Interacting Forces

Question 1

Which evolutionary forces are deterministic? Which are stochastic? What’s the difference?

Answer 1

• Drift- stochastic, meaning strong generalities but no certainty in individual outcomes
• Selection- deterministic, predictable and linear

Question 2

How do the basic pairwise interactions among forces play out? Thinking specifically about adaptive topographies, how do the forces other than selection act to move populations on these theoretical surfaces? How does the shape of the adaptive topography affect the balance of evolutionary forces?

Answer 2

• Drift works against selection pressures, and will move a population off of its fitness optima. It will have a greater effect on topographies with a lower slope
• Drift/recessive alleles- can remove recessive alleles before they can become useful or it can lead to them occuring at a high enough frequency that they take hold

Question 3

How does the relationship between drift and selection effect the outcome of new mutations that effect fitness?

Answer 3

Drift can lead to the removal of beneficial recessive alleles or can cause them to occur at a high enough frequency that they take hold and increase fitness

Question 4

What is migration load? How does it work? How can migration enhance adaptive evolution?

Answer 4

• Migration load- a lower average population fitness than predicted given selection regime due to migrants from populations from different selection regimes
• Well adapted populations send out a large number of migrants which then interfere with selection in the new population and hurt the fitness of the new population
• An influx of new genetic variation can enhance adaptation as there is a larger pool on which selection pressures can act

Question 5

What is Wright’s Shifting Balance Theory? What kind of adaptive evolution does it help explain? What is the progression of “shifts” in this model?

Answer 5

• Wright’s Shifting Balance Theory involves shifts in the predominance of the evolutionary forces acting on a species
• Useful in species where adaptation is comples, where more than one fitness peak may be seen (ie. rugged topographies) as well as subdivided populations
• Drift to selection, selection to migration, migration to selection

Question 6

How does epistasis relate to all of this? Can you interpret a simple three dimensional adaptive topography in way that shows you understand what it means?

Answer 6

Epistasis causes adaptive topographies to be rugged