Lecture 8: Evolution Across Space Flashcards
What is gene flow in simple terms?
Mixing of alleles between different populations
What is a cline?
A smooth transition in a trait across geographical space
What is clinal variation?
gradual changes in a trait or characteristic of a species across a geographic area
Cline vs clinal variation
a cline is the concept of gradual change, while clinal variation is the actual observed variation in traits across different geographical locations
How does clinal variation relate to white clover plants?
Some plants have high frequency for an allele that causes deletion of the gene that produces cyanide
What is Bergmann’s rule?
Body sizes of mammals and birds tend to increase with distance from the equator
Why do animals in cold climates have less heat loss?
larger body size –> lower surface to volume ratio
Are moose larger in Northern or Southern Sweden?
Northern
What is the Wales mine and common bent grass situation an example of?
Local adaptation
Did adult common bent grass or common bent grass grown from seed have higher copper tolerance near the mine?
Adult
What happened to tolerance of copper as distance from the mine increased?
Decreased
When do clines evolve? (2 answers)
1) When selection pressures change across space
2) When there is gene flow between populations
What are the two important roles of gene flow in evolution?
1) Makes populations more similar
2) Introduces new alleles into a population from other populations
What causes gene flow?
Dispersal
What is dispersal?
Movement of individuals or gametes
What affects the ability of toucan subspecies to disperse?
Rivers
What is migration rate (m)?
the fraction of individuals arriving from another population each generation
Basically, m is a measure of …
how quickly gene flow erodes genetic differences between populations
What is the equation for the change in allele frequency before and after migration? What do the variables stand for?
∆p = m(pₘ – p)
∆p: change in allele freq b/c of migration
pₘ: allele frequency in migrants
p: allele frequency in recipient pop
m: migration rate
When is migration variance (σₘ^2) used?
When there are no distinct populations and thus the populations are spatially continuous
What does σₘ represent?
The average distance between birthplace of a parent and its offspring
What does a variance of zero mean?
Measurements are all identical
Larger variance means (more/less) dispersal
more
Can variance ever be negative?
No
What is Fₛₜ and what does it measure?
Fixation index statistic, measures the fraction of total genetic variance across two or more populations resulting from genetic differences between them
Fₛₜ = 0 means …
two populations are identical
Fₛₜ = 1 means …
two populations are fixed for different alleles
Fₛₜ = 0.36 means …
36% of all the genetic variation in the two populations is caused by the differences among them
As the distance between pairs of populations increases, Fₛₜ ___
increases
selection and gene flow act ___ of each other
opposite
What would happen to allele frequencies if there was no selection or drift?
Gene flow would make allele frequencies uniform
Gene swamping
When gene flow overwhelms local adaptation
What can width of cline be used to measure?
Strength of selection
What happens to the cline as the migration variance increases?
Changes in allele frequency are less abrupt, so the cline becomes flatter
If selection is stronger than gene flow, then the cline becomes ____
steeper
Clines that form when there is selection against heterozygotes are very ___. Why?
narrow. Heterozygotes have reduced fertility
