Biol120 - Lecture 2:Macroevolution & Phylogenetics

Evolution & Natural Selection

• Darwin: all species share common ancestry; change occurs via natural selection (published $1859$)
• Natural selection prerequisites:
  • Heritable variation within populations
  • Differential survival & reproduction ("struggle for resources")
  • Result: advantageous alleles increase frequency each generation
• Key observations (Mayr paraphrase):
  • Populations could grow exponentially yet remain stable → limited resources
  • Individuals vary; traits are heritable
  • Inferences: not all offspring survive; those with favorable heritable traits leave more offspring → evolution by natural selection

Genetic Variation & Heredity

• Sources of variation: mutation, genetic recombination (crossing-over, independent assortment), sexual reproduction
• Gene: DNA segment coding for a protein; alternate forms = alleles
• Evolution requires allelic diversity; selection sorts existing variation (does NOT create it)

Mechanisms Driving Evolutionary Change

• Natural selection (only mechanism that consistently produces adaptation)
• Genetic drift (random allele frequency shifts, esp. in small populations)
• Gene flow (migration)
• Mutation (ultimate source of new alleles)

Macroevolutionary Patterns

• Evolutionary change at/above species level: speciation, extinction, stasis
• Driven by population traits + environmental change; evidenced in fossil record
• Large-scale diversifications & losses often tied to ecological interactions, climate shifts, "bad luck"

Species Concepts

• Biological Species Concept: interbreeding natural populations reproductively isolated from others
• Phylogenetic Species Concept: smallest diagnosable cluster sharing a common ancestor & unique derived traits

Classification & Taxonomy

• Linnaean hierarchy → nested taxa; binomial nomenclature (Genus species)
• Modern taxonomy aligned with phylogenetic relationships; avoids non-monophyletic groupings

Phylogenetics

• Tree = hypothesis of relationships; branches = lineages; nodes = common ancestors
• Monophyletic (clade): ancestor + all descendants (preferred)
• Paraphyletic: ancestor + some descendants
• Polyphyletic: excludes common ancestor of included taxa
• Shared derived characters (synapomorphies) diagnose clades
• Sister taxa: two lineages sharing immediate common ancestor
• Branch length may not indicate time unless specified

Reading / Chapters

• Campbell $12^{th}$ Ed.
  • Ch $23$: Concepts $23.3, 23.4$
  • Ch $24$: Concept $24.1$ (background)
  • Ch $26$: Concepts $26.1, 26.3$

Quick Misconception Check

• Natural selection acts on existing variation, not need-based traits
• Individuals do NOT evolve; populations do
• Evolution has no goal, progress scale, or moral value; adaptations are context-dependent
• Selection is constrained by ancestry (cannot "fix" past structures)

Exam Essentials

• Define natural selection & list its requirements
• Distinguish micro vs. macroevolution
• Identify sources of genetic variation
• Recognize monophyletic vs. non-monophyletic groupings on a tree
• Apply species concepts to given scenarios
• Spot common misconceptions quickly