Lecture 8 - Evolution Across Space Flashcards
What is a cline?
smooth change in a trait or allele frequency over space
What is Bergmann’s Rule?
body sizes of mammals and bird increases with distance from the equators (clines across large geographic areas)
What does it mean to have larger body sizes?
lower surface to volume ratio, less heat loss in cold climates - shows selection acting on body size across ecological gradient
What is the cline local adaptation with common bent grass?
- 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
When do clines evolve?
- when selection pressures change across space
- when there is gene flow btw populations
What is gene flow?
mixing of allele btw diff populations
How are gene flow and natural selection diff?
selection causes two pops to become either more similar or more different, but gene flow only makes them more similar
What are the 2 impt roles of gene flow in evolution?
- equalizes allele frequencies and erodes genetic differences btw populations
- introduces new alleles into a population from other populations where they already existed
What causes gene flow?
dispersal; mvmt of individuals or gametes
Does all migration occur in gene flow?
no; birds migrating back from tropics isn’t an example
What are two examples of gene flow?
wind blown pollen (mvmt of gametes) and ballooning spiders (which spink silk to use it carry them to distant areas)
What are key barriers to gene flow?
rivers and gene flow is “easiest” across diver head waters
What happens w a patchy environment and how is it quantified?
fraction of individuals arriving from another population each generation
- migration rate (m) quantifies it
What is migration rate?
how quickly gene flow erodes genetic differences btw pops
How do you calculate change in allele frequency because of migration?
delta p = m (pm - p)
pm: allele frequency in migrants
p: allele frequency in recipient population
How is migration measured if there are no distinct populations because of spatial continuity?
measure migration in variance and not migration rate
How is migration variance calculated?
(sigma m)^2
- sigma m: roughly equal to avg distance btw birthplace of parent and its offspring
What does variance measure?
measures the spread of a distribution around the mean (sigma squared)
What does a variance of zero mean?
all measurements are identical
What does a larger variance indicate?
more dispersion around mean
Can variance be negative?
no; as units are squared
What works against population differentiation or genetic divergence?
dispersal is a force that works against it
What measures genetic divergence?
Fixation index statistic (Fst)
What does Fst measure?
fraction of total genetic variance across two or more populations resulting from genetic differences btw them
What does Fst = 0 mean?
indicates that 2 populations are identicalW
What does Fst =1 mean?
that two populations are fixed for different alleles
What does Fst = 0.36 mean?
36% of all of the genetic variation in the two populations is caused by differences among them
What is the Fst formula for locus with two alleles?
Fst = = (Var(𝑝))/(𝑝̅(1− 𝑝̅ )
Var(p): variance of allele frequency among populations
𝑝̅: mean allele freq across all pops
How does Fst increase?
it increases as distance btw pairs of populations increases aka isolation by distance
- pops that are the farthest apart are the most divergent
What is Fst for human populations correlated with?
the distance btw them
How do gene flow and selection act in opposition to one another?
- local selection enhances genetic differences btw populations
- flow erodes genetic differences btw populations
Without selection or drift, what happens to gene flow?
gene flow would make allele frequencies uniform across space
What are clines and other spatial patterns compromises btw?
btw extremes of flow and selection (relative strengths of selection and migration)
What is gene swampping?
when migration onto an island is strong than selection
- freqs of local alleles evolves to 0
- gene flow overwhelms local adaptation
What can cline width be used to estimate?
the strength of selection
What does cline width equal?
𝑤𝑐 = 2.5√(𝜎𝑚2”/s” )
- 2.5 times sqrt of (migration variance/selection coefficient)
What happens when you have a higher migration variance?
less abrupt the change in allele freqs is across the cline
- lower variance (less migration) = more allele freqs abruptly change across cline
What happens to the cline when the strength of flow increases relative to selection?
cline becomes flatter, if selection is strong that we have steeper cline
What is a tension zone?
situation where selection maintains differences btw populations that are connected by gene flow but where fitness and thus selection varies across space
What clines form when there is a selection against heterozygotes?
tension zones - steep clines
Regardless of population size, what can happen for a neutral locus?
single migrant per generation prevents drift from causing much divergence
What kind of loci can be used to estimate Fst?
neutral loci
