Introduction to Natural Selection
Evolution is the change in allele frequencies in a population over time. Natural selection is the primary mechanism driving adaptive evolution.
Individuals in a population differ in their traits
Many variations are passed from parents to offspring
More offspring are produced than can survive
Some variants survive and reproduce better than others
Biological fitness = an organism's ability to survive AND reproduce. It's measured by the number of viable, fertile offspring an individual produces relative to others in the population.
Note: Fitness is NOT about strength or speed — it's about reproductive success!
Natural Selection
Natural selection occurs when individuals with certain heritable traits are more likely to survive and reproduce than others.
Favors ONE extreme phenotype
Example: Giraffe necks getting longer over time
Favors INTERMEDIATE phenotype
Example: Human birth weight (too small or too large = problems)
Favors BOTH extremes
Example: Bird beaks — very large or very small seeds
Artificial Selection
Artificial selection (selective breeding) is when HUMANS choose which individuals reproduce based on desirable traits.
• Environment determines fitness
• Traits that aid survival/reproduction increase
• Slow process over many generations
• Humans determine which traits are "desirable"
• Traits may not improve survival
• Can produce rapid changes
| Organism | Wild Ancestor | Selected Traits |
|---|---|---|
| Dogs | Wolves | Size, temperament, coat, behavior |
| Corn | Teosinte (small seeds) | Large kernels, many rows |
| Chickens | Jungle fowl | Egg production, meat |
| Brassica crops | Wild mustard | Broccoli, cabbage, cauliflower, kale (all same species!) |
Darwin used artificial selection as evidence that selection CAN change populations. If humans can cause dramatic changes in just a few generations, imagine what nature can do over millions of years!
Population Genetics
Population genetics studies allele frequencies in populations and how they change over time.
| Term | Definition |
|---|---|
| Population | A group of individuals of the same species in the same area that can interbreed |
| Gene pool | All the alleles for all genes in a population |
| Allele frequency | How common an allele is in the population (expressed as decimal, e.g., 0.3) |
| Microevolution | Change in allele frequencies in a population over generations |
In a population of 100 individuals (200 alleles for a gene):
- 50 have genotype AA (100 A alleles)
- 30 have genotype Aa (30 A alleles + 30 a alleles)
- 20 have genotype aa (40 a alleles)
Frequency of A: (100 + 30) / 200 = 0.65
Frequency of a: (30 + 40) / 200 = 0.35
Note: Allele frequencies must add up to 1.0!
Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle describes a theoretical population that is NOT evolving. It provides a baseline to detect when evolution IS occurring.
p = frequency of dominant allele, q = frequency of recessive allele
p² = freq of AA, 2pq = freq of Aa, q² = freq of aa
A population is in H-W equilibrium (NOT evolving) only if ALL five conditions are met:
No new alleles created
No mate preferences
All phenotypes equal fitness
No genetic drift
No migration in/out
Question: In a population, 16% of individuals are homozygous recessive (aa). What percentage are carriers (Aa)?
Step 1: q² = 0.16, so q = √0.16 = 0.4
Step 2: p + q = 1, so p = 1 - 0.4 = 0.6
Step 3: 2pq = 2(0.6)(0.4) = 0.48 = 48% carriers
Evidence of Evolution
Multiple independent lines of evidence support the theory of evolution:
| Evidence Type | Description | Example |
|---|---|---|
| Fossil Record | Shows changes in organisms over time; transitional forms | Tiktaalik (fish→tetrapod) |
| Comparative Anatomy | Similar structures in different species suggest common ancestor | Homologous limb bones |
| Comparative Embryology | Similar embryonic development patterns | Pharyngeal pouches in vertebrate embryos |
| Molecular Biology | DNA/protein sequence similarities | Cytochrome c conservation across species |
| Biogeography | Geographic distribution of species | Darwin's finches on Galápagos |
| Direct Observation | Evolution observed in real time | Antibiotic resistance in bacteria |
SAME underlying structure, different function
Due to common ancestry
Ex: Human arm, whale flipper, bat wing
DIFFERENT structure, same function
Due to convergent evolution
Ex: Bird wing vs. insect wing
Common Ancestry
All living organisms share a common ancestor. Evidence for this universal common ancestry includes:
All organisms use DNA
Same codons = same amino acids
Used by all organisms
Universal energy currency
Comparing DNA or protein sequences reveals evolutionary relationships:
- More similar sequences = more closely related = more recent common ancestor
- Highly conserved genes (like those for ribosomes) are similar across ALL life
- Pseudogenes (non-functional gene copies) provide evidence of shared ancestry
- Endogenous retroviruses in same location in related species' genomes
Continuing Evolution
Evolution is ongoing — we can observe it happening in real time, especially in organisms with short generation times.
| Example | Description |
|---|---|
| Antibiotic resistance | Bacteria evolve resistance to antibiotics within years; MRSA, drug-resistant TB |
| Pesticide resistance | Insects evolve resistance to pesticides like DDT |
| HIV evolution | Virus evolves rapidly within a single patient, evading immune system |
| Darwin's finches | Beak sizes changed measurably during droughts (Peter & Rosemary Grant) |
| Peppered moths | Color frequencies changed with industrial pollution |
Some bacteria have resistance genes
Antibiotic kills susceptible bacteria
Resistant bacteria survive & multiply
Resistance becomes common
Phylogeny
A phylogeny is the evolutionary history of a group of organisms. Phylogenetic trees (cladograms) visually represent these relationships.
Branching point = common ancestor
Line representing lineage over time
End points = current species or groups
Base = most recent common ancestor of all groups shown
Speciation
Speciation is the formation of new species. A species is a group of organisms that can interbreed and produce fertile offspring.
Geographic isolation separates populations
• Physical barrier (mountain, river, ocean)
• Populations evolve independently
• Eventually can't interbreed
Ex: Darwin's finches on different islands
No geographic isolation — populations in same area
• Polyploidy (plants)
• Habitat differentiation
• Sexual selection
Ex: Apple maggot flies on different fruits
| Prezygotic Barriers (prevent mating/fertilization) | |
|---|---|
| Habitat isolation | Live in different habitats |
| Temporal isolation | Breed at different times |
| Behavioral isolation | Different courtship rituals |
| Mechanical isolation | Reproductive organs incompatible |
| Gametic isolation | Sperm can't fertilize egg |
| Postzygotic Barriers (after fertilization) | |
| Hybrid inviability | Hybrid embryo doesn't survive |
| Hybrid sterility | Hybrid can't reproduce (mule) |
| Hybrid breakdown | F2 hybrids are weak/sterile |
Variations in Populations
Genetic variation in populations comes from multiple sources and is maintained by several mechanisms.
Ultimate source of new alleles
Crossing over, independent assortment
Migration brings new alleles
| Mechanism | Description | Effect |
|---|---|---|
| Natural Selection | Differential survival/reproduction | Adaptive; increases fitness |
| Genetic Drift | Random changes in small populations | Non-adaptive; random |
| Gene Flow | Migration of alleles between populations | Equalizes allele frequencies |
| Mutation | Creates new alleles | Introduces variation |
| Sexual Selection | Mate choice affects reproduction | May oppose natural selection |
Sudden reduction in population size (disaster)
Ex: Cheetahs, Northern elephant seals
Small group colonizes new area
Ex: Amish population, Afrikaner population
Origins of Life on Earth
Life on Earth began approximately 3.5-4 billion years ago. Several hypotheses explain how life could have arisen from non-living matter.
- No free oxygen (reducing atmosphere)
- Atmosphere: water vapor, CO₂, N₂, CH₄, NH₃, H₂
- Intense UV radiation (no ozone layer)
- Frequent volcanic activity and lightning
- Bombardment by meteors
| Experiment/Hypothesis | Description |
|---|---|
| Miller-Urey Experiment (1953) | Simulated early Earth conditions; produced amino acids from simple molecules |
| RNA World Hypothesis | RNA was the first genetic material (can store info AND catalyze reactions) |
| Deep-Sea Vent Hypothesis | Life may have originated at hydrothermal vents (energy + chemicals) |
| Panspermia | Life (or precursors) arrived from space on meteors |
Small organic molecules form
Monomers join to form polymers
Self-replicating molecules in membrane
Natural selection begins
| Time (BYA) | Event |
|---|---|
| ~4.6 | Earth forms |
| ~3.5-4 | First prokaryotes (bacteria/archaea) |
| ~2.7 | Cyanobacteria produce O₂ (Great Oxidation Event) |
| ~2 | First eukaryotes (endosymbiosis) |
| ~1.5 | Multicellular organisms |
| ~0.54 | Cambrian Explosion (animal diversity) |
Evolution by natural selection is the central organizing principle of biology. It explains the diversity of life, common ancestry, and ongoing adaptation. The Hardy-Weinberg equations help us detect when evolution is occurring, and multiple lines of evidence (fossils, molecular biology, biogeography) support evolutionary theory.