Unit 6 Concept 1 - Natural Selection

evolution

• The process of biological change in populations over time that makes descendants genetically different from their ancestors.

microevolution

Evolution can occur on a small scale affecting a single population

macroevolution

Evolution on a large scale affecting changes in species across populations

natural selection

• organisms with the “best” traits (adaptations) will live longer and reproduce more than others, causing changes in the population over time by acting on traits that are heritable.

Name 4 principles of natural selection

• overproduction of offspring

• variation

• adaptation

• descent with modification

overproduction of offspring

• Lots of offspring and limited resources causes competition for those resources.

variation

• differences in the physical traits of organisms

sources of variation

• Random mutations = ultimate source

• Genetic recombination during meiosis (crossing over)

• Migration (gene flow)

adaptation

• a feature that allows an organism to better survive in its environment.

descent with modification

• A change in gene frequency over time.

• Beneficial traits should become more common over time.

directional selection

increases the expression of an extreme version of a trait in a population.

disruptive selection

 a process that splits a population into two groups; removes individuals with average traits and favors the 2 extremes.

stabilizing selection

eliminates extreme expressions of a trait when the average expression leads to higher fitness.

allele frequency

• # of times the allele appears in a population (how common it is).

gene pool

• the combined alleles of all individuals in a population.

what does p represent

Frequency of the dominant allele

what does q represent

 Frequency of the recessive allele

equation for population with only 2 versions of a gene (dominant and recessive)

p + q = 1

mechanisms of microevolution

1. Mutations

2. Natural Selection (the MAIN mechanism!!!)

3. Genetic Drift

4. Gene Flow

5. Non-random Mating (Sexual Selection)

mutation

• any change in a DNA sequence.

  • Creates new genotypes and thus new phenotypes.

genetic drift

• random change in the frequency of alleles in a population over time.

  • Rare alleles in a pop. will decrease in frequency, while others increase.

  • Often results in a loss of genetic variation.

Changes may be more apparent in smaller populations.

gene flow

•  movement of genes into/out of a population. 

  • Occurs during migration.

  • Results in an increase in genetic variation in the population.

non random mating

• The selection of traits that aren’t necessarily good for survival fitness, but without them, you can’t pass on your genes at all because you can’t reproduce.

Genetic / Hardy Weinberg Equilibrium

• a state in which there are NO changes in the allele frequencies in a population over time.

Evolution will NOT occur if:

1. Population is very large. 

2. Must be random mating.

3. No migration.

4. No mutations.

5. No natural selection.

HWE equation

p² + 2pq + q² = 1

p²

genotypic frequency of homozygous dominant individuals

2pq

• genotypic frequency of heterozygous individuals

q²

• genotypic frequency of homozygous recessive individuals