1. The Evolution of Populations
Chapter 21

2. Levels of Organization
Atoms - CHNOPS
Molecules – Carbs, Proteins, Lipids, Nucleic Acids
Organelles – Nucleus, Ribosomes, ER, Golgi, vesicles etc
Cells – smallest level for life
Tissues
Organs
Organ Systems
Organism - Individual
Population – smallest unit of evolution
Community
Ecosystem
Biosphere

3. Population
Group of organisms that can interbreed to produce fertile offspring
Gene pool – total alleles in a population

4. Sources of Genetic Variation
Mutations
Sexual Reproduction
Crossing over
Independent assortment
Random fusion of gametes
Diploidy – recessive allele does not show
..\..\AP Bio 15-16\Evolution\MechanismsofEvolution.mpg

5. Sources of Genetic Variation
Outbreeding – mating with unrelated partners
Balanced polymorphism
Heterozygote advantage – sickle cell carriers
Hybrid vigor – plant species
Frequency – dependent selection – least common phenotypes have an advantage

6. Hardy-Weinberg Equilibrium
Evolution does NOT occur if the gene pool remains constant from one generation to the next.
Outside forces must act on a population for there to be change

7. NO Evolution = genetic equilibrium Evolution = no genetic equilibrium
Conditions
NO Evolution = genetic equilibrium
Evolution = no genetic equilibrium
No Mutations
Extremely Large population size – no genetic drift
No gene flow – no migration
Random Mating
No Natural selection
Mutations
Small populations – genetic drift
Gene flow – migration
Non Random mating
Natural Selection

8. Equation - Hardy-Weinberg Equilibrium
p2 + 2pq + q2 = 1
p + q = 1
p = dominant allele
q = recessive allele
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive

9. Example Problem #1
In a population the frequency of the recessive allele is 0.4.
What is the frequency of the dominant allele?
What frequency of the population will be homozygous dominant, heterozygous, and homozygous recessive?
What frequency of the population will demonstrate the dominant phenotype?

10. Example problem #2
In a population 25% of the individuals demonstrate the recessive phenotype.
What is the frequency of the recessive allele?
What is the frequency of the dominant allele?
What frequency will be homozygous dominant in the population?
What frequency will demonstrate the dominant phenotype?

11. Violations to H-W Equilibrium – cause evolution
Mutations
Small populations – genetic drift
Non random mating
Natural Selection
Migration – gene flow
23_08EvolutionaryChanges_A.swf

12. 1. Mutations Change in DNA’s nucleotide sequence.
Raw source for new genes and alleles
Most mutations are somatic cell mutations and do not affect offspring
Only gametic mutations affect a gene pool.
Mutation rates
Lower in organisms with a longer generation span
Plants and animals – 1/ genes
Higher in organisms with a shorter generation span
Bacteria and viruses

13. 1. Mutations Point Mutations – alter one nucleotide base only
Usually neutral
Sickle cell anemia
Chromosomal Mutations – alter many regions or loci of the entire chromosome
Gene duplication
Usually harmful, but when beneficial act as an important source of variation in a population

14. 2. Nonrandom Mating – sexual selection
Creates sexual recombination – joining of different alleles in a gene pool
Huge source of variation in a population
Gametes from different organisms contribute different alleles to the next generation.

15. 3. Natural Selection
Differential success in reproduction based on variation in a population.
Better suited organisms in an environment tend to produce more offspring than less suited organisms.
Fitness
Types
Directional
Disruptive
stabilizing
Sexual
Artificial

16. 4. Genetic Drift
Random fluctuation of allele frequencies from one generation to the next.
Greater occurrence in smaller populations b/c one organism who is homozygous recessive breeding more than expected can create a large increase in the frequency of the recessive allele, if the pop is small
Figure 23.7
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

17. 4. Genetic Drift Bottleneck effect
Sudden environmental change can drastically reduce the size of a population – only some survive
Fire, flood, human influence etc.
Reduces the genetic variation
in a population
Ex: An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

18. 4. Genetic Drift Founder effect
Occurs when a few individuals become isolated from the original population
The smaller population may not have the same gene pool as the original and speciation is likely

19. 5. Migration – Gene Flow
Addition or loss of alleles to or from a gene pool.
Caused by the movement of fertile individuals to or from a population – migration, a deer goes to live with another population
Scientists are studying the effects of cultivated crop’s (artificially selected crops) gene flow on wild populations

20. Genetic Variation
Differences in phenotypes between members of a population
Inherited in genotype
The raw source for natural selection within a population
Relative Fitness:
The # of offspring
An individual passes
To the next generation

21. Types of Selection
1. Disruptive – extreme phenotypes are favored and the average is selected against
2. Directional – One extreme phenotype only is favored
3. Stabilizing – Average individuals are selected for and extremes are selected against.

22. Types of Selection 4. Sexual Selection – mating selection
Intrasexual – within the same sex
Ex: males competing
Intersexual – between the sexes
Ex: Females choosing the males
5. Artificial Selection - Modification of species due to selective breeding to produce organisms with desired traits
Ex: pets

23. Question at end of notes
Copy figure 14.8 and explain how the concept of probability affects both gamete assortment and genetic drift. (goes with genetic drift video)