SHAPING EVOLUTIONARY THEORYSECTION 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 mating4. No mutations5. 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)