The
Theory of
Evolution
Background: Darwin
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.
Background: Darwin
Darwin’s Ideas
2. Common Ancestry
– All life forms share a
common ancestor
– We are all in the same “tree of life”
Background: Definition
The Definition of Evolution
• Descent with Modification
– Small-scale evolution: Changes in gene
frequency in a population from one generation
to the next
– Large-scale evolution: The descent of
different species from a common ancestor
over many generations.
• What exactly is being modified?
Background: Definition
Which of these represents
descent with modification?
Big Beetles go through a
drought and have limited food
90% of the beetles have the
gene for green color
Next generation is physically
smaller due to lack of food
30% of the beetles have the
gene for green color
Mechanism: 1. Mutation
How does this “descent with
modification” happen?
1. Mutation
• A change in a DNA sequence
– usually occurring because of errors in
replication or repair.
•
Mutation is the ultimate source of genetic
variation.
Mechanism: 2. Migration
How does this “descent with
modification” happen?
2. Migration
• Individuals from one group move into
another group.
• Making the gene more frequent in the
population.
Mechanism: 3. Genetic Drift
How does this “descent with
modification” happen?
3. Genetic Drift
In each generation, some individuals may,
just by chance, leave behind a few more
descendents (and genes, of course!) than
other individuals.
Mechanism: 3. Genetic Drift
Genetic Drift: A game of chance
•
Imagine a game in which you have a bag holding 100
marbles
–
•
•
You are allowed to draw 10 marbles out of the bag.
Now imagine that the bag is restocked with 100
marbles
–
•
50 of which are brown and 50 green.
with the same proportion of brown and green marbles as you
have just drawn out.
The game might play out like this:
Mechanism: 3. Genetic Drift
What effect does Genetic Drift have
on Evolution?
1. Drift reduces genetic variation in populations,
potentially reducing a population’s ability to
evolve in response to new selective pressures.
2. Genetic drift acts faster and has more drastic
results in smaller populations. This effect is
particularly important in rare and endangered
species.
3. Genetic drift can contribute to speciation. For
example, a small isolated population may
diverge from the larger population through
genetic drift.
Mechanism: 3. Genetic Drift
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
Mechanism: 3. Genetic Drift
Examples of
Genetic Drift
2. Founder Effect
– Changes in gene
frequencies that
usually accompany
starting a new
population from a
small number of
individuals.
Mechanism: 3. Genetic Drift
Driftworm Activity
• Put your IntNB to the side, and get out:
– Your colored pencils and
– The worksheet you picked
up when you came in
• Color in Generation 0 of your driftworms
Mechanism: 3. Genetic Drift
Driftworm Activity
• Each of these worms reproduces
asexually
• The population size is constant
• The generations do not overlap
• These are haploid organisms
Mechanism: 3. Genetic Drift
Driftworm Activity
1. Roll the die and put a dot next to the
corresponding worm.
2. Put a dot next to the corresponding
number (the one you land on)
3. Do this five times (for each individual in
the population)
Mechanism: 3. Genetic Drift
Driftworm Activity
4. Repeat this until one allele is “fixed” in
the population.
•
That is, until there is only one phenotype in
the population.
Mechanism: 3. Genetic Drift
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. Mating (Sexual Reproduction)
4. A large population (to avoid the effects of
genetic drift)
5. and…
Mechanism: 4. Natural Selection
How does this “descent with
modification” happen?
4. 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.
Mechanism: 4. Natural Selection
Basic requirements of Natural
Selection
1. There is variation in traits.
2. There is differential reproduction.
Since the environment can't support
unlimited population growth, not all
individuals get to reproduce to their full
potential.
3. There is heredity.
Mechanism: 4. Natural Selection
Types of Selection
• Artificial Selection:
– Farmers and breeders have been only
allowing plants and animals with desirable
traits to reproduce.
Mechanism: 4. Natural Selection
Types of Selection
• Stabilizing Selection:
– Genetic Diversity decreases as a population
stabilizes on a particular genetic trait.
–Extreme traits are
selected against.
Mechanism: 4. Natural Selection
Types of Selection
• Disruptive Selection:
– Selection that favors the extremes of the
distribution
Mechanism: 4. Natural Selection
Types of Selection
• Directional Selection:
– One allele is favored over another, and the
population shifts in one direction
Mechanism: 4. Natural 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
Mechanism: 4. Natural Selection
Directional Selection
• Industrial Melanism Activity:
Mechanism: 4. Natural Selection
Directional Selection
• We will collect individual and class data and
see if selection took place.
• Draw the following data table in your IntNB.
When on White Paper
Number of White
Moths Remaining
Number of Newspaper
Moths Remaining
When on Newspaper
Mechanism: 4. Natural Selection
What is the general way we refer to
those who survive?
• Fitness:
• a genotype’s success at reproducing
– (the more offspring the genotype leaves, the
higher its fitness).
• Fitness describes how good a particular
genotype is at leaving offspring in the next
generation relative to other genotypes.
Is this always the biggest, fastest and strongest?
Mechanism: 4. Natural Selection
Let’s explore this Cartoon to find
out…
• Take a minute to read the cartoon that was
just handed out.
• When you are through reading the
cartoon, turn it over to indicate that you
have read it.
Based upon your reading, does fitness always
mean “the biggest, fastest and strongest”?
No. The “fit” cricket has adapted a strategy for
reproducing without the “cost.”
Mechanism: 4. Natural 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.
Mechanism: 4. Natural Selection
Q: The following are examples of
what type of adaptation?
1. Katydids blending in with their substrate.
 Structural
2. A bird’s mating call
 Behavioral
3. A protein working at human body
temperature and denaturing at higher
temperatures.
 Functional
Mechanism: 4. Natural Selection
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!
Mechanism: 4. Natural Selection
What about odd features that don’t
seem to serve any selective
function?
1. Vestigial Structures
– A feature that an organism inherited from its
ancestor but that is now functionless and
usually less elaborate than in the ancestor.
– Formed when a lineage experiences a
different set of selective pressures than its
ancestors, and selection to maintain the
elaboration and function of the feature ends.
Mechanism: 4. Natural Selection
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
• Deleterious Genes:
– Sickle Cell Trait
Evolution in Action: Coevolution
Coevolution
• A process in which two or more different
species reciprocally effect each other’s
evolution.
– For example, species A evolves, which
causes species B to evolve, which causes
species A to evolve, which causes species B
to evolve, etc
Coevolution is the evolutionary “arms race”
Evolution in Action: Coevolution
Types of Coevolution
1. Predator/prey and parasite/host
2. Competitive species
3. Mutualistic species
•
Like plants and pollinators