Transcript Natural Selection PPT

WHAT CAUSES
EVOLUTION TO OCCUR?
DARWIN’S MECHANISM:
THE THEORY OF
NATURAL SELECTION
NATURAL SELECTION
• DEFINITION:
– the process by which organisms that are better suited to
an environment survive and reproduce in greater number
than those less suited.
• What causes natural selection?
• Environmental Change
• “Survival of the Fittest”
• (But what is fit????)
• Categorized into 5 Steps
STEP 1: Overproduction
• All species over reproduce creating a population with many
variations (alleles)
– the total of all alleles (or variations) in the population is called a gene pool
– What is the allele frequency for feather color in this population of birds?
STEP 2: Inherited Variation
•Variations within a population come from mutations &
random mating changing the populations gene pool.
MUTATIONS: Changes in an organisms DNA or genes
(Darwin was missing this key info on genetics)
RANDOM MATING: mixing of genes through reproduction
A change in the gene for feather color creates different alleles which creates
different coloration of bird feathers in the population
VARIATIONS IN BIRD COLOR:
• Most birds are born with traits (genes) for being bluegreen in color.
• Occasionally some birds are born with traits that make
them more green.
• Occasionally some birds are born with traits that make
them more blue.
STEP 3: Struggle to Survive
Organisms compete for resources such as
food, shelter, and mates. Those better
suited to meeting these needs will
survive.
The birds face the challenge of blending in to survive.
STEP 3: Struggle to Survive
Ugh!, so
full…
Not all members of the species will
survive this competition. Only
some will survive and have the
opportunity to pass on their
successful genes.
Since blue-green birds
stand out more in the
habitat than green birds,
more blue-green birds are
eaten
EEP
!
STEP 4: Successful Reproduction
Successful traits are passed to the next generation.
Now, more members of the population are being born with
the successful variation than those with the less suited trait.
How has the allele frequency in the population changed?
This is how a species changes over time (evolves)
Now more green chicks than blue/ blue-green are being born.
STEP 5: Adaptation of the Species
?
(Not included in the textbook)
Adaptation – process by which a species
becomes better suited to its
environment.
Hee, hee…
NATURAL SELECTION EXAMPLES
1. Giraffe’s long neck
2. Tuskless elephants (pg 122)
OR
Insecticide Resistance (pg 123)
3. Pick your own species adaptation
Additional Examples
•
Peppered Moth
•
Sickle Cell Anemia
NATURAL SELECTION EXAMPLE
Sickle Cell Anemia
NATURAL SELECTION EXAMPLE
Antibiotics vs. Bacteria
Salmonella (bacteria that
causes food poisoning)
vs
Salmonycin antibiotic
(fights against Salmonella
bacteria)
Discussion Questions
• According to Natural Selection, explain
why human overpopulation exists?
• Are humans evolving?
SPECIATION (Steps to a new species)
Speciation – The formation of a new species by the change of allele frequency
(genetic variation) due to natural selection.
Species: a group of organisms that can interbreed and produce fertile offspring.
Steps of Speciation
1.
Separation: part of a population becomes isolated from the rest.
•
What could cause separation?
2.
Adaptation: Natural selection improves the fit between species & environment
created from changes in the gene pool.
3.
Divergence: Differences in the gene pool accumulate between populations.
Enough differences accumulate no longer allowing the populations to
interbreed, which leads to a NEW SPECIES.
Example of Speciation
1
1. A population of fruit flies lay their eggs in
some rotting bananas along a beach.
2. Separation - A hurricane washes the bananas
and eggs out to sea and onto a nearby island.
• The flies hatch and are genetically isolated
from the mainland fruit flies.
2
3
4
• Adaptation - Environmental conditions are
different on the island, natural
so the island
selection
fliesselects
evolve
different
traits
helping
adaptations.
the species adapt to the island.
3. Divergence - Natural selection changes
physical features, food preferences, and mating
rituals.
4. If the two populations were to by chance meet
again they would unlikely mate with each other
due to the differences in their gene pools creating
differences in the characteristics listed above,
especially mating rituals.
Speciation
Madagascar – Examples of
Speciation
Speciation Example – Flightless Island Birds
Speciation Experiment
• Fruit flies were taken from a single
population and divided into two
groups living in different cages
(geographic isolation).
• One group lived on maltose (sugar)food, and the other group lived on
starch foods (differing environmental
conditions).
• After many generations, it was found
that some reproductive isolation had
occurred due to the different food
sources in the two isolated
environments:
– "maltose flies" preferred other "maltose
flies,“
– "starch flies" preferred other "starch flies"
Speciation in Action?
In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a
raft of uprooted trees. They rode the high seas for a month before colonizing
the Caribbean island, Anguilla. These few individuals are the first of their
species to reach the island.
Evolutionary biologists would love to know what happens next: will the
colonizing iguanas die out, will they survive and change only slightly, or will
they become reproductively isolated from other iguanas and become a new
species? We could be watching the first steps of speciation, but in such a short
time we can't be sure.
Factors Contributing to Speciation
Isolating Mechanisms
Genetic Drift / Founders Effect
Isolating Mechanisms
•
Genetic differences that prevent breeding between two species
–
Pre-mating Examples - mating rituals, songs or calls, time of mating
–
Post-mating Examples – hybrids are usually sterile
Genetic Drift
•
Loss of genetic variation (alleles) brought about by chance, not
adaptation. (occurs in small populations – Example: Founders
Effect – on next slide)
–
Examples
•
•
cheetahs have little genetic variation (population decline 1,000s yrs ago
endangered species lack genetic variation due to habitat loss
Mountain Gorilla less than 720 left,
due to habitat loss
Founders Effect
•
Loss of genetic variation due to an isolated small founding population (could
contain some rare variations (alleles) making the gene pool different from
the original)
–
–
Examples – Amish (founded by a few couples of German immigrants in
1770; unusually high frequency of dwarfism & polydactyly (extra
fingers)
Why? Because of continuous marring within the group, recessive gene
diseases are more likely to show up in offspring
At what pace does evolution occur?
GRADUALISM: states that evolution
occurs slowly over time and
includes many transitions
PUNCTUATED EQUILIBRIUM:
states that evolution occurs in short
bursts followed by long periods of
stasis (no evolving)