What is evolution?

A gradual change in the genes of a population of organisms

over time

What is meant by “last common ancestor?”

You Cousin

AuntMom

GrandparentLast common ancestor of you and

your cousin

Evolution is a theory.What does this mean?

• A theory in science is a well tested explanation of observable events supported by experimental evidence collected over time.

Charles Darwin’s Theory of Evolution

• Individuals with the best adaptations for their environment survive and pass on their genes (natural selection)

• All living species compete with each other for survival

• All living things change over time• All species on earth (both alive and

extinct) share a common heritage

Darwin’s Theory of Evolution by Natural Selection

• Charles Darwin devised the theory of evolution by natural selection and published it in 1858

• Natural Selection: organisms best adapted to their environment survive, reproduce, and pass on their traits

• Examples of natural selection

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Charles Darwin (1860 at age 51)

Bacterial Growth - no antibiotic present

Mutation makesbacteria resistant to drugs

Antibiotic is present

Drug-resistant bacteria survive and reproduce

ANTIBIOTIC RESISTANCE

Evidence for Evolution

• Fossil Record • Homologous and vestigial

structures• Comparative embryology • Biochemical (DNA, RNA,

proteins)

Fossil Record

• Shows changes in a species over the course of time

• Older fossils are in bottom rock layers

• Radioisotope dating is used to determine the age of the rock the fossils are in Older fossils are in lower layers;

younger fossils are in higher layers.

Homologous Structures

• Structures with the same design, but serve a different purpose (ex. Human arm, cat leg, flipper of a whale)

• Similarities in the structures suggest that the organisms shared a common ancestor.

Four homologous structures.Note the similarity in the structureof each

Vestigial Structures

A structure that is present in anorganism but no longer serves its

original purpose

examples

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Embryological similarities

• Early embryos of many species look very similar

• The genes for early development of embryos are the same or similar for many species

• Suggests a common ancestor whom had these developmental genes

Embryos of different vertebratespecies

Biochemical Comparisons

• Similarities in the genes and proteins of different species

• Closely related organisms share a high percentage of similarity in their DNA sequences

• Homeotic genes determine what structures develop where in an organism (ex. The placement and structure of the arms and legs)

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Evolution of PopulationsEvolution of Populations

Population GeneticsPopulation Genetics

Populations evolve, NOT individuals

Changes in the Gene Pool

• Evolution is caused by changes in the gene pool of a population

• A gene pool is all of the genes in a population of a species

• The frequency of an allele in the gene pool is known as allelic frequency

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What causes change in the Gene Pool?

• Mutations

• Natural Selection

• Gene flow (movement of genes into or out of the gene pool)

• Genetic drift (changes in allele frequency due to chance events)

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Gene flow between two populations

Speciation - the formation of a new species

• Members of a population become isolated from each other

• Each new population evolves through natural selection with no gene flow between populations

• Over time, the gene pool of each population changes due to natural selection and genetic drift

• When members from the two populations can no longer successfully reproduce with one another, they are said to be different species

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Speciation experiment withfruit flies (click picture for another example)

Isolating mechanisms that cause speciation

• Geographic - a natural barrier prevent two groups from interbreeding

• Temporal - different mating seasons prevent individuals from mating

• Behavioral - different behaviors prevent interbreeding between populations

* All of the above may lead to reproductive isolation (the two groups can no longer mate successfully)

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Geographic isolation by a barrier preventing gene flow between populations