Evolution

Natural selection requires variation, heritability, and differences in reproductive success. Selection can be directional, stabilizing, disruptive, frequency-dependent, or sexual. Selection acts on phenotypes, and allele frequencies change as a result.

Genetic drift is random change in allele frequencies. Its effect is strongest in small populations (bottlenecks, founder effects). Gene flow makes populations more similar, and mutation is the ultimate source of new alleles.

The biological species concept defines species by reproductive isolation. It does not work for asexual organisms or fossils, so other concepts (morphological, phylogenetic) are also used.

Prezygotic barriers (habitat, temporal, behavioral, mechanical, gametic) prevent fertilization. Postzygotic barriers (hybrid inviability, sterility, breakdown) act afterward. Allopatric speciation needs geographic separation. Sympatric speciation can occur through polyploidy, which is especially common in plants.

A phylogenetic tree shows relationships through common ancestry. How closely two taxa are related depends on their most recent common ancestor, not on how near their tips are drawn. Rotating branches around a node does not change the tree.

Monophyletic groups (clades) include an ancestor and all its descendants. Paraphyletic groups leave some descendants out (traditional "reptiles" without birds). Shared derived characters (synapomorphies) define clades. Parsimony picks the tree that needs the fewest changes.

The three domains are Bacteria, Archaea, and Eukarya. Key events include the origin of photosynthesis and atmospheric O₂, eukaryotes through endosymbiosis, multicellularity, the colonization of land, and mass extinctions.

For biosystematics, know the defining traits of the major plant groups (bryophytes, seedless vascular plants, gymnosperms, angiosperms) and animal groups (sponges, cnidarians, protostomes, and deuterostomes, including chordates).