Transcript Natural Selection - SBI3U

Natural Selection
The Mechanism of Existence
Natural Selection
• 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
1. The Struggle for Existence
• Drawing from Malthus’s ideas,
Darwin recognized that all
species produce excessive no. of
offspring.
• But due to limited resources in
nature, only a small percentage of
offspring survive in each
generation
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
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
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
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.
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 [Video]
• 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
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.
Evidence for Natural Selection
• Since Darwin, there have been several other
evidences for Natural Selection. Some of them are:
• Artificial Selection
• Changes in Beak Shape
• Antibiotic Resistance in Bacteria
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.
• Problems [Video]
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
• 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.
• New problem: Staphylococcus aureus,
human skin bacteria, cause pimples on
skin, pneumonia if they enter the body
• Methicillin-resistant strains are in
Ontario hospitals. Why?