🦎 Natural Selection and Evolution

Evolution Defined

Evolution: Change in allele frequencies in a population over time

Population: Group of individuals of same species in same area that can interbreed

Darwin's Theory of Natural Selection

Key observations:

1. Overproduction: More offspring than can survive
2. Variation: Individuals differ in traits
3. Heredity: Traits passed to offspring
4. Competition: Struggle for resources

Result: Natural Selection

• Individuals with advantageous traits survive and reproduce more
• Favorable alleles increase in frequency
• "Survival of the fittest" (reproductive success)

Mechanisms of Evolution

1. Natural Selection

Types:

Directional selection:

• One extreme favored
• Mean shifts one direction
• Example: antibiotic resistance, peppered moths

Stabilizing selection:

• Intermediate favored
• Reduces variation
• Example: human birth weight

Disruptive selection:

• Both extremes favored
• Increases variation
• Example: beak sizes in African seedcrackers

Sexual selection:

• Traits increase mating success
• May reduce survival (e.g., peacock tail)
• Examples: bright colors, large antlers, mating displays

2. Genetic Drift

Random changes in allele frequencies

• More effect in small populations
• Not related to fitness

Bottleneck effect:

• Population drastically reduced
• Survivors' alleles determine future
• Reduces genetic diversity
• Example: Northern elephant seals

Founder effect:

• Small group colonizes new area
• Limited genetic variation
• Example: Amish populations

3. Gene Flow (Migration)

• Movement of alleles between populations
• Increases genetic variation
• Can introduce new alleles
• Reduces differences between populations

4. Mutation

• Ultimate source of new alleles
• Random changes in DNA
• Provides raw material for evolution
• Usually neutral or harmful, rarely beneficial

Conditions for Hardy-Weinberg Equilibrium

Hardy-Weinberg: Population NOT evolving (allele frequencies constant)

Five conditions:

1. No mutations
2. Random mating
3. No gene flow (migration)
4. Large population (no genetic drift)
5. No natural selection

If conditions met: p² + 2pq + q² = 1 and p + q = 1

• p = frequency of dominant allele
• q = frequency of recessive allele
• p² = homozygous dominant
• 2pq = heterozygous
• q² = homozygous recessive

Real populations: Always evolving (conditions rarely met)

Evidence for Evolution

1. Fossil Record

• Shows change over time
• Transitional forms
• Age determined by radiometric dating

2. Comparative Anatomy

Homologous structures:

• Same structure, different function
• Common ancestry
• Example: vertebrate forelimbs

Vestigial structures:

• Reduced, no longer functional
• Evidence of evolutionary past
• Example: human appendix, whale pelvis

Analogous structures:

• Different structure, same function
• Convergent evolution (not common ancestry)
• Example: bird and insect wings

3. Molecular Biology

• DNA and protein similarities
• More similar = more recent common ancestor
• Universal genetic code
• Cytochrome c comparisons

4. Biogeography

• Geographic distribution of species
• Islands have unique species
• Related to continental species
• Example: Darwin's finches (Galápagos)

5. Direct Observation

• Bacterial resistance
• Pesticide resistance in insects
• Changes in populations over time

Speciation

Speciation: Formation of new species

Species: Group that can interbreed and produce fertile offspring

Reproductive isolation:

• Prezygotic barriers (before fertilization)
• Postzygotic barriers (after fertilization)

Allopatric speciation:

• Geographic separation
• Most common type

Sympatric speciation:

• No geographic separation
• Polyploidy in plants

Key Concepts

1. Evolution = change in allele frequencies over time
2. Natural selection favors advantageous traits
3. Three types: directional, stabilizing, disruptive
4. Genetic drift = random changes (bottleneck, founder effect)
5. Hardy-Weinberg describes non-evolving population
6. Evidence: fossils, anatomy, molecular, biogeography, observation
7. Speciation creates new species through reproductive isolation