Transcript IB-T5-4-Evolution - bio

5.4: Evolution
Topic 5: Ecology & Evolution
Miss Friedman
5.1: Definition
Evolution is the cumulative change in the
heritable characteristics of a population
►
Darwinian Evolution is not simply based on
natural selection but was in fact composed of at
least five different “sub theories”
1.
2.
3.
4.
5.
Evolution
Common descent
Gradualism
Multiplication of species
Natural selection
1. Evolution
► All
life is and has been perpetually
changing. This contrasts strongly with
notions that all forms of life are constant
and unchanging
2. Common descent
► All
living things share a common ancestor if
traced back far enough
3. Gradualism
► Evolutionary
change takes place slowly and
gradually. This contrasts with saltation in
which changes are sudden and extreme
4. Multiplication of species
► Diversity
of life is a consequence of
speciation. Populations adapting to locations
and becoming reproductively isolated from
other populations
5. Natural selection
►A
two stage process in which:
 Producing genetic variation
 selection
5.4.2: Evidence of evolution
► Evolution,
like any scientific theory, requires
evidence.
► Evidence shows that organisms change over
time and even result in the production of
new species of organisms
► Types of evidence
1. Fossil record
2. Homologous structures
1. Fossil Record
A fossil is the ancient preserved remains of an organism. The
fossil can be dated from the age of the rock formation.
► Sequences of fossil show the gradual change of an
organism over geological time
► Although the planet Earth has extensive ocenas for most of
its existence, fish fossils have only been found in rocks for
500 million years (less than 15% of the Earth’s age)
► No top predators such as bears and orcas existed in the
time of dinosaurs
► Very few organisms today have identical form today as
hundreds of years ago
2. Homologous structures
Similarities between anatomical structure
which are similar in form and function but
which are found in seeming dissimilar
species.
The classic example is the pentadactyl
(penta=five) and (dacytl=fingers) limb of
the vertebrate
a)
b)
c)
Humerus
Radius
Ulna
•In each example the bones are modified and adapted to the
locomotion of the animal
•The fact that the basic shape and position of the bones is
similar suggest that they have a common ancestor
•In the whale there is no real need for the fingers as they can
still swim without them, this could suggest common ancestry
with the other five-fingered organisms
Divergence
► The pentadactyl limb structure shows
adaptation and modification from a common
limb (ancestor) structure
Convergence
► Two organisms with different ancestors have
a limb structure that fulfills the same function
but has evolved from different origins.
► Examples are wing of a bird and the wing of
an insect
Selective Breeding
► Man
has selectively bred animals and plants
for thousands of years.
► If an animal posses a characteristic that is
considered useful or valuable then this
animal is selected for breeding.
► The hope then is that this characteristic will
be present in the next generation and at a
higher frequency than before.
► In subsequent generations it may even then
be possible to select from an even more
advantageous characteristic
Selective breeding
continued
► The
observations of artificial selection
(selective breeding) suggested that natural
populations would:
 Show phenotypic variation
 Be subject to natural selection pressures
 There would be selection of those individuals
possessing the advantageous characteristic
5.4.3: Population size &
evolution
► The
population produces more offspring than the
carrying capacity of the environment can support:
 Offspring/population compete for limited resources
(Intraspecific competition)
 Some individuals have characteristic (or combination)
that gives them a competitve advantage
 These individuals are more likely to successfully
reproduce (offspring survive)
 Through inheritance the frequency of these
characteristics become greater in the next generation
 By definition these characteristics have a genetic basis
5.4.4: Population size & survival
► Plant
and animals produce far more
offspring then could ever survive
 Fish will lay hundreds of eggs yet only a handful
survive
► Why
are animal’s valuable resources wasted
if they are never going to give offspring?
► Competition is set up for resources so only
the best adapted organism will survive
5.4.4
continued
► It should be noted that the “struggle for
survival” in this model is a consequence of
over-population
► The struggle takes the form of individuals in
the population being “selected for” or
“selected against”
► Survivors form the new breeding population
► The frequency of advantageous alleles has
increased
► The change in the heritable characteristics is
by definition evolution
5.4.5: Variation in a species
► Populations
of a species show variation
► Variation means differences in phenotypes
► Notice that no two humans are alike (or
other animals)
5.4.6: Sexual reproduction &
variation
► Asexual
(bacteria) and sexual populations both
experience mutation which increases the variation
within the members of a population
► Sexual reproduction increases variation by the
selection of mate or mutation
 A fish with a better shaped mouth might be able to feed
on coral that cannot be accessed by other fish, he will
have a better chance of survival into adulthood
 A fish mouth could be inadequately adapted and as a
result die of starvation
5.4.6 continued
► Sexually
reproducing populations also
experience significant additional sources of
variation
► There are two sources of genetic variation
in populations:
1. Meiosis
2. Fertilization
1. Meiosis
► When
is an egg is made during meiosis,
only 50% of the mother’s information is
needed.
► Due to random distribution of the
chromosomes during meiosis, each egg has
a different combination and therefore two
eggs are never alike
2. Random fertilization
► Increases
the variation in the population a
second time
► Of the many sperm cells that exist, only one
will fertilize the egg
Conclusion
► Genes
get mixed up as a double lottery
once in meiosis and a second time in
fertilization
“… can we doubt (remembering that many
more individuals are born than can
possible survive) that individuals having
any advantage, however slight, over
others, would have the best chance of
surviving and procreating their kind? On
the other hand, we may feel sure that any
variation in the least injurious would be
rigidly destroyed. This preservation of
favourable variations and the rejection of
injurious varations, I call Natural
Selection” Darwin C. (1859) The Origin
of Species
5.4.7: Natural selection
Natural selection is a two stage process:
1. Production of variation
2. Selection
5.4.7
continued
► When a population evolves there is a
cumulative change in the heritable
characteristics of the population
► Natural selection can act on a population
without speciation occurring
► In effect the genetic profile of the
population is adapting to changes in local
conditions
► Ever phase in the process of evolution is
affected by variation and by selection
Factors in a
Model of
Natural
Selection
5.4.8: Examples of evolution
1, Antibiotic resistance in bacteria
2. Peppered Moth
3. New Zealand Kaka
1. Antibiotic resistance in bacteria