The
Theory of
Evolution
Darwin’s Ideas
1. Natural Selection
– A process in which some individuals have
genetically-based traits that improve survival
or reproduction
– Thus, they have more offspring surviving to
reproductive age than other individuals.
Darwin’s Ideas
2. Common Ancestry
– All life forms share a
common ancestor
– We are all in the same “tree of life”
The Definition of Evolution
Evolution is….
• Descent with Modification
– Small-scale evolution: Changes in gene
frequency in a population from one generation
to the next. (microevolution)
– Large-scale evolution: The descent of
different species from a common ancestor
over many generations. (macroevolution)
of thesewhat
represents
InWhich
this example,
exactly is
descent
withmodified?
modification?
being
Phenotypic traits of a population are
modified over time.
This is measurable as changes in
Big Beetles go through a
Next generation is physically
“allele
frequencies”
drought and have limited food
smaller due to lack of food
or
90% of the beetles have the
gene for green color
30% of the beetles have the
gene for green color
A change in allele frequencies
in a population is evidence of
microevolution
Let’s further study how allele
frequencies can change in an
activity called
“Breeding Bunnies”
Basic requirements of
Micro-evolution
1. There is variation in traits.
(diversity)
2. There is differential reproduction.
Due to selective forces in the
environment, not all individuals get to
reproduce to their full potential.
3. There is heredity.
How do populations introduce
variation or diversity?
1. Mutation
• A change in a DNA sequence
–
•
usually because of errors
in replication or repair.
Mutation is the ultimate source of
genetic variation.
How do populations introduce
variation or diversity?
2. Migration (Gene Flow)
• Individuals from one group move into
another group.
–
Brings in new genes or changes frequency
of genes in the population.
How do populations introduce
variation or diversity?
Let’s
consider the
But
does
genetic
In each generation, some individuals may,
following
drift
always
just by chance, leave behind a few more
game
of
increase
descendents (and genes, of course!) than
chance to
other
individuals.
diversity?
find out
3. Genetic Drift
Genetic Drift: A game of chance
•
•
•
•
Imagine a game in which you have a bag holding 100
marbles
th
How
does
the
diversity
of
the
4
–
50 of which are brown and 50 green.
generation
to that
You
are allowed tocompare
draw 10 marbles
out of of
the the
bag.
Now imagine
that the bag
is restocked with 100
starting
population?
marbles
decreased
The
variation
has
__________
in
–
with the same proportion of brown and green marbles as you
have just drawn out.
response
to selective pressures
The game might play out like this:
What effect does genetic drift have
on smaller populations?
1. Genetic drift acts faster and has more drastic
results in smaller populations. This effect is
particularly important in rare and endangered
species.
Examples of Genetic Drift
1. Population Bottleneck
– An event in which a population’s
size is greatly reduced
Northern Elephant Seal
Initial Diverse
Population
Event that causes the
population to be
reduced in size
Final
population
Cheetah
Examples of
Genetic Drift
2. Founder Effect
– Changes in gene
frequencies that
usually accompany
starting a new
population from a
small number of
individuals.
Driftworm Activity
• Turn your intNB to the driftworm activity,
and get out:
– Five colored pencils
• Color in Generation 0 of your driftworms
Driftworm Activity
• Each of these worms reproduces
asexually
• The population size is constant
• The generations do not overlap
• These are haploid organisms
Driftworm Activity
1. Roll the die.
2. This number represents which worm
survives (by chance)and can be colored in
for the next generation.
3. Do this four more times.
Driftworm Activity
4. Repeat this in as many generations as
possible
•
That is, until there is only one phenotype
“fixed” in the population.
What factors prevent an allele from
becoming “fixed” in a population?
1. Mutation
2. Migration (Immigration and Emigration)
 this is sometimes called “gene flow”
3. Meiosis and Sexual Reproduction
4. A large population (to avoid the effects of
genetic drift)
Genetic variation
(diversity) is
required for what
micro-evolutionary
process?
Natural Selection
•
•
A process in which some individuals
have genetically-based traits that
improve survival or reproduction
They have more offspring surviving to
reproductive age than other individuals.
Natural Selection
fittest
It is often called “survival of the _________.”
“Fit” often refers to a phenotype’s success in
reproducing.
But… is this term accurate?
Is it always the biggest, fastest, strongest?
Let’s read a cartoon about natural selection to
find out…
Types of Selection
• Artificial Selection:
– People choose plants and animals with
desirable traits to reproduce.
– Nature does NOT select.
Types of Selection
• Directional Selection:
– One allele is favored over another, and the
population shifts in one direction
Types of Selection
• Stabilizing Selection:
– Genetic Diversity decreases as a population
stabilizes on a particular genetic trait.
–Extreme traits are
selected against.
Types of Selection
• Disruptive Selection:
– Selection that favors the extremes of the
distribution
Overview of the types of
selection
Which of type of selection is/can be
caused by the following…
1. Human birth weight
 Answer: Stabilizing Selection
2. The finches Darwin observed on the
Galapagos Islands
 Answer: Disruptive Selection
3. Lighter moths being selected by
predators after the trees became
covered with soot (post Industrial
Revolution)
 Answer: Directional Selection
Natural Selection: Adaptations
• Adaptations: Come in many forms and
help the organism survive. It could be:
– Behavioral: Behaviors that an organism does
to survive
– Functional: An adaptation in which one
aspect of the organism has increased function
in an environment
– Structural: Physical features on an organism
that enable it to survive.
Q: The following are examples of
what type of adaptation?
1. Katydids blending in with their
surroundings.
 Structural
2. A bird’s mating call
 Behavioral
3. A protein working at human body
temperature and denaturing at higher
temperatures.
 Functional
Q: The following are examples of
what type of adaptation?
1. Echolocation in bats searching for food
 Behavioral
2. A bird’s beak
 Structural
3. The non-poisonous milk snake has a
banding pattern similar, but not the same
as a poisonous coral snake.
 Structural: This is mimicry!
What about odd features that don’t
seem to serve any selective
function?
Vestigial Structures
– An inherited feature that is now functionless
and usually less elaborate than in the
ancestor.
– Formed when a species experiences a
different set of selective pressures than its
ancestors, and selection to maintain the
elaboration and function of the feature ends.
Misconceptions about Natural
Selection
• It is not “Survival of the Fittest” - really it is
“Survival of the ‘fit enough’”
– Recessive alleles for disease can stay
unnoticed in a population
• Organism does not choose
• Perfection is not obtained
Coevolution
• A process in which two or more different
species reciprocally effect each other’s
evolution.
– species A evolves, which causes species B to
evolve, which causes species A to evolve,
which causes species B to evolve, etc.
– For ex. Co-Evolution of Computer – virus
Types of Coevolution
1. Predator/prey and parasite/host
2. Competitive species
3. Mutualistic species
•
Like plants and pollinators
Macroevolution
(Speciation)
• Form of large-scale evolution
• Descent of different species from a
common ancestor
What evidence is
there for
macroevolution?
1. Fossil Record
2. Geographic
Distribution of
Living Species
3. Homologous Body Structures
4. Vestigial Organs
5. Similarities in
Early Development
Mechanism of
Macroevolution
For macroevolution to
occur, two populations must
be reproductively isolated
Green
(or separated) from one
blends
another
in with
grass
The separated population
experience different selective
pressures
Over time, they evolve in
separate directions, into different
species that cannot interbreed
Brown
blends
in with
tree
trunks
What is a
species?
Two populations that can
mate and produce fertile
offspring.
Ex. donkeys and horses are
considered different species
because they produce
mules, that are not fertile.
But… all breeds of dogs are
the same species
“mutts” are still fertile
Does speciation occur gradually or
suddenly?
Punctuated Equilibrium:
Gradualism:
- Species stay relatively
unchanged over time
- Phenotypic changes due to
evolution accumulate slowly
over time
- Speciation is driven by major
events that drastically change
the environment, forcing rapid
evolution
- Evolution occurs in rapid
bursts, as supported by gaps
in the fossil record.
- Speciation occurs gradually
Does speciation occur gradually or
suddenly?
• Speciation occurs both gradually and
punctually
• There is evidence to support both
ideologies