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Natural Selection
Charles Darwin
 Theory of Natural
Selection
 Species that are better
adapted to their
environment will live
longer and reproduce
creating more offspring
like them
 “Survival of the fittest”
How did Darwin form his
theory?
 1831: naturalist on
the H.M.S. Beagle for
a 5 year voyage
around the world
 Galapagos Islands
 Finches
 13 species of finches
 Beak size and shape
varied, most likely due
to their island locations
and available food
 Finches had adapted to
best obtain food in their
environment
Darwin’s Finches
Natural Selection
Darwin’s Breakthrough Theory
The Struggle for Existence
 Darwin believed that there is a constant
struggle for survival in the natural world
 The faster or “smarter” the predator the
more likely it is to capture food
 The better camouflaged, protected, or
cunning the prey the more likely it would
not be eaten
Survival of the Fittest
 Fitness – how well an individual is able to
survive and reproduce in a specific
environment
 Adaptation – an inherited trait which makes
an individual more fit in its environment
 Darwin called this natural selection because of
its similarity to artificial selection
How Natural Selection
Works
 All populations have genetic diversity
(they are not 100% identical)
 If an individual is born/produced that
has trait which make it more fit it then
is more likely to survive and
reproduce
 When it reproduces there is a higher
chance that the beneficial traits will be
passed on to its offspring
Natural Selection
 Over very long periods of time this
selection can lead to the entire
population having the similar beneficial
traits which then makes the entire
population more fit
 Peppered moth activity
 Survival of the Sneakiest
What is variation?
 Variation: differences
between members of the
same species
 Variation is important to
survival!
Imagine that you go
to a corn field and
sample beetles.
80% of the genes in
the population are for
green coloration
20%of them are for
brown coloration
You go back the next year,
repeat the procedure, and
find a new ratio:
60% green
40% brown genes.
A change in gene
frequency over time means
that the population has
evolved.
Genetic Flow
 Gene migration
 The transfer of alleles of genes from
one population to another
gene flow: beetles with
brown genes immigrated
from another population,
or some beetles
carrying green genes
emigrated.
Genetic drift
 Random fluctuations in the frequency of
the appearance of a gene in a small
isolated population
 Leaving gene frequency up to chance
Genetic Drift
 In small populations if an individual that carries
a particular trait and by chance has more
offspring, then the trait or allele can become
more common (by chance, not selection)
 If this happens when a small group of
individuals break away from the main group
and colonize a new habitat it is called the
founder effect
Genetic drift: when the
beetles reproduced, by
luck more brown genes
ended up in the offspring.
Natural Selection: beetles
with brown genes escaped
predation and survived to
reproduce more than
beetles with green genes,
so more brown genes got
into the next generation.
Also known as: survival of
the fittest or only the strong
survive
Mutation
 Genes randomly mutate.
 This changes the genetic makeup of certain
organisms phenotype allowing for changes
that can be harmful or beneficial
MUTATIONS
 Some “green genes” randomly mutated
to “brown genes” and were passed on
 *organisms do not mutate because it
will benefit them.
 MUTATIONS CAUSE VARIATION!
Natural selection
happens because of:
 Mutations
 Variation
 Environmental influence
 Let’s look at an example
Example:
In the U.S., where people
use shampoos with
particular chemicals in
order to kill lice, we have a
lot of lice that are
resistant to the chemicals
in those shampoos.
Resistant means
unaffected
Hypothesis A: Exposure to
lice shampoo actually
caused mutations for
resistance to the shampoo.
Hypothesis B: Resistant
strains of lice were always
there—and are just more
frequent now because all
the non-resistant lice died
a sudsy death.
Which of these hypotheses is
correct?
 Hypothesis B
Why?
Variation is there already
Shampoo killed the “weaker” lice
Other lice live and reproduce to make
more resistant lice
MUTATION ARE NOT
CAUSED!
 Lamarck was an evolutionist who
believed in hypothesis A.
 He was proven wrong.
 He called his theory “Acquired
Characteristics”.
DARWIN
 Hypothesis B is what Darwin
discovered.
 He called these mutations or
changes ADAPTATIONS
Evolution
What is evolution?
 Process by which
organisms pass on
traits from generation to
generation
 Explains the origin of new
species and diversity
 The living species of today
are related to each other
through common traits
Evidences of Evolution






Fossil Records
Homologous Structures
Analogous Structures
Vestigial Structures
Embryology
DNA / Amino Acid Homology
Fossil Records
 Fossil- A piece
of organism or
imprint left
behind.
 Fossils show
that species
have changed
over time.
Homologous
structures a structure like a
bone, that is shared
by a group of
different species
because it is
inherited from a
common ancestor
Analogous
Structures
 two structures are
said to be analogous
if they perform the
same or similar
function by a similar
mechanism but
evolved separately.
 Did you know that whales have feet?
 Well, sort of….
Fossils help us determine whale evolution
60 million
years ago
50 million
years ago
40 million
years ago
Yesterday
Vestigal Organs
 “left over” structures from a common
ancestor
Bat
eyes
Similarities in Embryology
 The early stages of development of
animals with backbones are very similar
(Means the same!)
DNA/Amino Acid Homology
 Human
5’A-T-C-T-T-A-C-G-A-A-T-C-A-T-G-C-C-C-T-A-AC-T-T-C-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’
 Whale
5’A-C-C-T-T-A-C-C-A-A-T-C-A-T-T-C-C-C-T-A-AC-T-T-A-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’
 Frog
5’A-C-C-A-T-G-C-G-T-G-T-C-A-T-C-C-C-C-G-AG-C-T-T-C-G-G-T-A-T-T-A-C-G-C-A-A-G-C-3’
 Gorilla
5’A-T-C-T-T-A-C-G-A-A-T-C-A-T-G-C-C-C-T-A-AC-A-T-C-G-G-C-A-T-T-A-C-G-C-T-A-G-C-3’
Defining a Species
These spiders are from the
same species
* They look different because all
species show VARIATION
Speciation
 Species: a population of individuals that
can interbreed and produce FERTILE
offspring
 Speciation: The process in which new
species are formed
Isolating Mechanisms
 As new species evolve, populations
become reproductively isolated from
each other
 When individuals of two populations
cannot produce fertile offspring, then
reproductive isolation has occurred
 Three ways this can happen…
Geographic Isolation
 Physical separation of
members of a population
Geographic Isolation
 Populations are separated by geographic
barriers such as rivers, mountain ranges,
ect…
 The Abert squirrel population was split
into two groups about 10,000 years ago
by the Colorado River. A new species
formed from on one side of the river, the
Kaibab, which are significantly different
from the original population.
Reproductive Isolation
 Prezygotic
 Inability to breed because of being
geographically isolated from the other
species, breeding times being
contradictory,
or the animals behavior not being
consistent
with the other animals.
Reproductive Isolation
 Postzygotic
 Mishaps after the egg is fertilized.
 This includes the zygote not surviving
because it is rejected by the mothers body,
hybrid sterility, gametic isolation, and
hybrid inviability.
Behavioral Isolation
 Two populations are capable of
interbreeding but do not because of
differences in courtship rituals or
reproductive strategies
 The eastern meadowlark and western
meadowlark are capable of interbreeding
but do not due to differences in mating
songs
Temporal Isolation
 When populations reproduce at
different TIMES
 Three species of orchids all live in the
same rainforest. Each one only releases
pollen for one day, and each species
does this at different times throughout the
year, so they do not interbreed
Evolution of
populations
Natural Selection on
Polygenic Traits
 When a trait is controlled by two or more
genes natural selection can affect a
population in one of three ways.
 Directional Selection
 Stabilizing Selection
 Disruptive Selection
Directional Selection
 When individuals at one end of the
curve (one extreme OR the other) have
the advantage and the population
shifts in one direction.
Stabilizing Selection
 When individuals near the center of
the curve are selected for (average),
keeps most individuals in a population
near the average
Disruptive Selection
 When individuals near the middle are
selected against, and the population
splits so that each extreme grows
Founder effect
 This effect is the loss of
genetic variation that
occurs when a new
population is
established by a very
small number of
individuals from a larger
population.
Bottleneck effect
 This effect happens when a significant
percentage of a population or species is
killed or otherwise prevented from
reproducing
Evolution vs. Genetic
Equilibrium
 The Hardy-Weinberg Principle states that
allele frequencies will remain constant
unless one or more factors cause those
frequencies to change
 When the allele frequencies remain
constant it is called genetic equilibrium
and the population will not evolve
Genetic Equilibrium
 In order for a population to be in genetic
equilibrium five conditions have to be met
 Random mating, very large population, no
movement in or out, no mutations, no natural
selection
 If one of these conditions are not met the
pop. will evolve
Cladogenesis
Cladogenesis
 The process by which species split
into two distinct species
 Cladogram - diagram used in cladistics
which shows relations among organisms
Punctuated Equilibrium
• Equilibrium is broken by
rare events of large
change away from the norm
phenotype, characterized
by rapid events of
branching speciation
called cladogenesis.
Gradualism
 Gradualism- Homeostasis is broken by small
changes that occur, or ought to occur, slowly in
the form of gradual steps over a long time
Divergent Evolution
 The accumulation of differences between
groups which can lead to the formation of
new species.
 Ex: Homologous Species
Convergent Evolution
 The acquisition of the same biological
trait in unrelated lineages.
 Ex: Analogous Species