Natural Selection
Today’s Objectives: SOL BIO.8b-d

TSW investigate and understand how
populations change through time,
including:
– How variation of traits, reproductive
strategies, and environmental pressures
impact on the survival of populations
– Recognizing how adaptations lead to
natural selection
– How new species emerge
Charles Darwin
On the Origin of Species
 Sailed with the HMS Beagle
 Observations made in the Galapogos
Islands
 These observations helped him form the
theory of how species change over time
called natural selection

What is Natural Selection?

Natural selection is governed by the
principles of genetics.
Types of Adaptations

Protective Coloring
– Camouflage
– Mimicry

Physiological Adaptations
– Reproductive Changes
– Other changes

Behavioral Adaptations
Evidence for Evolution
Fossil record
 Anatomy

– Homologous structures
– Analagous structures
Vestigial structures
 Embryology
 Molecular biology (DNA differences)

What is a Population?
Populations evolve over many
generations, individuals don’t
 Populations are groups of interbreeding
individuals that live in the same place at
the same time
 Individuals in a population compete for
resources with each other

How Does Evolution Work?
Populations produce more offspring
than the environment can support
 The unequal ability of individuals to
survive and reproduc leads to the
gradual change in a population over
many generations

Mechanism for change in a
population of organisms
Animals who have greater fitness
survive in environment and live to
reproduce
 Random changes (mutations) can lead
to greater or less fitness
 Adaptations allow an organism to
survive better in their environment

Mutations & Variety Produced by
Sexual Reproduction Allow for
Diversity within a Population

Genetic drift (caused by chance)
– Bottleneck
– Founder effect

Genetic equilibrium
– Hardy-Weinberg law
– In H-W equilibrium, does not occur
Adaptations

Can arise in response to environmental
pressures
– Temperature
– Antibiotic resistance in bacteria
– Pesticide resistance
– Morphological changes in peppered moths
Types of Selection

Directional
– Extreme form favored by natural selection

Stabilizing
– Middle form most successful

Disruptive
– Two extreme forms successful in separate
environments
How are new species created?
Geographic isolation
 Reproductive barriers
 Change in chromosome numbers
 Adaptive radiation

Types of Evolution

Convergent evolution
– Dolphins & fishes
– Wings of bees & bats

Divergent evolution
– Darwin’s finches
– Adaptive radiation
How fast does evolution occur?

Gradualism
– Darwin
– Species change slowly over time

Punctuated Equilibrium
– Gould & Lewontin
– Species can make rapid “leaps” in
evolution

Modern Synthesis
– Parts of both are correct