Lecture 8 - Evolution Across Space

Question 1

What is a cline?

Answer 1

smooth change in a trait or allele frequency over space

Question 2

What is Bergmann’s Rule?

Answer 2

body sizes of mammals and bird increases with distance from the equators (clines across large geographic areas)

Question 3

What does it mean to have larger body sizes?

Answer 3

lower surface to volume ratio, less heat loss in cold climates - shows selection acting on body size across ecological gradient

Question 4

What is the cline local adaptation with common bent grass?

Answer 4

• common bent grass has locally adapted to high copper concentrations in the soil around a mine
• tolerance for copper declines over a short distance across 2 transects out into surrounding area
• on mine: tolerance is higher for adult plants grown from seeds and off the mine there is lower tolerance

Question 5

When do clines evolve?

Answer 5

• when selection pressures change across space
• when there is gene flow btw populations

Question 6

What is gene flow?

Answer 6

mixing of allele btw diff populations

Question 7

How are gene flow and natural selection diff?

Answer 7

selection causes two pops to become either more similar or more different, but gene flow only makes them more similar

Question 8

What are the 2 impt roles of gene flow in evolution?

Answer 8

• equalizes allele frequencies and erodes genetic differences btw populations
• introduces new alleles into a population from other populations where they already existed

Question 9

What causes gene flow?

Answer 9

dispersal; mvmt of individuals or gametes

Question 10

Does all migration occur in gene flow?

Answer 10

no; birds migrating back from tropics isn’t an example

Question 11

What are two examples of gene flow?

Answer 11

wind blown pollen (mvmt of gametes) and ballooning spiders (which spink silk to use it carry them to distant areas)

Question 12

What are key barriers to gene flow?

Answer 12

rivers and gene flow is “easiest” across diver head waters

Question 13

What happens w a patchy environment and how is it quantified?

Answer 13

fraction of individuals arriving from another population each generation
- migration rate (m) quantifies it

Question 14

What is migration rate?

Answer 14

how quickly gene flow erodes genetic differences btw pops

Question 15

How do you calculate change in allele frequency because of migration?

Answer 15

delta p = m (pm - p)
pm: allele frequency in migrants
p: allele frequency in recipient population

Question 16

How is migration measured if there are no distinct populations because of spatial continuity?

Answer 16

measure migration in variance and not migration rate

Question 17

How is migration variance calculated?

Answer 17

(sigma m)^2
- sigma m: roughly equal to avg distance btw birthplace of parent and its offspring

Question 18

What does variance measure?

Answer 18

measures the spread of a distribution around the mean (sigma squared)

Question 19

What does a variance of zero mean?

Answer 19

all measurements are identical

Question 20

What does a larger variance indicate?

Answer 20

more dispersion around mean

Question 21

Can variance be negative?

Answer 21

no; as units are squared

Question 22

What works against population differentiation or genetic divergence?

Answer 22

dispersal is a force that works against it

Question 23

What measures genetic divergence?

Answer 23

Fixation index statistic (Fst)

Question 24

What does Fst measure?

Answer 24

fraction of total genetic variance across two or more populations resulting from genetic differences btw them

Question 25

What does Fst = 0 mean?

Answer 25

indicates that 2 populations are identicalW

Question 26

What does Fst =1 mean?

Answer 26

that two populations are fixed for different alleles

Question 27

What does Fst = 0.36 mean?

Answer 27

36% of all of the genetic variation in the two populations is caused by differences among them

Question 28

What is the Fst formula for locus with two alleles?

Answer 28

Fst = = (Var(𝑝))/(𝑝̅(1− 𝑝̅ )

Var(p): variance of allele frequency among populations
𝑝̅: mean allele freq across all pops

Question 29

How does Fst increase?

Answer 29

it increases as distance btw pairs of populations increases aka isolation by distance
- pops that are the farthest apart are the most divergent

Question 30

What is Fst for human populations correlated with?

Answer 30

the distance btw them

Question 31

How do gene flow and selection act in opposition to one another?

Answer 31

• local selection enhances genetic differences btw populations
• flow erodes genetic differences btw populations

Question 32

Without selection or drift, what happens to gene flow?

Answer 32

gene flow would make allele frequencies uniform across space

Question 33

What are clines and other spatial patterns compromises btw?

Answer 33

btw extremes of flow and selection (relative strengths of selection and migration)

Question 34

What is gene swampping?

Answer 34

when migration onto an island is strong than selection
- freqs of local alleles evolves to 0
- gene flow overwhelms local adaptation

Question 35

What can cline width be used to estimate?

Answer 35

the strength of selection

Question 36

What does cline width equal?

Answer 36

𝑤𝑐 = 2.5√(𝜎𝑚2”/s” )

• 2.5 times sqrt of (migration variance/selection coefficient)

Question 37

What happens when you have a higher migration variance?

Answer 37

less abrupt the change in allele freqs is across the cline
- lower variance (less migration) = more allele freqs abruptly change across cline

Question 38

What happens to the cline when the strength of flow increases relative to selection?

Answer 38

cline becomes flatter, if selection is strong that we have steeper cline

Question 39

What is a tension zone?

Answer 39

situation where selection maintains differences btw populations that are connected by gene flow but where fitness and thus selection varies across space

Question 40

What clines form when there is a selection against heterozygotes?

Answer 40

tension zones - steep clines

Question 41

Regardless of population size, what can happen for a neutral locus?

Answer 41

single migrant per generation prevents drift from causing much divergence

Question 42

What kind of loci can be used to estimate Fst?

Answer 42

neutral loci