The Evolution of Populations & Speciation

1. The Evolution of Populations & Speciation Chapter 16

2. Genetic Equilibrium • Population Genetics: the study of evolution from a genetic viewpoint • Most popltns have variation amongst their traits • i.e. different heights, skin color, etc. • If graphed, looks like a bell

3. Effects of Natural Selection • Natural Selection can cause a shift in traits within a population • Depends on which traits are favorable in that environment • Those traits will increase over time • 4 Different Effects: • Stabilizing Selection • Directional Selection • Disruptive Selection • Sexual Selection

4. Stabilizing Selection • Occurs when the average form of a trait is considered the highest fitness level (best for that environment) • i.e. height • Causes decrease in variation

5. Directional Selection • Occurs when one extreme is considered highest fitness level • i.e.: an anteater’s tongue length • Causes a shift in variation

6. Disruptive Selection • Occurs when both extremes are considered highest fitness level • i.e.: black/white limpets on barnacles • Causes splitting of phenotypes

7. Sexual Selection • Occurs when female organisms select a mate based on phenotype • May require extreme male traits • i.e. bright colors, unique mating calls, etc.

8. Formation of Species • Speciation: process of forming new species • Results in many related popltns • May remain similar or become quite different Manchester Terrier Dandie Terrier

9. Isolating Mechanisms • Speciation occurs when members of a species stop interbreeding and split • Become isolated from each other • 2 Types Isolating Mechanisms: • 1) Geographic • 2) Reproductive

10. Geographic Isolation • Geographic Isolation: the physical separation of members of a popltn • i.e.: by canyons, mountains, rivers, etc. • Once physically separated, will continue to diverge until they can no longer mate

11. Reproductive Isolation • Reproductive Isolation: members of a popltn no longer able to mate successfully • i.e. due to disruptive selection, changes in mating habits, etc. • Once separated, will continue to diverge

12. Rates of Speciation • Gradualism: some species evolve gradually & at stable rate • Although speciation may take millions of years, it is not always gradual • Punctuated Equilibrium: when species evolve in rapid spurts of great change • Involves long periods of no change between “growth spurts” • Usually due to natural disasters