The Evolution of Populations 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; may lead to formation of new species

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.

Speciation Chapter 16

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

Concepts of “Species” 1) Morphological: species defined by appearance Does not account for individual variations 2) Biological: species defined by reproduction patterns Does not apply to ancient organisms 3) Modern: species are similar in appearance and can interbreed to produce fertile offspring Concept currently used in Biology

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 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

The Gene Pool Gene Pool: the total genetic information available in a popltn All the possible alleles available Allele Frequency: # certain allele total # alleles in gene pool Used to predict phenotypes of future offspring

Hardy-Weinberg Principle “Allele frequencies in a popltn will remain constant unless acted upon by an outside force” AKA: Evolution will not occur unless… 5 conditions must be met for this to hold true….

Hardy-Weinberg Conditions 1) No Mutations can occur BUT, mutations occur constantly (can’t control) 2) Individuals may not enter or leave BUT, individuals immigrate & emigrate constantly 3) Population size must be VERY large BUT, popltn size is limited by many factors i.e.: space, food source, competition, etc.

Hardy-Weinberg Conditions (con’t) 4) Mating must be completely random BUT, mate selection is influenced by proximity, physical charac., relation, etc.

Hardy-Weinberg Conditions (con’t) 5) Natural Selection does not occur (all traits considered “favorable”) BUT, “favorable” traits are determined by environment 4 Types Natural Selection: Stabilizing Directional Disruptive Sexual