Natural Selection

•You will be able to describe Darwin’s theory of
natural selection as well as give examples that
support the theory.
•You will be able to describe the conditions
required for natural selection.

Recall
•Evolution: changes
that transform life over
an immense time.
•Darwin published , The
Origins of Species
making two main points
for the mechanisms of
evolution…
1. Descent with
modification
2. Natural selection

Decent with Modification
 Species living today descended from ancestral species
 Species spread into various habitats over millions of
years
 Species accumulated different modifications, or
adaptations, to diverse ways of life
 Results in diversity of species

Natural Selection
 Process by which individuals
with inherited characteristics
well-suited for their
environment leave more
offspring on average than do
other individuals
 Survival and reproduction of
the fittest

Theory of Natural Selection Flow
Chart Population of
Organisms
Overproduction Variation
Struggle for
existence
Differences in
reproductive success
Evolution of
adaptations

What have you learned?
 What are the two main points that Darwin made in is
book to ague the mechanisms of evolution?
 Descent with modification and natural selection
 What is natural selection?
 Process by which individuals with inherited
characteristics well-suited for their environment leave
more offspring on average than do other individuals
 Describe some conditions that could lead to a
struggle for existence among individuals in a
population.
 Over population, limited resources, changes occurring
in the environment

Natural Selection Game
http://www.sciencechannel.com/games-and-
interactives/charles-darwin-game/

Evidence of Evolution
 The fossil record= shows patterns of evolution over
millions of years
 Comparative Anatomy
 Comparative Embryology
 Comparative Biochemistry: amino acid tables
 Geographic Distribution: similar organisms separated
by land

Fossil Record
 Fossils provide a
record of species
that lived long
ago.
 Fossils show that
ancient species
share similarities
with species that
now live on
Earth.

Comparative Anatomy
 Homologous structures= anatomically similar
structures inherited from a common ancestor

Comparative Embryology
 Similar phases of development

Comparative
Biochemistry
 Common ancestry can be seen
in the complex metabolic
molecules that many different
organisms share.
 Organisms with closely related
morphological features have
more closely related molecular
features.

Geographical
Distribution
 The distribution of
plants and animals
that Darwin saw first
suggested evolution
to Darwin.
 Patterns of migration
were critical to
Darwin when he was
developing his theory.
 Evolution is
intimately linked with
climate and
geological forces.

 What are the five things that provide evidence for
evolution?
 Fossil Record
 Comparative anatomy
 Comparative embryology
 Comparative biochemistry
 Geographical Distribution
 In your foldable if you missed something update it
with the new information.

You will be able to discuss mechanisms of
evolution other than natural selection
such as genetic drift and gene flow.

Gene Pool
Consists of all the
alleles in all the
individuals that
make up a
population.
The process of
meiosis (specifically
crossing over) and
fertilization shuffle
alleles in the gene
pool giving us
genetic diversity

Genetic Drift
 A change in the population due to chance
 All populations are subject to some genetic drift;
however, the smaller the population the greater the
impact.
 Bottleneck effect- disasters can reduce the size
of a population. By chance some alleles are
represented more than others while others are
completely eliminated
 Founder effect- when a few individuals colonize
an isolated habitat. There is low genetic make-
up

Demonstration for Genetic Drift
 Hypothesis:
 What would the populations look like generations
from now if there were no environmental changes
and the gene pool remained the same?
 Analysis:
 What are some deadly “chance” events?
 How did the removal of some of the marbles
(change in the gene pool) affect each population?
 How did the impact of genetic drift contrast
between the small population and the larger
population?

Gene Flow
 The exchange of genes with another population
 Occurs when fertile individuals or their gametes
migrate between populations
 Tends to reduce genetic differences between
populations

 What is a gene pool?
 All of the alleles in all of the organisms that make up a
population
 What are the two main forces of evolutionary change
in gene pools other than natural selection?
 Genetic drift and gene flow
 How can genetic drift cause a substantial change in
allele frequencies in small populations?
 It can reduce genetic variations

Natural Selection

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Natural Selection