Natural Selection I

Question 1

What are the three conditions for evolution by natural selection?

Answer 1

1. Individuals within populations vary in their phenotypic traits
2. At least some of the variation in traits has a genetic basis and is therefore heritable
3. Variation in these traits influences reproductive success (fitness), in terms of survival and/or the production of offspring

Question 2

What does this equation represent? Define all variables.

Answer 2

Equation for selection differential (S)
x* = mean trait value for the individuals that survive to reproduce after selection event
x = mean trait value of entire population before selection event

Question 3

What is measured by the selection differential?

Answer 3

Strength of selection

Question 4

How does the selection gradient differ from the selection differential?

Answer 4

Gradient offers a more general and flexible way of measuring selection because any measure of fitness can be used (survival, babies, etc.)
* Also used to measure selection on many traits simultaneously

Question 5

Outline the process for measuring the selection gradient.

Answer 5

1. Calculate absolute fitness (Wi) for each individual:
   Survival: 0 = dead/non-breeder
   1 = lived/breeder
   Reproduction: # babies (seeds, eggs, live young, etc.)
2. Calculate relative fitness (wi) for each individual
3. Regress wi vs. the trait value (xi) of each individual. The slope of the best fit line is the strength of directional selection (β)

Question 6

What is this equation for? Define all variables.

Answer 6

Formula for relative fitness for an individual (wi)
Wi = absolute fitness for each individual
W = mean absolute fitness for population