Natural Selection & Speciation

Mechanisms and Modes of Selection

• Evolution's engines: natural selection (adaptive), genetic drift (random — bottleneck and founder effects), gene flow (homogenizes populations), and mutation (raw material).
• Selection modes: stabilizing favors the average and shrinks variance (birth weight); directional shifts the mean toward one extreme (antibiotic resistance); disruptive favors both extremes and can make the distribution bimodal, seeding sympatric divergence.
• Fitness is relative reproductive success — viable, fertile offspring — not strength or lifespan. Sexual selection (mate choice or male competition) can fix survival-costly traits like the peacock's tail when mating gains outweigh predation losses.

Speciation

• Biological species concept: interbreeding groups producing fertile offspring. Allopatric speciation follows geographic separation; sympatric speciation occurs in place (polyploidy in plants is the classic instant example).
• Prezygotic barriers prevent fertilization: temporal, behavioral, habitat, mechanical, gametic. Postzygotic barriers reduce hybrid fitness: inviability, sterility (the mule), hybrid breakdown.

Reading Evolutionary History

• Homologous structures (human arm, whale flipper) mark common ancestry and divergent evolution; analogous structures (bird vs. insect wing) mark convergence; vestigial structures are evolutionary leftovers. Gradualism contrasts with punctuated equilibrium — long stasis broken by rapid bursts.

Key Takeaways

• Classify any selection scenario by what happens to the mean and variance.