Modes of Selection

Evo Bio Exam 3

Negative Directional Selection

• large individuals = less fitness

• smaller guys stick around

• decrease in mean through the production of variance

• variance decreases

• fitness decreases with trait value

Variance

• describes the distribution or spread of observations/ individuals in population

• high variance = high spread

• low variance = low spread

Positive Directional Selection

• fitness increases with trait value

• mean increases

• variance decreases

• changes in mean happen initially through shifts in variance

Flower size in high elevation alpine skypilots is what type of selection?

• positive directional selection

• more flare of the flower = more fit

  • produce more pollen

Lizards body sisze in proportion to the egg survival is what type of selection?

• positive directional selection

• big males produce eggs that survive better than small males

Stabilizing selection

• individuals w/ intermediate trait values have the highest fitness

• extreme no good

• after selection, you lose ends w/ extremes

• reduction in variance with no change in mean

  • population is already at highest fitness so mean doesn’t want to change

• most common in nature

What can you say about the selection differential (S) for stabilizing selection?

• its very close to 0

Stabilizing selection example #1

• fly lays egg on weed —→ causes goldenrod gall growth

  • egg hatches out in larvae and lives in gall

  • provides protection from wasp

  • small gall does not protect well from wasp

• Big gall:

  • larvae runs to other side and is safe, protects itself

  • bigger galls better if you are a fly

  • positive directional selection

• Wood peckers:

  • notice bigger galls, peel open and eat larvae

• Result:

  • high variance at first

  • + and - directional selection combine to create a net stabilizing selection

  • middle point = compromising fitness

  • most common in nature

  • net stabilizing selection

• different selective processes can work at different points in the overall function

Why is stabilizing selection most common in nature?

• selection forces combine

• often already removed unfit variants, moving population to a mean close to that with the highest fitness

  • measuring selection in nature can be difficult

• new mutations happen every generation and often reduce fitness

• no phenotypic variation in the population, so selection cannot be measured

• might appear as though there is no relationship between trait and fitness

How can you measure selection when natural phenotypic variation is low?

• create phenotypic variation

Widowbird tail length example

• males have long tails + elaborate display where they fly slowly

• but everyone has long tails

• Experiment:

  • measured tails + divided birds into groups randomly

  • bird with long tail, chopped tail off and put on another bird so they would have super long tails

  • positive directional selection

  • females like to mate with males w/ long tails

Disruptive selection

• individuals with extreme trait values have the highest fitness

• mean value of the trait does not change

• variance increases