Post on 30-Dec-2015
BIG IDEA 1: EvolutionEvolution as a Process
Selection as a MechanismEvidence for Evolution
HW EquilibriumSpeciation
Origin of Life
Living things change, or evolve over generations & living species descended from earlier life-forms: descent with modification
I have called this principle, by whicheach slight variation, if useful, is
preserved, by the term Natural Selection.
—Charles Darwin from "The Origin of Species"
Population Variation Selection Reproduction Change in population over time leading to new species
Natural selectionResults in favored traits being represented more and more and unfavored ones less and less in ensuing generations of organisms
Selection Against or in Favor of Extreme Phenotypes
• Stabilizing Selection– Intermediate forms of a
trait are favored
– Alleles that specify extreme forms are eliminated from a population
Birth Weight and Clutch Size
Stabilizing Selection
Coloration of snails
Light snailseliminated
Dark snailseliminated
Num
ber o
f ind
ivid
uals
Coloration of snails
Snails withextreme
coloration areeliminated
Num
ber o
f ind
ivid
uals
Average remains the sameNumber of individuals with intermediate coloration increases
Selection against extremes
Natural selection
Selection Against or in Favor of
Extreme Phenotypes
• Disruptive Selection– Both forms at
extreme ends are favored
– Intermediate forms are eliminated
Summary of Evolution of Life
Formationof the
earth’searly
crust andatmosphere
Small organic
moleculesform inthe seas
Large organic
molecules(biopolymers)
form inthe seas
First protocells
form inthe seas
Single-cellprokaryotes
form inthe seas
Single-celleukaryotes
form inthe seas
Variety ofmulticellular
organismsform, firstin the seas and lateron land
Chemical Evolution(1 billion years)
Biological Evolution(3.7 billion years)