Rucete ✏ AP Biology In a Nutshell
17. Types of Selection
This chapter reviews the evidence of evolution and explains the major types of selection—natural, artificial, and sexual—that act on genetic variation in populations and lead to evolutionary change.
Evidence of Evolution
• Molecular evidence: DNA and protein sequence comparisons show relatedness. – Example: GAPDH gene is over 99% similar in humans and chimpanzees, but only 91% in dogs.
• Morphology: Homologous structures (e.g., human hands, bat wings, whale fins) suggest common ancestry.
• Fossils: Provide records of extinct species and transitional forms.
• Vestigial structures: Nonfunctional traits inherited from ancestors (e.g., human tailbone).
• Convergent evolution: Similar adaptations evolve in unrelated species in similar environments (e.g., dolphins and sharks).
• Biogeography: Geographic proximity of species supports common ancestry (e.g., island vs. mainland species).
• Current observations: Bacteria evolving antibiotic resistance, mosquitoes evolving pesticide resistance.
Natural Selection
• Core principles: – Genetic variation exists in populations. – Competition for limited resources leads to differential survival. – Favorable traits increase survival and reproduction. – These traits become more common in the population over time.
• Example 1: Antibiotic resistance in bacteria: pre-existing resistant individuals survive and reproduce.
• Example 2: Peppered moths: wing color variation + environmental change → directional selection.
Types of Natural Selection
• Directional selection: One extreme phenotype is favored. – Example: darker moths during the Industrial Revolution.
• Stabilizing selection: Intermediate phenotype is favored. – Example: clutch size in birds—too few or too many eggs reduces survival.
• Disruptive selection: Both extremes are favored over the intermediate. – Example: mice with light or dark fur surviving better than those with medium fur in mixed terrain.
Artificial Selection
• Humans select individuals with desired traits to reproduce.
• Example: Brassica oleracea selectively bred into kale, cauliflower, broccoli.
• Example: dog breeds from domesticated wolves; cattle breeds from aurochs.
Sexual Selection
• Certain traits increase mating success.
– Intersexual selection: Mate choice based on traits like color or behavior (e.g., peacocks, blue-footed boobies).
– Intrasexual selection: Competition between members of one sex (e.g., dominance in males).
In a Nutshell
Evolution acts on populations through mechanisms like natural, artificial, and sexual selection. Environmental pressures shape which traits are favorable, while human intervention and mating behavior also influence trait frequency. These processes lead to changes in allele frequencies and drive the evolution of species.