Processes of Evolution – Summary
Allele Frequency
• Allele Frequency – the number of alleles for a specific trait
that occur in a population
•Ex. The number of A’s vs a’s
• Natural selection
shifts the phenotypic
distribution away
from the phenotype
that is selected
against
Mechanisms of Evolution
 Natural Selection “survival of the fittest”
 Mutations create new alleles
 Migration moves alleles into and out of a population
 Genetic drift happens when random events change
the allele frequencies of small populations
 Sexual Selection (non-random mating)
Natural Selection
 http://www.youtube.com/watch?v=aTftyFboC_M
Species
 A group of organisms
that can interbreed in
nature and produce
fertile offspring
Speciation –
How New Species Form
 Must be isolated from other
populations for a long period of
time.
 different selection pressures
cause different traits to be
selected for or against.
 http://www.youtube.com/watch?v=2oKlKmrbLoU
Geographic Isolation
• Species are separated from each other by a physical barrier
Geographic Isolation
• The Kaibab and Albert squirrels are separated by the Grand Canyon
Behavioral Isolation
• Species cannot interbreed because they have different
mating rituals or behavior
Temporal Isolation
• Species mate at different times of the day or the year
Mass Extinctions
• When many species
disappeared from the Earth
•Provides opportunity for
new species to evolve and
establish themselves
• Mammals might not have
“taken over” if the
dinosaurs hadn’t gone
extinct!
Adaptive Radiation
• A single species
evolves into many
species
• Occurs as new
opportunities for
survival arise
Convergent Evolution
• Unrelated organisms
begin to look like each
other because of similar
selection pressures
• These organisms
would have analogous
structures
Convergent Evolution
• Sharks (fish) and dolphins (mammals) have evolved
similar adaptations because of similar selective pressures
Coevolution
• When two species evolve together in response to
changes in each other
• Example: Bees and Flowers
Evidences of Evolution
Homologous Structures
 Similar body structures that are adapted in different
ways for different organisms
 Provide evidence that related organisms descended,
with modifications, from a common ancestor
Vestigial Structures
 The structures of
organisms that remain
from a common
ancestor, but may no
longer affect its ability
to survive and
reproduce
 Example: Appendix,
Tail Bone
Analogous Structures
 Unrelated organisms
in similar
environments develop
similar adaptations.
 Example: Flight of
Insects vs. Birds vs.
Bats
Fossil Record
 The fossilized remains of organisms found in the layers of rock and
soil
 Shows that the diversity of organisms has changed over time
Embryology
 Organisms with a
common ancestor
share similar stages of
development.
DNA Analysis
 All organisms use the
same four bases for their
DNA (genetic code).
 The more similar two
sequences of DNA are, the
more recent their
common ancestor, or the
more closely related the
organisms are.
DNA Analysis
• Modern
biologists compare
DNA sequences to
determine which
organisms are
most-closely
related to each
other