SHAPING EVOLUTIONARY THEORY SECTION 15.3 Mrs. Pachuta

Hardy-Weinberg Principle  Genetic equilibrium (no evolution) will occur with the following conditions: 1. No genetic drift 2 Types: 1. Founder Effect 2. Bottleneck 2. No gene flow (due to immigration/emigration) 3. Random mating 4. No mutations 5. No natural selection

Hardy-Weinberg Principle  Genetic Drift – any change in allele frequencies in a population that is due to chance  In larger populations, enough alleles ‘drift’ so that allele frequency stays fairly constant from one generation to another  In smaller populations, the effects of ‘drift’ are more pronounced

Hardy-Weinberg Principle  Founder Effect – An extreme example of genetic drift; occurs when a small sample of a population settles in a location separated from the rest of the population.  Result: Uncommon genes might become more common in this group Example: Polydactyly in Amish population

Polydactyly in Amish Population

Hardy-Weinberg Principle  Bottleneck – another extreme example of genetic drift; occurs when population declines to very low number, then rebounds.  Result: The gene pool of rebound population often genetically similar to population at its lowest level – which means decreased diversity. Example: Cheetah population in Africa – almost died out 10,000 yrs ago and again 100 yrs ago

Bottleneck  Cheetahs are now so genetically identical, that they appear to be inbred.  This decreases fertility and increases the chance for extinction.

Hardy-Weinberg Equation

 Directional selection – an extreme version of the trait is favored.

Types of Natural Selection  Stabilizing selection – the average phenotype is favored

 Disruptional selection – favors both extreme versions of the trait; divides population into 2 groups

 Sexual selection – males and females differ significantly in appearance  Darwin was intrigued that qualities of attractiveness to the opposite sex appeared to decrease chance of survival Example: peacocks

PATTERNS OF EVOLUTION

Divergent Evolution (diverge = moving apart)  Also called “adaptive radiation”  One species gives rise to many species as result of different environmental conditions  Habitat  Food source

Convergent Evolution (converge = “coming together”)  Occurs when unrelated species evolve similar traits because they live in a similar environment

Coevolution  Species in a close relationship evolve in response to one another  Example: plants and pollinators

RATE OF SPECIATION

Gradualism  Theory that says evolution proceeds in small, gradual steps.  (left side)

Punctuated Equilibrium  Theory that says evolution occurs through abrupt changes  (right side)