Transcript Natural Selection - Alex LeMay – Science

Natural
Selection
…how evolution works.
Natural selection is…
–Natural selection says that the
organisms best suited to the
environment will live to reproduce
while other organisms that are not as
well suited will die.
What does Natural Selection
mean?
Organisms that are better adapted
to their environment tend to
survive longer. (Nature has
“selected” them to survive because
they have more useful traits.)
The survivors tend to produce more
offspring than the less well adapted,
so those useful genetic traits
become more common, until, after
many generations, all of that
This
species has the useful trait.
process of change over
time is called evolution
Natural Selection
Darwin noted 5 “mechanisms” that drove
natural selection:
1.
2.
3.
4.
5.
Overproduction
Variations
Competition
Selection
Environmental Change
Steps of natural selection
1. Overproduction - There are more creatures
born than will survive to addulthood and
reproduce.
2. Variation - Individuals of the same species
are genetically little different from each
other.
Steps of natural selection
5. Competition - They will pass those traits on
to the next generation.
Steps of natural selection
5. Selection - Over time, the traits seen in a
population will have those that help
individuals survive
6. Environmental Change -If the environment
changes, the usueful traints, and thus the
population changes.
Overproduction
Simply stated: more offspring are produced
than can survive in a species.
Variations
• These are differences among individuals in a
population.
Competition
• Competition: Individuals will compete to
survive and reproduce.
Indirect Competition:
Using the same resources.
Zebras and Wildebeast
eating the same grass.
Direct Competion: Hurting or
killing another organism to
obtain a resource. Lions and
Hyenas fighting over a kill, or
males fighting for mates. ->
Selection
• Different traits (variations) make organisms
more or less likely to survive.
• The environment “selects” organisms (lets them
live and reproduce) based which have the most
useful variations.
- Organisms that are
better suited to live in
an environment will
survive, reproduce and
pass on their traits.
- Over time, this can
lead to changes in a
species, aka, evolution
Selection
A real example, the Arctic Fox
Sometimes there are beneficial genetic
behaviors as well.
Helen?
Helen?
Environmental Change
Changes in the environment can affect the whether certain individuals in
a species survive.
Pre Industrial Revolution
Post Industrial Revolution
Environmental changes can make other traits more useful (so
they become dominant) or they might wipe out a species entirely if there
is not a variation that can survive the change.
“When the
monster came,
Lola, like the
Peppered Moth,
remained
motionless and
undetected.
Harold, of course,
was immediately
devoured.”
Natural selection at work…
THEN…
THEN…
Background (normal) Extinction vs. Mass (holy crud!) Extinction
Mass Extinction…
A Little History
Darwin, Evolution, and
Natural Selection
Charles Darwin
I’m a naturalist.
• Came up with the
idea that evolution
happens through
natural selection.
• Is credited with
being the father of
evolution
• Accompanied the captain of
the H.M.S. Beagle on a journey
around the world.
• Observed many plants and
animals.
• Observed some very
interesting (weird) animals in
the Galapagos Islands that
helped him discover the
process of natural selection.
Darwin’s Journey
Iguanas don’t swim.
Islands provide
environments you
can’t leave. You
must adapt to if you
are an animal.
Darwin’s Discovery
• Noticed that animals seemed to be specially
adapted to the places that they lived.
He especially noticed this in a type of bird,
a finch, that lived in the Galapagos
Islands, but had weird beaks.
All of their
beaks were
specifically
suited to the
type of food
that they ate.
Aha!
• Darwin determined that the finches that had the
beaks that got the most food lived longer and
reproduced more than the ones that didn’t get as
much food.
• That led him to his idea of natural selection, or
”survival of the fittest,” (those best suited to the
environment are likeliest to survive, reproduce,
and pass on those traits.)
Darwin Thought..
• That the finches had adapted to being on each of
the islands because each island had different food
• Eventually these finches lived apart from each other
for so long and changed so much that they could no
longer breed with each other.
That is what makes a new species!
Darwin’s Picture of how he thought the
finches were related
Cladogram- a branching diagram that
represents the proposed evolutionary history
of a species
Taxonomy – how we
classify things
When he got home…
• Married his cousin
• Was afraid to publish his findings, thought people,
like his wife, would be upset with him.
• Studied more of his idea by watching pigeons.
• Finally published his book On the Origin of Species
in 1858 because another scientist (Alfred Wallace)
was going to beat him to it.
Darwin
• In 1859, Darwin published his findings in a
book titled “The Origin of Species”
Darwin’s idea had two parts:
• 1. Descent with
modification- animals
and plants on the
earth today are just
changed versions of
what was here long
ago (new animals
don’t just come to life
out of nowhere).
Darwin’s Idea had two parts:
• 2. Modification by
natural selectionstates how the
evolution occurs.
Basically those who
are most suited to the
environment pass on
their DNA, those that
die…don’t.
Lamark Proposed First Hypothesis
• Thought
giraffes had
long necks
because they
stretched them
to reach
leaves.
• Was proven to
be wrong
Why Lamark Was Wrong
• DNA is
passed to
offspring,
not acquired
traits.
Evolution
Evidence for Evolution
1. Fossil Record
2. Embryos
3. Structures
1. Homologous
2. Analogous
3. Vestigial
4.Geological
5.Cellular Evidence
6.Experiments/
Artificial selection
7. DNA
Fossil Record
Fossil Record
Fossil Record
Above left, the Cretaceous snake Pachyrhachis problematicus
clearly had small hindlimbs. The drawing at right shows a
reconstruction of the pelvis and hindlimb of Pachyrhachis
Embryos
Homologous Structures- Structures
that are similar due to common
ancestry.
Analogous Structures
Shows that
animals in
same habitat
tend to
develop
similar features
due to natural
selection.
Geological Evidence
• Law of superposition
• Dating of rocks
Vestigial Structures- Structures that used
to function, but no longer have a purpose.
•Tailbones in People
•Appendix in People
•Ear muscles that no longer function
in people
•Hip bones in Snakes
Geological Evidence
Common ancestor was on a supercontinent;
decendents moved with the plates.
Cellular Evidence
Experiments/Artificial Selection
DNA
Cladogram- a branching
diagram that represents
the proposed
evolutionary history of a
species
Rate of evolution
• Different kinds of organisms evolve at
different rates
– Bacteria evolves very quickly due to short lifespan
and fast reproductive rates
– While lungfish have changed little over the past
150 million years
Rate of evolution
• There are two theories for the rate of
evolution
• Gradualism- species formation is constantly
occurring by accumulating small differences
• Punctuated equilibrium- species formation
occurs in burst, separated by long periods of
stasis.
End of Eighth Grade Material
• …H. Bio follows.
Molecular clock
• Molecular clock is a model that is used to
compare DNA sequences from two different
species to estimate how long the species have
been evolving since they diverged from a
common ancestor
Genetic change within populations
• Population genetics- study of the properties of
genes in populations
• Alleles- alternative forms of a gene
• Allele frequencies- how often a specific allele
is found within a population
Why allele frequencies change
• Five evolutionary forces have the potential to
significantly alter allele frequencies in
populations
– Mutation
– Migration
– Genetic drift
– Nonrandom mating
– Selection
Why allele frequencies change
• Mutation- when an error in replication of
nucleotide sequence in DNA
• Migration- movement of individuals from one
population into another
– This can lead to introduction of new alleles to a
population
Why allele frequencies change
• Genetic drift- change in the frequency of an
allele in a population by random sampling
– Most dramatic in small populations
– Natural disasters can cause genetic drift for
example a strong hurricane hitting a small island
can destroy large portion of a population causing
a loss in alleles
Genetic drift
• The surviving individuals form a disaster is a
random sample of the original population and
will begin to repopulate the environment
• This idea is know as bottleneck effect
– Which can cause a loss in allele
Genetic drift
• Founder effect- loss of
genetic variation that
occurs when a new
population is
established by a very
small number of
individuals from a large
population
Example of founder effect
• This Amish women is
holding child with Ellisvan Creveld syndrome.
This disorder was
introduced in the Amish
community by one of its
founders in 18th century
and persist to this day.
Why allele frequencies change
• Nonrandom mating- individuals with certain
genotypes sometimes mate with one another
either more or less commonly than would be
expected on a random basis
• Inbreeding- mating with relatives
Why allele frequencies change
• Selection- when certain traits or alleles of
genes segregating within a population
• Artificial selection- the breeder selects for the
desired characteristics
Forms of selection
• Stabilizing selectionwhen selection acts to
eliminate both
extremes from an array
of phenotypes
• Results in an increase in
frequencies already
common intermediate
phenotype
Forms of selection
• Disruptive selectionselection that acts to
eliminate the
intermediate type
• Results in separating
the population into two
phenotypically distinct
groups
Forms of Selection
• Directional selectionwhen selection acts to
eliminate one extreme
from an array of
phenotypes
• Results in the gene
determining this
extreme become less
frequent in population
The biological species concept
• Speciation- the way natural selection leads to
the formation of new species
• Biological species concept- group of
interbreeding natural population that is
reproductively isolated from other groups
biological species concept
• Reproductively isolated mechanisms- barriers
that prevent genetic exchange between
species
– Leads to speciation
Types of Reproductively isolated
mechanisms
• Prezygotic isolating mechanisms- prevent the
formation of zygotes
• Postzygotic isolating mechanisms- prevent the
proper functioning of zygotes after they have
formed
Prezygotic isolating mechanisms
• Geographical isolation- species that exist in
different areas are not able to interbreed
– Physical boundaries like river or mountain range
Prezygotic isolating mechanisms
• Ecological isolation- species occur in the same
area but they occupy different habitats.
• Survival of hybrids is low because they are not
adapted to either environment of their
parents
Prezygotic isolating mechanisms
• Behavioral isolation- species differ in the
mating rituals
– Song birds
– Dances
– How they attract their mates
Prezygotic isolating mechanisms
• Mechanical isolation- structural differences
between species that prevent them from
mating
– Flowers pollen can be different sizes and shapes
Prezygotic isolating mechanisms
• Prevention of gamete formation- gametes of
one species function poorly with the gametes
of another species or within the reproductive
tract of another species
Postzygotic isolating mechanisms
• Development of any species is a complex
process
• The structures for hybrids between two
species may be so different that they cannot
function together normally in embryonic
development
Divergence
• Allopatric divergence- geographical separation
can lead to wide variety of organism
– Populations can appear much more likely to have
evolved differences leading to speciation
Speciation
• Sympatric speciation- is one species splitting
into two at a single location without being
geographically separated