1. The Major Forces of Evolution
Chapter 6

2. Reminders… Exam 1 next Week Study Guide is up
Zoo day set…......Nov. 5th

3. Evolution
Evolution is a change in gene frequency in a population over time
What causes this change?
Natural selection
Migration (gene flow)
Genetic drift
Mutation

4. Evolution
The first three result in changes in frequency of genes in a population by “redistributing” the existing alleles
Mutation is the only one to introduce new information.
Another way to add new info is through nonrandom mating

5. Natural Selection
Darwin and Alfred Wallace independently formulated the theory of evolution
They discussed the struggle for existence, extinction of species, adaptation and variation
He looked at the work of Malthus, who said that resources are limited but population growth is not
Darwin realized that selection acts upon an individual
This led him to formulate the principle of natural selection

6. Natural Selection Here are the steps:
More individuals in a population are produced that can survive
There is variation and some individuals are better adapted to their environment
Individuals compete for limited resources
The better adapted ones will survive and reproduce, passing down their genes
This results in gene frequencies changing over time
If populations become isolated, this may result in enough genetic differences to create new species

7. Natural Selection For natural selection to occur, traits must be:
Variable: variation of genes is crucial for selection
Heritable: traits must be inherited through genes passed by parents

8. Natural Selection
Remember, natural selection acts on individuals, not species
Each individual has differential reproductive success and this results in a change in gene frequencies as well
The fitness (reproductive success) of any variation will change as the environment changes.
A result of natural selection is adaptation

9. Mutation
A heritable change in the genetic material (DNA) is a mutation
Natural selection, together with mutation, can change gene frequency more rapidly
Mutations are random

10. Migration (Gene Flow)
Migration occurs constantly among populations within a species
Also need reproduction
If we talk about a population being made up of genes rather than individuals, we are referring to a gene pool
This is all the genes in the population
As individuals of each species move among populations, gene flow causes frequencies to change

11. Genetic Drift
One generation’s frequencies may not accurately represent the frequency of it’s previous generation’s genes
Two causes are founder effect and population bottleneck
Genetic drift is similar to sampling error because the smaller the sample, the less likely it will represent the population at large

12. Genetic Drift
A small sub-population that separates itself from the rest of the population to “found” a new population will not represent accurately the frequency of the parent population
this is founder effect
If there is a drastic reduction in the number of individuals in a population
This is population bottleneck

13. Genetic Drift This can result in
A higher proportion of recessive genes
A greater chance of 2 recessive alleles coming together
More recessively expressed traits
Loss of genetic diversity

14. Hardy-Weinberg Used to predict allele frequencies
Uses a null hypothesis…tries to say evolution is not happening, which shows that it does
Homozygous recessive is aa or rr
Homozygous dominant is AA or RR
Heterozygous is Aa or Rr
Dominants are always expressed, even in heterozygous form

15. Hardy-Weinberg Equation: p=all dominant alleles
q=all recessive alleles
p+q=1, or 100% of population
p2+2pq+q2 = 1

16. Assignment We will practice Hardy-Weinberg Lab 6.1 Self Test 6.1
Maybe WS…..
Test Review