Descent with Modification

Descent with Modification

Heritable Variation

• Variation within species and between species
• Phenotype: Physical characteristics (shape, function, behavior)
• Genotype: Specific combination of genes and alleles
• Two requirements
• Traits must be expressed
• Traits must be heritable

Sources of Variation

• Mutation: Random errors in DNA replication
• Meiosis: Sexual reproduction which can introduce genetic variation
• Recombination: chromosomes cross and exchange sections
• Independent Assortment: Offspring receive one of each chromosome at random from the parent
• Fertilization: Combining two parental genomes

Selection

• Acts on the individual
• Agent of selection: environmental feature that consistently produces differential survival
• Causes the weaker / less adapted to die off Example: Heat, drought, salinity
• Selective pressure: How intensely the environment is selecting a trait Example: What fraction of the population dies each year due to drought
• Biotic: living pressure Example: Prey, Predators
• Abiotic: Non-living pressure Example: Rainfall, Temperature, Nutrients

Natural Selection

• Natural Selection depends on the environment
• Context-dependent
• Does not move towards perfection
• Requirements
• Heritable Variation
• Limited Resources
• Differential survival
• Given these properties, the population will adapt

Evidence for Evolution

• Fossils Records: see species change gradually through time
• Comparative Embryology: distantly related species have similar embryonic stages (due to shared ancestry)
• Biogeography: the distribution of related species matches evolutionary theory
• Vestigial Structures: some traits have no current function and are slowly disappearing Example: Whale pelvis, Human tailbone
• Genomics: DNA is more similar between closely related species
• Artificial Selection: We have selectively bred traits to produce desired phenotypes Example: Corn, Dogs
• Resistance in Pests: Pests have become resistant to our methods of control