Transcript 3-4 Natural Selection and Evidence(edit).

Natural Selection
The Mechanism of Evolution
Natural Selection
• Darwin based his theory of natural selection on 3
sets of observations:
• The Struggle for Existence
• Natural variations within the same species
• Environment’s role in evolution
1. The Struggle for Existence
• Drawing from Malthus’s ideas,
Darwin recognized that all
species produce excessive # of
offspring.
• But due to limited resources in
nature, only a small percentage of
offspring survive in each
generation
• Carrying capacity (K) of a
population: max. # of
individuals that a particular
habitat can support
Carrying Capacity (K)
•
Availability of resources
(water, food, sunlight, shelter,
space, oxygen)
•
Build up of waste such as
excrement or carbon dioxide
•
Predation
•
Disease
Do you think the Earth has a carrying capacity? What do you think this means?
2. Variation
•
Variation refers to differences
among the members of the
same species.
•
Individual variation is
widespread in all species.
•
Much of this is heritable i.e. it
passes from one generation to
the next
•
Effects how successful an
organism is.
Where is the variation
coming from?
1. Sexual Reproduction
•
Crossing over leads to genetic
recombination or “gene mixing”
•
Daughter cells are different from
the original cell
•
Fertilization: male gamete
contributes ½ of the
chromosomes and the female
gamete contributes the other ½.
Where else…
2. Mutations
• may produce genes that
lead to certain diseases (e.g:
phenylketonuria; PKU)
• may lead to more
advantageous traits
• Only few genes mutate and
most are neither useful nor
harmful
3. The Role of the Environment
• Darwin observed that a key factor in the
survival of an organism was how well it
was suited for the environment.
• The environment selected those
individuals with variations that were best
suited for that environment.
• E.g. Darwin would’ve said that in a
population, the environment favored the
individuals born with longer necks so they
survived and reproduced and passed their
characteristics to the next generation
Ecological Niche
• An organism’s ecological niche
select whether or not it will survive
and reproduce.
• An Ecological Niche is the sum
total of a species’ use of the biotic
and abiotic factors in its
environment.
• Biotic Factors: Predators or prey
• Abiotic Factors: Water availability,
weather, etc.
So…
• Natural Selection is the process by
which individuals with inherited
characteristics well suited for the
environment leave more offspring on
average than individuals with
adaptations less suited for the
environment
• When this happens over many
generations, each new generation has a
higher proportion of individuals with
advantageous traits
• This would cause a population to
change over time
Simulation!!
• Activity and discussion
• Darwin based his theory of natural selection on 3
sets of observations:
• The Struggle for Existence
• Natural variations within the same species
• Environment’s role in evolution
Changes in Beak Shape
• Peter and Rosemary Grant
(Princeton University) studied
Daphne Major Island finches
for 30 years
• They found that average beak
and body size of the medium
ground finch changes as El
Niños come and go
• El Niño is a warming of the
ocean current that passes
along the coasts of Peru and
Ecuador and affects global
weather patterns
Changes in Beak Shape
• The medium ground finch
prefers smaller seeds and due
to their abundance in wet
years, eat few large seeds
• During dry years due to El
Niños, all seeds are in short
supply (due to fewer plants)
and large seeds are the bulk of
the diet
• Only birds with large and
tough beaks survive these
years.
• With the return of the wet
years, the average beak size
diverges again
Antibiotic Resistance in Bacteria
Antibiotics: medication used to kill or inhibit the growth
of bacteria
Steps leading to antibiotic resistance:
1. A person is infected with a bacterial infection
2. Antibiotics are prescribed to kill the bacteria
3. The infection is largely destroyed and the patient gets better
4. One bacterium survives because of a random mutation
5. The bacterium begins to multiply, once again making the patient sick
again
6. The patient returns to the doctor and receives the same antibiotic
7. The patient remains sick
8. The doctor prescribes a different antibiotic
Resistance
1.
Mutation
2. Plasmid Transfer
•
The donation of
genetic information to
another in a ring of
nucleotides (plasmid).
Antibiotic Resistance in Bacteria
• Tuberculosis (TB) is severe
bacterial chest infection caused by
Myobacterium tuberculosis.
• In 1940s, Streptomycin was
developed to fight it and by 1970s
it was almost wiped out
• Resurgence in 2006 in South
Africa and by 2008 had spread to
49 countries. In 2009 1/3 of
world population had TB. 5%
new infections were resistant to
antibiotics
• Why?
Antibiotic Resistance in Bacteria
• Bacteria have many variations. Some
can be destroyed by antibiotics, others
are resistant.
• TB is treated by a 6-9 month course of
4 different antibiotics. Why?
• Patients may stop the treatment
prematurely and not take all the
antibiotics needed to kill all bacteria.
Survival of the Fittest?
• Fitness of an individual is
its ability to survive and
reproduce in its specific
environment.
• Natural selection does not
make organisms “better”. It
doesn’t have a goal.
• If the environment changes
in some way, another
adaptation might be favored.
Evidence for Evolution
• Evolution has left its marks
on all aspects of life:
• The Fossil Record
• Geographical distribution
• Anatomy
• Development of species
• Molecular Biology
The Fossil Record
• Most fossils are sound in
sedimentary rocks where younger
layers of rock, or strata, are
deposited on top of older ones
• Fossil Record is the chronological
collection of life’s remains in the
rock layers recorded over time
• Paleontologists are scientists who
study fossils
• Oldest fossils (Prokaryotes) were
found in rocks 3.5 Billion years old
The Fossil Record:
Whale Evolution
• Fossils of aquatic fauna are usually the best preserved
• Fossil evidence supports the hypothesis that whales
evolved from land-dwelling ancestors with four limbs
The Fossil Record:
Origins and Extinctions
• Fossil evidence also provides
evidence of the rise and fall of some
species.
• Dinosaur-like animals first appeared
230 Million years ago and went
extinct some 65 Million years ago
[Video]
• Drawback: Species without hard
tissue (like bones or shells) rarely
become fossilized and so its little
help in establishing the evolutionary
history of those organisms.
Geographic Distribution: Closely
Related but Different
• Darwin’s study of finches
showed that they were all
descendents of the single
ancestral species but today
all finch species in the
Galapagos has a beak
adapted to eating a specific
type of food
• Same for the tortoise species
that live on those islands
Geographic Distribution:
Distantly Related but Similar
• Similar environments will
select for similar adaptations
• E.g. the flightless birds of
grasslands look similar to
each other but their body
structure indicated that they
descended from different
types of birds
Geographical Distribution:
Marsupials vs Placental Mammals
• Marsupials: Their fetus leave the
uterus early and completes further
development in a pouched
• Placental mammals: The fetus is
protected by a placenta and
develops inside the uterus
• The reason Australia has so many
marsupials is because they all
evolved from marsupial ancestors
on an island isolated from
placental mammals in the distant
past
Comparative Anatomy:
Homologous Structures
• Similar structures in species
that share a common
ancestor are called
homologous structures.
• The structures that
originally functioned in one
way in an ancestral species
are now modified for new
functions.
• E.g. limbs of various
mammals
Comparative Anatomy: Vestigial
Structures
• Vestigial Structures are
remnants of structures that may
have had important functions in
an ancestral species but have no
clear function in some modern
descendents. E.g.
• Whales have vestigial hip bones
• Humans have appendices
Comparative Anatomy:
Analogous Structures
• Analogous structures are
structures, in distantly related
species, that are anatomically
different but serve the same
function. E.g. wings of
different kinds
• The evolutionary idea of
descent is based on structure
not function
• Convergent evolution
Comparative Development
• Comparing the various
stages of development of
various organisms shows
embryos of closely related
species with similar stages
in development.
• E.g. Vertebrate embryos are
all show pharyngeal gill
pouches, even in animals
with no gills.
Molecular Biology
• DNA provides a record of an organism’s
ancestry.
• The greater the number of differences in
DNA, the less likely it is that they share a
close common ancestry. E.g. siblings have
very similar DNA
• Cytochrome c sequence in the
mitochondrial DNA is found in all
oxygen dependent organisms.
• This sequence is highly conserved and the
amount of difference in sequence is used
to build phylogenetic trees.
Artificial Selection
• Artificial Selection is the selective
breeding of domesticated animals
to produce offspring with genetic
traits that humans value.
• Breeders breeding allow only those
plants or animals with useful traits
to reproduce.
• Artificial selection can produce a
great deal of change in a species in
a short time.
Artificial Selection
• Farmers breed cows that excel in
milk production
• Not a random process because the
farmers leave nothing to chance.
• In selective breeding,
microevolution can occur at a very
fast pace
• BUT also reduce the genetic
variability within the population
leading to genetic disorders
• E.g. Pedigree dogs