1. Mechanisms of Evolution
The “Process” in Populations

2. Lamarck’s Theory: acquired characteristics
Evolution: change through time
All species come from pre-existing species with modifications
Over time there are differences in ancestral & descendant species

3. Mechanisms of Evolution
Natural selection
Changes in allele frequencies
Natural Gene Variation
Genetic variation is the “rule”
Polymorphism
Traits display variation
DNA sequence polymorphism
Sequencing DNA
Variations in coding & non-coding sequences

4. Hardy-Weinberg Principle
1908
G.H. Hardy & G. Weinberg
Original proportions of genotypes in a population will stay constant if…
Population is large
Random mating occurs
No mutations
No genes are input from other sources
No selection occurs

5. Hardy-Weinberg Equation
p - # of dominant alleles
q - # of recessive alleles
p2 - # of homozygous dominant individuals
2pq - # of heterozygous individuals
q2 - # of homozygous recessive individuals
p + q = 1
p2 + 2pq + q2 = 1

6. Examples

7. Within a population of butterflies, the color brown (B) is dominant over the color white (b). 60% of all the butterflies are brown. Calculate the H-W frequencies of BOTH alleles and the different genotypes.

8. In a large population of biology instructors 396 are red-sided and 557 are tan-sided. Assume red-sided is a totally recessive trait.
What are the allele frequencies of each allele.
The expected genotypic frequencies
The number of heterozygous individuals.
Conditions happen to be really good this year for breeding and next year there are 1245 instructors. If all H-W conditions are met, how many of these would be red-sided? Tan-sided?

9. H-W Continued…
After a few generations the population of biology instructors was re-assessed and 450 are red-sided and 505 are tan-sided. Assume red-sided is a totally recessive trait.
What is the NEW p & q?
Has evolution occurred? How do you know?

10. What does this mean?
Are the conditions that Hardy & Weinberg set up realistic?
So what is the relevance?
This equation allows us to measure evolutionary change.
If frequencies do not stay constant then there has been evolution

11. 5 Agents of Evolutionary Change
Mutation
Gene flow
Non-random mating
Genetic drift
Selection
** These follow Hardy-Weinberg criteria!!

12. Mutation So low  little effect Ultimate source of genetic variation
Not affected by natural selection
1-10 times per 100,000 cell divisions
Very slow process – environments change too rapidly

13. Gene Flow Movement of alleles from one population to another
Powerful agent
Movement of an organism to a new place
Drifting of gametes of plants or immature stages of animals
Mating of individuals in adjacent populations
Can counter natural selection

14. Nonrandom Mating
Certain genotypes mate with another genotype more frequently than random
Inbreeding - increases proportion of homozygotes
Self-fertilizing plants  homozygous
Outcrossing  leads to heterozygotes
Increases expression of recessive alleles

15. Genetic Drift Frequencies of alleles change by chance alone
Small populations
Bottleneck Effect
Flooding, drought, disease, war
Lose genetic variability
Founder Effect
Individuals disperse to new areas
Some alleles from original pop. may be lost

16. Selection Artificial – breeder selects Natural – nature selects
3 conditions
Variation must exist between individuals
Variation results in differences in # of offspring surviving
Variation must be genetic

17. What does Nature Select For?
Avoid predators
Match climate conditions
Pesticide resistance

18. Natural Selection & Evolution
Not the same thing!!
Natural selection – process
Evolution – historical record of change through time (outcome)
Process may lead to outcome
Not the only thing that can lead to outcome (evolution)

19. Heterozygote Advantage
Natural selection  removes alleles
Heterozygote advantage favors both alleles
Maintains both alleles in population
Sickle Cell Anemia – homozygous recessive
Heterozygotes  advantage against the affects of malaria parasite
Recessive allele is maintained in the heterozygous

20. Limits?? Genetic variation is required
Selection can move faster than variation
May have no genetic basis
Selection acts on phenotypes – do not eliminate recessive alleles

21. Critical Thinking
Apply Darwin’s idea of natural selection & the Theory of Evolution to explain the following situation…
Many species of lemurs have large eyes which allow them to survive as nocturnal creatures. Lemurs that are not nocturnal have smaller eyes. Use Darwin’s ideas of natural selection to explain how the nocturnal lemurs evolved to have large eyes.
Ring-Tailed Lemur
Nocturnal Lemur

22. Types of Selection Disruptive Directional Stabilizing Fitness
Eliminates intermediate
2 phenotypically distinct groups
Directional
Eliminates one extreme
Stabilizing
Eliminates both extemes
Favors intermediate
Fitness
# of surviving offspring left in the next generation
Attracting mates
# offspring prodeuced per mating

23. Summary – Darwin’s Ideas
What things/conditions are required of a population or in a population in order for natural selection to occur?