Examples of Evolution

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Allele Frequencies

There are dominant and recessive alleles.

Hardy and Weinberg (scientists) determined that

dominant alleles do NOT automatically replace recessive

alleles.

Alleles in a population only change if selection acts upon

them

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http://campus.murraystate.edu/academic/faculty/terry.derting/bio116/hardy_weinberg.jpg

HARDY-WEINBERGPRINCIPLE:

The frequencies of alleles in a population do not change

unless evolutionary forces act on the population.

http://www.maropeng.co.za/images/uploads/fossil_lg.jpg

Hardy-Weinberg

• Holds true for:

• large populations in which members do not mate with relatives AND

• as long as evolutionary forces are not acting on the population

5 Evolutionary Forces

1.Mutations

2.Gene Flow

3.Nonrandom mating

4.Genetic drift

5.Natural Selection

#1 Mutations

• Mutation rates are slow in nature

• Not all result in phenotype changes (remember the codon wheel)

• Mutation IS the source of variation though & makes evolution possible

• http://www.youtube.com/watch?v=MkmtodFQbQM

#2 Gene Flow• The movement of individuals to or

from a population (MIGRATION) creates gene flow.

• Immigrants = arriving individuals

• Emigrants = departing individuals

#3 Nonrandom Mating

• In-breeding alters Hardy-Weinberg

• Also occurs when organisms choose their mates

#4 Genetic drift

• Small populations that are isolated from one another can differ greatly because of genetic drift

• Fires - landslides - etc.

# 5 Natural Selection

• The frequency of an allele will increase or decrease depending on the allele’s effects on survival and reproduction

• N.S. is one of the most powerful agents of genetic change.

http://evolution.berkeley.edu/evolibrary/article/evo_39

Natural Selection affects the Distribution of Phenotypes in 4 Ways!

1 – Stabilizing Selection 2 – Directional Selection3 – Disruptive Selection4 - Sexual Selection

1- Stabilizing Selection

• Individuals with the average form of a trait have the highest fitness.

2 – Directional Selection

• Individuals at one end of the curve have higher fitness and are favored more than individuals in the middle or other end.

3 – Disruptive Selection

• Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait.

4 – Sexual Selection

• The preferred choice of a mate based on a specific trait.

Peacocks

-- females choose

males based on

certain traits

V. I .S .T.

• V = Variation: All life forms vary genetically within a population. It is this genetic variation upon which selection works.

• I = Inheritance: Genetic traits are inherited from parents and passed on to offspring.

• S = Selection: Organisms with traits that are favorable to their survival get to live and pass on their genes to the next generation.

• T = Time: Evolution takes time. It can happen in a few generations, but major change, such as speciation, often take very long periods of time.