Transcript What is evolution?

What is evolution?
A gradual change in the genes
of a population of organisms
over time
You
What is meant by “last
common ancestor?”
Cousin
Mom
Aunt
Grandparent
Last 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 Antibiotic is present
no antibiotic present
Drug-resistant bacteria survive
and reproduce
Mutation makes
bacteria resistant to drugs
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 structure
of each
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
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
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 with
fruit 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