envs lecture 8 Flashcards
what are 4 main topics
gene flow, genetic divergence; gene flow, selection, and drift; evolution of dispersal and species ranges
what is one of the most common features of life on earth
species vary geographically across space
how do species vary
with a climate or host of other factors that shape this spatial variation we see in phenotypes
what does studying these patterns of variation allow for
us to understand interplay b/w nat selection, genetic drift, and gene flow
gene flow
movement of individuals
cline
smooth transition in a trait across geographical space
Bergmann’s rule
body sizes of mammals and birds increase w/ distance from the equator (clines across large geographic area)
where esle do we see clines of variation
in phenotypic features like body size
why does body size increase w/ distance from equator
SA to volume ratios
what ratios do large animals have
lower SA to volume ratios (high volume)
what ratios do smaller individuals have
high SA to volume ratios
compare large vs small individuals
large individuals have lower SA to V ratios, lose less body heat than smaller ppl
who loses more body heat small or large ppl
smaller
what is Bergmann’s rule an adaptation to
living in cold climates; selection acting on body size
what is selection acting on, and where
selection acts on body size, across this ecological gradient
when do clines evolve
(1) when selection pressures change across space AND (2) when there is gene flow b/w populations
what is gene flow
mixing of alleles b/w diff populations
what is 1st important role of gene flow in evolution
equalizes allele frequencies and erodes genetic differences b/w populations
what is 2nd important role of gene flow
introduces new alleles into a population from other populations where they already exist
what can natural selection do to 2 populations
can make them either more or less similar
what does gene flow do to 2 populations
only makes them more similar
what does gene flow introducing new alleles into a population play a similar role as
mutation in increasing variation in each population
what causes gene flow
dispersal
what is dispersal
movement of individuals or gametes
what does variation in environment do
affects cline shape and placement
what do rivers to to toucans
key barriers; affect ability to disperse
where is gene flow ‘easiest’
across river head waters
what is migration rate (m)
fraction of individuals arriving from another pop in each generation
what is m basically
how quickly gene flow erodes genetic differences b/w populations
what is diff in an allele’s frequency before and after migration
delta p = m(pm-p) [change in frequency of p equals migration rate * diff b/w allele p’s frequency in migrants - frequency in population]
how do we measure migration if no distinct populations b/c of spatial continuity
measure migration in migration variance
what is sqrt of migration variance equal
avg. distance b/w birthplace of parent and offspring
what does variance measure
(sigma squared), measures spread of distribution around mean
what does variance of zero mean
all measurements are identical
what does larger variance mean
more dispersion around mean
can variance be negative
nope (sigma squared)
what can migration lead to over many generations
can cause genes to diffuse across landscape, migration variance measures this spread (or diffusion)
what is dispersal
force that works against pop differentiation or genetic divergence
how do we measure genetic divergence
Fst (fixation index statistic)
what does Fst measure
fraction of total genetic variance across 2+ populations resulting from genetic differences b/w them
what does Fst = 0 mean
identical
what does Fst = 1 mean
fixed for diff alleles
who came up w/ fixation index
R.A. Fisher
what does Fst increase with
as distance b/w pairs of populations increases
what are the most divergent populations
those that are farthest apart
what is isolation by distanec
variance in Fst w/r to distance b/w populations (Fst increasing as distance b/w pairs of populations increases)
how do gene flow and selection act
in opposition to one another
what does local selection do to genetic differences
selection enhances differences b/w populations
what does gene flow do to genetic differences
gene flow erodes/erases genetic differences b/w populations
what would happen without selection/genetic drift
gene flow would make allele frequencies uniform across space
what are compromises b/w these 2 extremes
clines, other spatial patterns
what happens if selection is much greater than migration
A2 will be fixed
what happens if migration is stronger than selection
A2 will be lost
how can we estimate the selection coefficient
if you have independent measures of allele frequencies in 2 populations and migration rate b/w them
gene swamping
when immigration onto an island is stronger than selection, frequency of local alleles evolves to zero
basically what is swamping
when gene flow overwhelms local adaptation
how can we calculate cline width
when we have clines of variation b/w phenotypes
what can we use cline width to figure out
estimate strength of selection
what is cline width
constant * sqrt of migration variance/selection coefficient
what can cline width be used for
to estimate strength of selection
what does higher migration variance mean
less abrupt change in allele frequencies across cline
what does lower variance mean
more allele frequencies abruptly change across cline
when does cline get flatter
as strength of gene flow increases relative to selection
what happens if selection is relatively stronger
steeper cline
what are tension zones
steep clines
when do tension zones form
when there is selection against hterozygotes
basically what are tension zones
situation where selection maintains differences b/w populations that are connected by gene flow, but where fitness & selection varies across space
what can be used to estimate Fst
neutral loci
what are neutral loci
those that aren’t undergoing selection
what happens if Nm is less than 1
Fst is 1
what does it mean if Fst is 1
populations are differentiated
what happens if Nm is greater than 1
Fst is 0
what does it mean if Fst is 0
populations are identical
basically what does a single migrant per generation prevent
prevents drift from causing too much divergence regardless of pop size
what are 6 possibilities for species living along env. gradient responding to climaete change
OG population, (2) distribution shifts w/ climate change, (3) declining habitat size, (4) dispersal barriers, (5) limited dispersal abilities, (6) range expansion by adaptation
what are dispersal barriers
prevent some species from tracking the moving envelop of suitable habitat thus distribution reduces
what happens to species w/ limited dispersal abilities
may not be able to keep up w/ rapidly changing conditions, thus evolve reduced distributions
what is range expansion by adaptation
species adapts to new conditions, so range expands
of all 6 possibilities, which one includes evolutionary change
range expansion via adaptation
is adaptation to climate change likely or unlikely
unlikely
