinate in very different kinds of environment so there can be a lot of plasticity in sedentary organisms

and in animal behaviour
- eg. if we’re hot we tend to have plasticity around that we might sweat more or go to the shade as a behavioural response

Question 41

do all phenotypic plasticity result from adaptation

Answer 41

no. phenotypic plasticity often is an adaptation to unpredictable environment BUT not all results from adaptation.

extra:
can be advantageous to have a different phenotype depending on the environment BUT when we see differences between organisms and how they look in different environments, the reason why they look different is not because of local adaptation when talking about plasticity. Also important to remember that not all phenotypic plasticity is adaptive, some might be maladaptive specially if organisms are experiencing environments that they didn’t evolve in or unexpected to evolve in. so some plasticity but not all evolved and been adaptive

Question 42

what is done in reciprocal transplant studies + what is done using phenotype? + what does this tool help us distinguish + how in terms of, what does it enable?

Answer 42

Growing equivalent genotypes in different environments and comparing their relative performance

this tool helps us distinguish the role of phenotypic plasticity vs the role of local adaptation

1. can separate phenotypic variation into genetic and environmental components
2. this enables measurement of selection against non-local genotypes

Question 43

Reciprocal transplant studies provide evidence of?

Answer 43

local adaptation

Question 44

what components does Reciprocal Transplant Studies separate phenotypic variation into?

Answer 44

genetic and environmental components

Question 45

what were the conclusions in the Clausen-Keck-Hiesey Transplant?

Answer 45

Differences between populations are due to BOTH plasticity and genetics

Evidence from widespread local adaptation because local populations had the highest fitness

Question 46

what does high UV radiation select for in terms of skin pigmentation? what does it interfere with?

Answer 46

May select for increased pigmentation

Interferes with folate (the natural form of vitamin B9, water-soluble and naturally found in many foods)

Question 47

what does low UV radiation select for in terms of skin pigmentation? what does it reduce the making of?

Answer 47

Reduced making of vitamin D

Increase selection for reduced pigmentation

Question 48

is there a single best phenotype across the globe? why

Answer 48

No single best phenotype across the globe due to tradeoffs

Question 49

what was the history of Local adaptation on skin pigmentation?(4) NADl

Answer 49

Numerous genes are known to affect skin pigmentation.

Alleles of these genes show rapid allele frequency change over time using ancient genomes (indicates natural selection).

Alleles of these genes show higher between population differentiation than most other genes.

Evidence supporting a history of local adaptation