Forces that Drive Evolution

Mutation, Natural Selection,Genetic Drift and Gene Flow

Forces that Drive EvolutionObjectives:

• 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

• 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.

Darwin’s Evidence for Evolution

Yeah, but what actually causes

evolution, anyway?!?

• 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

Forces That Drive Evolution

U.S. Department of Energy Human Genome Program, http://www.ornl.gov/hgmis

Mutations Affect Genes, Proteins, Traits

Natural Selection

From “Understanding Evolution”

What is “Natural Selection”?

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)

H.E. Hoekstra

Natural Selection: Pocket Mouse Video

H.E. Hoekstra

Natural Selection Activities• Survival of the Skittles http

://salsa.nescent.org/activity-2-survival-of-the-skittles/)

• “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 – Change in gene pool due to chance

Genetic Drift

From “Understanding Evolution”

Genetic Drift – Change in gene pool due to chance

Genetic Drift

The “jar of marbles” example

From “Understanding Evolution”

Genetic Drift – Change in gene pool due to chance

Genetic Drift

The “jar of marbles” example

From “Understanding Evolution”

A population’s size is reduced for at least one generation

Genetic Drift - Bottlenecks

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…

Climate change Evolving pathogens

or…

Change in allele frequency due to migration

Gene Flow

From “Understanding Evolution”

Questions?