What is evolution?

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What is evolution?
A gradual change in the genes of a population of
organisms over time
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What is meant by last common ancestor?
You
Cousin
Mom
Aunt
Grandparent
Last common ancestor of you and your cousin
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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.

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Charles Darwins 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

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Darwins 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

Charles Darwin (1860 at age 51)
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Bacterial Growth - no antibiotic present
Antibiotic is present
Drug-resistant bacteria survive and reproduce
Mutation makes bacteria resistant to drugs
ANTIBIOTIC RESISTANCE
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Evidence for Evolution

• Fossil Record
• Homologous and vestigial structures
• Comparative embryology
• Biochemical (DNA, RNA, proteins)

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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.
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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 structure of each
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Vestigial Structures
A structure that is present in an organism 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 vertebrate species
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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 Populations

• Population Genetics

Populations evolve, NOT individuals
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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)

Gene flow between two populations
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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

Speciation experiment with fruit flies (click
picture for another example)
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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)

Geographic isolation by a barrier preventing
gene flow between populations