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

Evidence for Evolution

Vestigial Structures

Analogous structures- same structural origin but has evolved different functions

Convergent Evolution

Artificial Selection

Pesticide application

Survivor

Cladograms:

Creates new alleles

Chance!

Exchange of alleles

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)