The Evolution of Populations & Speciation Chapter 16

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

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

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

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

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

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.

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

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

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

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

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