Natural Selection as the Mechanism for Evolution
3 Mechanisms for selection and an introduction to Hardy-Weinberg
Lyell’s Influence
In attempt to explain the past in terms of present day processes, Darwin went to local farmers and
animal breeders and observed…
 ___________________________________________________________________________
 After many generations, ______________________________________________________
Darwin called this process ARTIFICIAL SELECTION
2 examples of artificial selection
Natural Selection
1. Variation in Nature
 __________________________________________________________
2. Struggle for Existence (Malthus)
 __________________________________________________________
 __________________________________________________________
3. Survival of the Fittest
 __________________________________________________________
4. Reproduction of Viable Offspring
 __________________________________________________________
Understanding Histograms
 A histogram is a ______________________________________________
_____________________________________. It is an estimate of
the probability distribution of a continuous variable (quantitative variable)
Three mechanisms for selection
What does the x-axis on the
histogram show?__________
________________________
________________________
_
What does the y-axis on the
histogram show?__________
________________________
________________________
_ original populations
The original populations are shown below. Draw in the evolved population.
Stabilizing Selection
Directional Selection
Disruptive Selection
Stabilizing Selection
Examples?
What happens to the variation with a population when it encounters stabilizing selection?
Directional Selection
Examples?
What happens to the variation with a population when it encounters stabilizing selection?
Disruptive Selection
Examples?
What happens to the variation with a population when it encounters stabilizing selection?
Microevolution: A Population’s Gene Pool
 A gene pool is all the ____________________________________________
 Microevolution_________________________________________________
Hardy-Weinberg Equilibrium
 It is a condition in which ______________________________________
 The equation used to ________________________________________
Example 1: In one hypothetical Zebra Mussel (Dreissena polymorpha) population, most of the
individuals have dark, zebra-striped shells (below left). However, solid light-colored shells (below
right, caused by a homozygous recessive gene, aa) occur in 1 of every 10,000 individuals.
Example 2: The Coquina Clam (Donax variabilis) is highly polychromic: (Polymorphism expressed as
existing in several different colors. (adj. polychromic)) (with shells of many different colors.) In a
population of 2,000 clams, 1,920 are solid colored, whereas the remainder has radiating color bands.
Solid color occurs in homozygous dominant (BB) and heterozygotes (Bb); color banding only occurs
in homozygous recessive individuals (bb).
Problem: Calculate gene frequencies and numbers of BB and Bb.