Evolution
Change in allele frequency in a population over time.
Natural Selection
Individuals best suited (most fit) for their environment reproduce at higher rate.
Fitness
Reproductive success.
Adaptation
Trait that increases fitness.
Darwin’s 4 Postulates
- Preexisting Variation
- Increased fitness
- Heritable (passed onto offspring)
- More offspring are produced than can survive.
Typological Thinking
perfect, unchanged, unrelated
Population Thinking
Change, related not perfect (Lamark- mechanism wrong, Darwin)
3 Modes of Balancing Selection
- Heterozygous advantage (Aa- anemia example)
- Good to be rare (frequency dependent- predators won’t go after what they haven’t seen before)
- Time and place dependence (black mice in rocks vs. white mice in sand)
Can individuals adapt?
No, only populations can. Individuals can acclimate (change in phenotype over lifetime that aren’t heritable).
4 Modes of Natural Selection + Effects on Variability & Fitness
- Directional V- F+
- Stabilizing V- F+
- Balancing V= F+
- Disruptive V+ F+
5 Evolutionary Processes
- Natural Selection: V depends F+
- Genetic Drift: V- F-
- Gene Flow: V+- F+-
- Mutations: V+ F-
- Horizontal Gene Transfer:
Evolutionary Process: Natural Selection
Increases the frequency of alleles that contribute to reproductive success in a particular environment.
Gives rise to adaptations
Evolutionary Process: Genetic Drift
Causes allele frequencies to change randomly between small populations.
a. Founder’s effect- occurs when a group of individuals establishes a new population in a new area.
Decrease fitness, decrease variation
b. Population Bottle neck- A sudden decrease in population size in a large population.
V- F-
Evolutionary Process: Gene Flow
Occurs when individuals leave one population, join another, and breed.
Scenario 1- Some from population 2 move to population 2. There is more variation in population 1 than 2 now. Fitness can both increase (1) and decrease (2).
Scenario 2- Two populations intermingle/join together. Produces one variation so variation decreases fitness can increase or decrease depending on if they get an advantageous variation of not.
Evolutionary Process: Horizontal Gene Transfer
The transfer of genetic material between organisms that are not parent and offspring
Evolutionary Process: Mutation
Modifies allele frequencies by continually introducing new alleles.
Ultimate source of variation. The rate of mutations stays constant.
Beneficial: increase the fitness of an organism which should increase in frequency in a population due to natural selection
Neutral: do not affect an organism’s fitness. Occurs when a point mutation does not change the amino acid sequence
Deleterious: decrease the fitness of the organism. Tend to be eliminated by negative/purifying selection (Asexual- horizontal gene transfer. Sexual- 2Fold+ cost of sex)
Modes of Natural Selection : Directional
Changes the average phenotype in the population in one direction (tends to reduce the genetic diversity of populations).
Modes of Natural Selection: Stabilizing
Reduces genetic variation in a trait but does not change average value of a trait over time (favors intermediate genotypes).
Modes of Natural Selection: Disruptive
Intermediate phenotypes are selected against, and extreme phenotypes are favored (can cause speciation)
Modes of Natural Selection: Balancing
Occurs when no single allele has a distinct advantage – allelic variation is maintained.
