Download Notes: Microevolution Part 1 (Evolution of Populations)

AP Biology B Lecture 4: Microevolution Part 1 (Evolution of Populations)
Some Vocabulary
•Microevolution – Evolutionary change below the species level
–Example: change in a population’s allele frequencies over generations
•Macroevolution – Evolutionary change above the species level including origin of a new group of organisms or a
shift in the broad pattern of evolutionary change over a long period of time
–Examples: appearance of new organism features, mass extinction and recovery of diversity
Populations Evolve
•Natural selection acts on individuals
–differential survival
•“survival of the fittest”
–differential reproductive success
•who bears more offspring
•Populations evolve
–genetic makeup of population changes over time
–favorable traits increase fitness & become more common in
Individuals DON’T evolve…
-Individuals survive or don’t
-Individuals reproduce or don’t
Individuals are selected
Populations evolve
Fitness
•Survival & Reproductive Success
–individuals with one phenotype leave more surviving offspring
Variation & Natural Selection
•Variation is the raw material for natural selection
–there have to be differences within population
–some individuals must be more fit than others
Where Does Variation Come From?
•Mutation
–random changes to DNA
•errors in mitosis & meiosis
•environmental damage
•Sexual Reproduction
–mixing of alleles
•recombination of alleles
–new arrangements in every offspring; new combinations = new phenotypes
–spreads variation; offspring inherit traits from parent
5 Agents of Evolutionary Change
1. Mutation (it’s not always as bad as it sounds)
•Change from one allele to another
–Mutation changes the DNA sequence
–It may alter the resulting amino acid sequence which could effect the corresponding protein/enzyme
•Alters proportion of alleles in population
–Generally low mutation rates in a population
–Ultimate source of genetic variation making evolution possible
2. Gene Flow
•Movement of individuals & alleles in & out of populations
–seed & pollen distribution by wind & insect
–migration of animals
•sub-populations may have different allele frequencies
•causes genetic mixing across regions
•reduce differences between populations
Human Evolution Today
•Gene flow in human populations is increasing today
–transferring alleles between populations due to increased mobility
3. Non-Random Mating
•Sexual selection – some individuals have features which make them more attractive than others
4. Genetic Drift
•Effect of chance events
–founder effect: small group splinters off & starts a new colony
–bottleneck: some factor reduces population to small number & then population recovers & expands again
5. Natural Selection
•Differential survival & reproduction due to changing environmental conditions
•climate change; food source availability; predators, parasites, diseases; toxins
–combinations of alleles that provide “fitness” increase in the population
•adaptive evolutionary change
How Do Populations Change?
•The 5 agents of evolutionary change create opportunities for changes in allele frequency in a population
•Population Genetics is the study of the properties of genes within a population
•Using the Hardy Weinberg Rule a scientist can set up a reference equilibrium point to monitor genetic frequency
changes in a population.
Hardy Weinberg Equilibrium
•Under these conditions no evolution should take place & allele frequencies should remain constant in a population:
–No Mutation (no modifications to the gene pool)
–No Natural Selection (no differences in survival or reproductive success)
–Extremely Large Population (no chance for genetic drift)
–Random Mating (no one is more/less attractive)
–Isolated Population/No Gene Flow (no Immigration - entering or Emigration - exiting)
•A mathematical equation exists to track genotype and allele frequencies in a population
•The Hardy Weinberg Equilibrium Equation looks like this:
–p2 + 2pq + q2 = 1
p2 + 2pq + q2 = 1
p + q = 1
•p2 = frequency of homozygous dominant genotype
•This is the second part of the Hardy Weinberg equation
•p = frequency of dominant allele
–EX: 25% of population with “AA” genotype
•q2 = frequency of homozygous recessive genotype
–EX: 25% of population with “aa” genotype
•2pq = frequency of heterozygous genotype
–EX: 50% of population with “Aa” genotype
–EX: 50% have an “A” allele
•q = frequency of recessive allele
–EX: 50% have an “a” allele