Forces that Drive Evolution
Mutation, Natural Selection,
Genetic Drift and Gene Flow
Forces that Drive Evolution
Objectives:
• Understand that evolution is change over time
• Understand that there are four main forces
that “drive” evolution:
– Mutation
– Natural selection
– Genetic drift
– Gene flow
• Be able to explain each, and how they work
Evolution – A Definition (or three)
• Variation of inherited traits within a
population over time as a consequence of
mutation, genetic recombination, or gene
flow
• Descent with modification
• Change over time
Charles Darwin – The Theory of Evolution
• English naturalist (1809-1882)
• Traveled the world (including
the Galapagos Islands) on the
Beagle, making observations,
collecting samples, formulating
the Theory of Evolution
• Published On The Origin of
Species over 20 years after his
voyage on the Beagle.
• It was meant to explain the
diversity of life, not the origin.
Why is Darwin so important?
• Provided evidence for
descent with modification
(i.e., change over time, i.e.,
evolution)
• Proposed a natural
mechanism for how
species evolve (i.e., natural
selection)
Evolution helps explain…
• The diversity of life on Earth
• How new species are formed
• Why organisms look and behave
the way they do
• How organisms respond to
changes in their environment
• Why some groups of organisms
are more diverse than others
Darwin’s Evidence for Evolution
• Animal distributions - Closely related
organisms found in geographical proximity
Darwin’s Evidence for Evolution
• Fossils - Sequential emergence of different
groups and transitional forms
Darwin’s Evidence for Evolution
• Homology - The spatial and developmental
equivalence of traits that may look quite
different
Darwin’s Evidence for Evolution
• Vestigial features - Speaks
to imperfect “design”.
Best explained by the
persistence of shared
traits.
Yeah, but what
actually causes
evolution,
anyway?!?
Forces That Drive Evolution
• Mutation – Random
changes in DNA
sequence
• Natural selection
• Genetic Drift – Change in
gene pool due to chance
• Gene Flow – Change in
allele frequency due to
migration
Mutations Affect Genes, Proteins, Traits
U.S. Department of Energy Human Genome Program, http://www.ornl.gov/hgmis
Natural Selection
From “Understanding Evolution”
What is “Natural Selection”?
H.E. Hoekstra
Individuals in a population
that are better adapted
(variation) to survive in an
environment (competition)
pass on their traits to more
offspring in the next
generation (differential
reproductive success,
heritability)
Natural Selection: Pocket Mouse Video
H.E. Hoekstra
Natural Selection Activities
• Survival of the Skittles http://salsa.nescent.org/activity-2-survival-of-theskittles/)
• “Clip Birds” (http://www.ucmp.berkeley.edu/education/lessons/clipbirds/)
• Teddy Graham Evolution
• Several online simulations…
– EvoDots (http://faculty.washington.edu/herronjc/SoftwareFolder/EvoDots.html)
– AIPOTU (http://aipotu.umb.edu/)
– PhET NS Simulator (http://phet.colorado.edu/en/simulation/natural-selection)
– McGraw-Hill Natural Selection Simulator
http://www.mhhe.com/biosci/genbio/virtual_labs/BL_12/BL_12.html
• ENSI Evolution Lessons (http://www.indiana.edu/~ensiweb/evol.fs.html)
Genetic Drift
Genetic Drift – Change in gene pool due to chance
From “Understanding Evolution”
Genetic Drift
Genetic Drift – Change in gene pool due to chance
The “jar of marbles” example
From “Understanding Evolution”
Genetic Drift
Genetic Drift – Change in gene pool due to chance
The “jar of marbles” example
From “Understanding Evolution”
Genetic Drift - Bottlenecks
A population’s size is reduced for at least one
generation
From “Understanding Evolution”
Genetic Drift – Founder Effect
Establishment of a new population (colony) by
just a few individuals
Question:
Why are genetic drift, bottlenecks and
founder effects important in
evolution?
Answer:
They result in reduced genetic
variation.
And why does this matter???
Because reduced genetic variation can
mean decreased ability to adapt to
new selective pressures, such as…
or…
Climate change
Evolving pathogens
Gene Flow
Change in allele frequency due to migration
From “Understanding Evolution”
Questions?