Transcript Natural Selection

Ch 23 Mechanisms of Evolution - Which one would not promote change over time?
Natural Selection
a major mechanism for evolution
Which organisms will be selected, depends
on their direct ability to survive & reproduce
What’s the premise of Natural Selection - coined by Charles Darwin…
1. There is a struggle for existence
Organisms must compete for limited resources - food, space, a mate.
2. Many more offspring are born each season than can possibly survive
to maturity (overpopulation)
3. Their survival is based on beneficial phenotypes (inherited adaptations)
that give them an advantage, increasing their likelihood of survival and
production of more viable offspring.
4. Alleles (variations of a trait) tend to increase over time when
“selective pressure” favors that phenotype.
5. Over long periods of time, and given a steady input of new variation into
a population, these processes lead to the emergence of new species
Which premise do these photos demonstrate?
What is the selective pressure ? – and what allele or phenotype is favored in each photo?
Survival of The Fittest
Rank these apes by fitness level (#1 being the best)
Justify your rationale. What is the selective pressure?
Name
Boris
Rex
Hank
Abe
Age at Death
13 years
12 years
16 years
10 years
# offspring
fathered
18
20
25
20
# offspring
surviving into
adulthood
15
14
14
19
Size
6’2”
5’8”
5’10”
5’8”
Which premise do these photos demonstrate?
Advantages/Disadvantages of each and compare.
Emperor Scorpion
r/K Selection - Quality vs Quantity
- at parental expense
Thornbug Treehopper
Variation
(key to evolution) - comes about in two ways
1. Sexual reproduction creates variation
causes recombination of alleles into new
arrangements in every offspring
Genetic shuffling is a source of variation
2. Mutation creates variation
new mutations are constantly appearing
not all bad) causes changes DNA sequence
(change amino acid sequence? -- protein? -- phenotype? -- fitness??)
Some fish adapt by evolving antifreeze proteins.
(…and it’s
Princess Diana, Prince William & Harry
Jonas Brothers
Baldwin Brothers
Venus & Serena
Reproduction recombines
alleles into new arrangements
in every offspring
Madonna & daughter
Luke & Owen
John Lennon’s sons
Cameron & Chimene Diaz
Penelope Cruz
Martin Sheen and sons
Kennedy’s
Mariska Hargitay & mom
What are the beneficial phenotypes?
What “drives” the phenotype?
What is the selective pressure that ensures these alleles are kept in the population?
Predatory Selection Defensive adaptation
Predatory Selection Cryptic Coloration (camouflage)
Magnificent
Frigatebird
Sexual Selection - desire to mate
Physiological Selection - desire to
survive in their environment
In a Directional Selection, the
shift in the population is
against one extreme variant
toward a preferred extreme the longer neck giraffes are
more physically fit and have a
better chance of survival and
reproduction rate than do the
medium or smaller giraffes.
What is being selected against?
What is being selected for?
What’s the selective pressure?
Natural selection can act in a number of directions
Use one animal from your illustrative examples to
create a Title and
In a Directional Selection, the
identify the Independent & Dependent
shift in the population is
variables on the x & y axes
against one extreme variant
toward a preferred phenotype
What is being selected against?
What is being selected for?
What’s the selective pressure?
In a Stabilizing Selection, the
shift in the population is
toward the moderate variant this graphs shows that the two
extremes have been reduced
in the population, and the
medium sized lizards have the
best chance at survival &
reproduction.
What is being selected against?
What is being selected for?
What’s the selective pressure?
Natural selection can act in a number of directions
Use one animal from your illustrative examples to
create a Title and
In a Stabilizing Selection, the
identify the Independent & Dependent
shift in the population is
variables on the x & y axes
toward the moderate variant against/eliminating both
extremes
What is being selected against?
What is being selected for?
What’s the selective pressure?
In a Disruptive Selection, the
shift in the population is
against the mean/middle
selecting for toward both
extremes - this graphs shows
that the two extremes have
been extenuated in the
population, those birds with
either long beaks or really
short beaks have the best
chance at survival &
reproduction.
What is being selected against?
What is being selected for?
What’s the selective pressure?
Natural selection can act in a number of directions
Use one animal from your illustrative examples to
create a Title and
In a Disruptive Selection, the
identify the Independent & Dependent
shift in the population is
variables on the x & y axes
selection against the mean,
toward both extremes
What is being selected against?
What is being selected for?
What’s the selective pressure?
Changes in populations
 Evolution of populations is measuring changes in allele frequency

all the genes & alleles in a population = gene pool
 Factors that alter allele frequencies in a population are:
1. natural selection - survival of the fittest
2. genetic drift - evolution is also driven by random, chance events
a. founder effect - small group splinters off to start a new colony
b. bottleneck effect - some factor (usually a disaster) reduces a population to a
small number, then it recovers and expands again.
3. gene flow -
immigration and emigration allow for the movement of genes, creating a
more diverse gene pool
Looking at the following pictures, which factor has created a change in the allele frequencies
b. Gene flow
 Population spread over large area


migrations = individuals move from one area to another
sub-populations may have different allele frequencies
 Migrations cause genetic mixing across regions =
gene flow


new alleles are moving into gene pool
reduce differences between populations
Genetic Drift - as per Founder Effect
What is this allele frequency?
Some individuals from an original population break away,
find a new place and start their own colony - they survive
and reproduce
Record the allele frequencies for each population.
Descendants A
Founder A
Founder B
What is this allele frequency?
Descendants B
What is this allele frequency?
Genetic Drift - as per Bottleneck Effect
When large population is drastically reduced by a disaster
 famine, natural disaster, loss of habitat…
 loss of variation by chance
 alleles lost from gene pool
 reduces variation
 reduces ability to adapt
 puts populations at risk
Any Questions??
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