The Major Forces of Evolution

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